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1. 20 kV 0 2 kV mm Table 2 11 Rule of thumb values for the selection of kilo voltage 102 Examples 15 mm steel 100 15x 8 220 kV 12mmaluminium 50 12x2 74kV 10 mm plastics 20 10x 0 2 22 kV In the range 200 400 kV only a significant change in voltage say 30 40 kV will cause a noticeable difference in defect discernibility Selection of gamma source As it is not possible to vary the radiation energy emitted by a gamma ray source it is necessary to indicate a range of thickness which may be satisfactorily examined with each type of radio isotope The upper limit is decided by the source strengths commercially available and the maxi mum tolerable exposure time the lower limit is determined by the decrease in contrast and the related reduced image quality The lower limit therefore depends on the required degree of defect discernibility When this is insufficient in comparison to what is achievable by the use of X ray equip ment another type of isotope providing a reduced energy radiation could be selected Table 3 11 shows the thickness range usually recommended for various gamma sources The table applies to steel If for reasons of convenience gamma rays are used on thin specimens which could also be X rayed it should be understood that the resulting radio graphs will be of poorer quality compared to X radiographs Source type Standard sensitivity High sensitivity technique in mm technique in mm Co6
2. If the aim is to make geometric unsharpness U equal to the value of U the source to film distance F required can be calculated from the following formula minimum distance t U ie 4s source to source side object mm source size mm U distance source side object to film mm 5000 Instead of calculating F various code based procedures and guidel ines provide graphs from which minimum distance F pin can be determined Figure 5 11 shows a nomogram on the basis of EN 1435 from which the minimum focus distance for two quality levels cate gory A and B can be extracted Fig 5 11 Nomogram for minimum source to film distance fmin according to EN 1435 criteria Catagory A less critical applications general techniques Catagory B techniques with high requirements of detail discernability In which a 3000 F source to film distance 2000 U total unsharpness t thickness of the object s effective source size fa T 9 50 50 All measurements in mm a 2 5 S S s 5 of of Y Y oO Oo Rz 2 ES ES The above graph appears enlarged in the appendix 101 11 3 Other considerations with regard to the source to film distance Inverse square law As explained in the previous section the effect of U can be reduced by increasing the focus to film distance F One of the properties of electromagnetic radia tion is that its intensity is inversely proportion
3. This document harmonises image file formats ASTM E 1475 02 regards DICONDE file formats DICONDE is the acronym for Digital Imaging and COmmunication in Non Destructive Evaluation This document includes a tamper proof file protocol to eliminate potential concerns In any case data is recorded in unprocessed format 16 10 Selection of CR and DR methods In radiography as in other areas of NDT no one method or technique will serve all situa tions of need There are a number of factors to be considered when evaluating a radiogra phic imaging system beginning with the size shape and the flexibility of the sensor Contrast sensitivity or grey scale range e g 8 bit versus 12 bit and resolution pixel size are major factors in determining the imaging performance and scan rate All these factors must be traded off against size mobility and cost In storage on phosphor based imaging plates for computed radiography the conversion of radiation into an image is a two step process The DR technique however immediately during exposure or within seconds following exposure produces an image on the screen of the workstation see figure 33 16 This makes DR extremely useful in automated robotic production processes Although DR with the correct exposure parameters offers a higher relative image quality than CR flat panel detectors are less suitable for field use and for appli cations with difficult access requirements due to their physical
4. Image contrast required facilitating ease of defect detection Sometimes this can be automated when it concerns common defects Time restraints number of objects to be examined per unit of time Budget e Space available e Installation and specimen dimensions e Sufficient safety measures A number of these factors also influence the choice of detector system Some of the options are Phosphorescent screen afterglow with TV camera and display monitor CCTV at a remote safe location e Fluorescent screen instant image with CCTV system at a safe location e X ray image intensifier with conversion screen in combination with a CCTV system 183 e CCD camera as a substitute for the relatively slow conversion screen e Photo array detector minimal size per diode pixel approx 100 microns to inspect slowly moving objects airport luggage checks e Flat panel detector consisting of millions of light sensitive pixels Although the image intensifier is still most commonly used the flat panel detector is becoming more and more attractive Flat panel detectors provide various pixel sizes with extensive image dynamics a very wide density range far greater than is possible with film Since the signals received by the computer are digital the screen image can be optimised for interpretation contrast brightness sharpness magnification filtering noise suppression and subsequently stored These advanced systems also o
5. In practice the following formula pro duces the best approximation for film unsharpness U U U Broadly if one value of unsharpness U or Ur is more than twice the value of the other the total unsharpness is equal to the largest single value if both values of unsharpness are equal total unsharpness is about 2 1 4 times the single value If necessary U can be reduced by increasing the focus to film distance This can only be done to a limited extent because due to the inverse square law exposure times would become extremely long As a compromise an optimum focus to film distance F is chosen whereby U U 100 11 2 Selection of source to film distance Preceding paragraphs of this chapter described the effects of geometric unsharpness and the possibility to influence this by adjusting the source to film distance This section will expand on this To obtain a radiograph which is as sharp as possible so as to show maximum detail the total unsharpness should be kept to a minimum The radiation energy level selected for making the radiograph see chapter 9 can serve as a lead It is after all determined by the thickness of the material to be radiographed but is at the same time also responsi ble for film unsharpness Ug which can be extracted from table 1 11 and or figure 4 11 It is no use to try and keep geometric unsharpness U far below the value of Ur as in that case Ur determines the total unsharpness anyhow
6. nishment and accurate temperature control of the solutions in the processors Agfa systems ensure top quality results The Agfa NDT U processor is equipped with an infrared film drier while its functions are controlled by a microprocessor Its throughput depends on the required cycle time adjus table between 1 5 and 12 minutes and film size All normal film sizes including roll film can be processed When set for an 8 minute cycle 100 seconds immersion time for exam ple approximately 100 films of size 10 x 48 cm can be processed per hour 91 NDT E economy film processor In order to limit any detrimental effects on the environment Agfa has developed the Eco Ecology and economy designated processors Here too equipment and chemicals are carefully matched thus complying with strict ecological requirements such as a maximum of 50 mg silver per square metre of processed film for the disposal of rinse water This figure for silver content is at least fifteen times lower than for conventional developing systems This is achieved through the considerably improved cascade fixing process which additionally results in a bigger quantity of recovered silver Furthermore measures have been taken to save on energy chemical and water usage the reby making the eco range of film processors as environmentally friendly as possible Figure1 10 shows the schematic lay out of this high tech processor The S eco version has
7. 100 to 450 kV and a tube current of up to 20 mA Unipolar tubes In these shorter tubes as shown in figure 1b 5 the anode is held at earth potential and the cathode only has a potential difference to earth This makes anode cooling a simpler operation It also means that for low medium kilo voltage sets up to approx 300 kV as often used in ambulant applications a single simpler high voltage supply source will suffice The radiation window is placed asymmetric which can be advantageous in practice 39 Special types of X ray tubes Unipolar X ray tubes with a long hollow anode as shown in fig 1c 5 are generally known as rod anode tube and can be inserted into pipes or vessels These tubes produce an annular panoramic beam over 360 so allowing a complete circumferential weld to be radiographed in one exposure Figure 2 5 shows the conical anode of a 360 panoramic tube which allows a circumferential weld to be radiographed centrally hence uni formly from within With this anode the axis of the electron beam must strike the top of the ne A cone in such a way that the centre of the generated X ray beam is perpendicular to the longitudinal axis of the tube cathode filament Note Anodes shaped so that the cen tre of the generated X ray beam is not perpendicular oblique to the centre line of the tube which was acceptable in the past are no longer allowed when work is to be perfor med to official stan
8. Fig 2 16 Structure of the CR imaging plate 148 Image development As a result of incident X ray or gamma ray radiation on the storage phosphor part of its electrons are excited and trapped in a semi stable higher energy state This creates the latent image These trapped electrons can be released by laser beam energy This stimula tion causes visible light to be emitted which can then be captured by a PMT Photo Multiplier Tube The wavelength of the laser beam 550 nanometres and that of the emit ted visible blue light 400 nm are of course different to separate the two Rotating Red light 550 nm n arte Fig 3 16 Schematic S of CR imaging process A D Converter Display Light guide Blue light 400 nm Storage phosphor plate CR SCANNER OR READER WORK STATION The laser scanning device used to scan develop the latent image contains the PMT and its electronics which digitises the analogue light signal that is generated This process as illustrated in figure 3 16 takes place in the phosphor scanner or so called CR scanner or reader The plate is scanned ina linear pattern similar to the formation of a TV image and identical to the film digitisation process Fig 5 16 CR drum type scanner Fig 4 16 Automated CR scanner Scanners Readers There are various types of scanners In the most professional scanners all that needs to be done is to insert the cassette in the input tray and the machine a
9. G135 and G335 and by a higher temperature of the solutions 28 C instead of 20 C The shortest processing time of 1 5 minute is essential for the development of the special films used on board lay barges where the results must be available quickly The chemicals used are more active at higher temperatures The higher temperature of the solutions makes the emulsion layers swell resulting in a faster diffusion of the liquid through the layers and consequently more rapid action of the chemicals Swollen emulsion coatings do however have the disadvantage of being softer and hence more vulnerable to damage a compromise between the advantages and drawbacks is rea ched by adding a carefully determined proportion of hardening ingredients to the fixer Chemicals for use in automatic processors also have additives to inhibit oxidation of the solutions and formation of fog in the emulsions Automatic film processing not only makes the results available sooner it also standardises improved reproducibility uniformity the development process and consequently the exposure technique This increases the quality and reliability of radiography as a method of non destructive testing GE Inspection Technologies supplies integrated Agfa systems in which X ray films chemi cals and processing equipment are all adapted to each other Through the uniform charac teristics of its films carefully formulated chemicals continuous agitation automatic reple
10. and the Compton backscatter technique for example would not exist without them 17 1 Image magnification techniques 17 1 1 Common image magnification technique By positioning an object between an X ray tube and film or detector as illustrated in figure 1 17 a magnified image is obtained As a consequence any defect will be magnified as well The sharpness of the image is dependent of focal spot size the smaller the spot size the better the sharpness Ug geometric unsharp ness focal spot density detector brightness image intensifier Fig 1 17 Image magnification Any unsharpness as illustrated in figure 1 17 is determined by the relationship between F and F and the size of U The effective unsharpness is calculated as follows U Ur F2 Fi Fi For example An X ray tube with a focal spot size of 20 microns with focus to object distance F of 50 mm and focus to film distance F2 of 550 mm will have a geometric unsharpness of 0 02 550 50 50 0 20 mm The magnification factor is F F 550 50 11 The magnification technique is mainly used in combination with a radiation sensitive device such as fluorescent screen image intensifier or flat panel detector A computer workstation may be used for image processing and or enhancement prior to interpretation on the screen 79 17 1 2 High resolution X ray microscopy Magnification factors For a number of years magnification facto
11. pixel values can be interpolated with data from neighbouring pixels In document ASTM 2597 a number of terms and definitions are given for malfunctioning pixels or pixel clusters Some flat panel DR detectors are also subject to some memory effect in jargon called ghosting This is due to hysteresis of the scintillation layer after exposure The image slowly fades away particularly in case of high energy and dose levels This hysteresis causes a certain dead time of the system from seconds to minutes depending on the application during which the detector cannot immediately be re used 16 6 Image quality and exposure energy 16 6 1 Exposure energy To achieve the best image quality the maximum X ray tube voltage or energy of the isotope selected should be as low as possible This applies for both film and digital radiography Figure 17 16 shows a graph taken from EN 14784 2 showing the optimum energy versus wall thickness for different materials COPPER TITANIUM ALUMINUM According to EN14784 2 5 6 78910 20 30 40 50 60 70 8090100 mm Material thickness Fig 17 16 Optimum radiation energies for best image quality 158 16 6 2 Determination of image quality To determine the quality of a digital image existing codes require two different IQIs in analogy to radioscopy One wire or plaque IQI for contrast and one duplex wire IQI for the determination of spatial resolution unsharpness iN 20 DIP
12. 1 at 200 keV 40 at 30 to 40 MeV of the kinetic energy of the electrons is converted into X radiation the remainder is transformed into heat electron beam Cooling the anode _ The heat which accompanies the production of X radiation is quite considerable so that C the anode has to be cooled This can be done in a variety of ways 1 by natural radiation 2 by convection 3 by forced circulation of liquid or gas 4 by conduction The focal spot X fay beam The area of the target which is struck by the electrons see figure 2 4 is called the focal spot FIG Gla EA or the focus It is essential that this area is sufficiently large to avoid local overheating which might damage the anode From the radiographic point of view however the focus has to be as small as possible in order to achieve maximum sharpness in the radiographic image This focal loading is expressed in Joule mm A tungsten target can take a maximum loading of 200 Joule mm A higher loading might damage the anode 32 33 Effective focal spot size The projections of the focal spot on a surface perpendicular to the axis of the beam of X rays is termed the effective focal spot size or focus size see figure 2 4 The effective focus size is one of the parameters in radiography see section 11 1 The effective focus size principally determining the sharpness in the radiographic image has to be as small as pos sible in order to achieve
13. Correction factor d H d The dimension of the radiographic image Dg on the film is multiplied by this correction factor The true diameter of the reinforcing steel is therefore D Dr d H d 18 6 On stream inspection profiling technique On stream inspection can be carried out on pipes valves vessels and distillation columns while in operation in order to esta blish the degree of deterioration of the system either the projection or the tangential technique can be used Since the introducti f source to insulation on of digital radiography the CR method eae using storage phosphorplates is increasingly becoming an alternative for traditional film in case of on stream exposures see chapter 16 The main advantage being that it reduces the exposure time by a factor of 5 to 10 or if lower energies Iridium192 instead of Cobalt60 can be applied it results in a redu ced controlled area which is very attractive in cramped spaces and personnel nearby e g on offshore platforms t S source insulation Projection technique f b 2 a correction factor or The projection technique is most commonly Df f b used With this technique the two walls are projected on film simultaneously as shown in figure 5 18 The image projected is larger than the actual object dimensions It is important to know the degree of magnification so as to be able to determine the true wall thickness If both walls of the pipe are
14. NF A04 304 includes an addendum which defines the index of visibility For each radiograph a record is made of 1 the number of visible holes a 2 the number of holes b of a diameter equal to or greater than 5 of the material thickness being radiographed The index of visibility N is given by the formula N a b The value of N may be positive zero or negative Image quality improves as the value of N increases Duplex IQIs 13 1 Fig 5 13 Duplex wire IQI 116 Duplex IQI s are described in norm EN 462 5 The duplex IQI consists of a number of pairs duplex of wires or thin strips made of platinum or tungsten of increasingly smaller size and diminishing distances for each pair Figure 5 13 shows such an IQI made up of pairs of wires The duplex IQI has been in existence for decades but is no longer current in conventio nal film radiography because of their high cost and limited possibilities of application It is however increasingly used in digital radiography because it is perfectly suited to determine contrast and un sharpness 13 4 Position of the IQI To be of any value in checking the factors defining sharpness and quality the IQI must be placed on the source side of the specimen If the source side is not accessible the IQI is placed on the film side In the latter position visibility is no longer an indication of geometric unsharpness but still a check on the developing process and radiatio
15. Radiation images The intensity of a beam of X rays or gamma rays undergoes local attenuation as it passes through an object due to absorption and scattering of the radiation On a uniform object attenuation of the primary beam will also be uniform and the film evenly exposed If the object contains defects or is of variable thickness the surface of the film will be unevenly exposed resulting in a shadow image of the object and the defects in it When the film is processed the variations in radiation intensity show up as varying film densities higher radiation intensity producing higher film density resulting in a negative X ray image as shown in figure 1 6 When the primary beam is partly absorbed in the object some radiation as shown in figure 2 6 will be scattered and reach the film as secondary radiation by an indirect path The quality of the radiograph is reduced by this scattered radiation and it is important to keep its effects to a minimum At any point P on the film therefore the total radiation reaching that point is made up of some transmitted primary radiation forming the image of cavity N the image forming or direct radiation intensity Ip and some secondary non image forming scattered radi ation intensity Is Hence the total radiation intensity at P is I I The ratio Is Tp is called the scattered radiation factor and can be as high as 10 for great wall thicknesses which means that the scatter
16. Relative exposure times for NDT1200 RCF and lead screens It is clear from the above tables and graphs that there are many ways to reduce the expo sure time or radiation dose needed The required image quality is decisive a higher expo sure rate automatically means reduced image quality and next the economic factors for example the cost of the screens against time saved need to be weighed 57 a layer of hardened gelatine b emulsion layer c substratum bonding layer d cellulose triacetate or polyester base c substratum b emulsion layer a layer of hardened gelatine Fig 1 7 Schematic cross section of an X ray film total thickness approx 0 5 mm 58 The X ray film and its properties 7 1 Structure of the X ray film An X ray film total thickness approx 0 5 mm is made up of seven layers see figure 1 7 consisting of A transparent cellulose triacetate or polyester base d On both sides of this base are applied e alayer of hardened gelatine a to protect the emulsion e emulsion layer b which is suspended in gelatine sensitive to radiation e avery thin layer called the substratum c which bonds the emulsion layer to the base The normal X ray film therefore has two coatings of emulsion doubling the speed compared to a film with a single emulsion layer Photographic emulsion is a substance sensitive to ionising radiation and light and consists of microscopic particles o
17. a 50 higher production capacity than the U version A very useful option is its suitability for use in daylight in combination with a matching film feeding system Fig 1 10 Schematic layout of the Agfa NDT S eco processor 1 Film feeder 7 Infrared dryer 2 Film surface scanner 8 Film exit 3 Developer tank 9 Film receiving tray 4 Rinse tank 10 Fixer pump 5a b Fixer tanks 11 Developer pump 6 Final wash tank 12 Overheating protection 92 10 6 Checking the development process and film archiving properties Besides exposure technique many aspects influence the quality of the final radiograph An important factor is the development system Monitoring and quantifying the proper functioning of a development system is an essential part of quality control as a properly exposed radiograph can be spoilt if the processes that follow are performed incorrectly For the monitoring of the development process and archival properties of X ray films Agfa has produced two methods the so called PMC strips and the Thio Test Both methods are based on the international standard ISO 11699 part 2 and the European standard EN 584 part 2 which describe a standard development process and the means to control its execution PMC strips to check the developing process To facilitate ongoing quality control and ensure compliance with existing standards on systems classification certified PMC strips are used to monitor the development process PMC is s
18. as small as possible Among the many factors deciding a source suitability for non destructive testing are the wavelength and intensity of its radiation its half life and its specific radia tion In fact only a few of the many artificial radio isotopes available have been found to be suitable for industrial radiography 4 5 Advantages and disadvantages of artificial radioactive sources Advantages 1 require no electric power supply easy to use in the field 2 can be obtained in a range of source diameters so that if necessary a very short source to film distance with a small diameter source can be used for example for pipes of small diameter 3 awide variety of radiation hardnesses 4 higher radiation hardness more penetration power than those of conventional X ray equipment can be selected Disadvantages cannot be switched off the energy level radiation hardness cannot be adjusted the intensity cannot be adjusted limited service life due to source deterioration half life less contrast than X ray equipment de ele ee 36 4 6 Properties of radioactive sources Activity source strength The activity of a radioactive substance is given by the number of atoms of the substance which disintegrate per second This is measured in Becquerels Bq 1 Becquerel corresponds to 1 disintegration per second 1 Bq 1 s Specific activity The specific activity of a radioactive source is the activity of this substance per wei
19. current 2008 phosphor plates this is considerably more approximately 10 microns 152 16 5 Genuine Digital Radiography DR One step digital radiography Digital radiography DR for short is also known as direct radiography to indicate the dif ference with CR which is a two step and thus slower process With DR technology there is an immediate conversion of radiation intensity into digital image information Similar to common digital photo cameras the radiographic image is almost immediately available Exposure and image formation happen simultaneously allowing near real time image capture with the radiographic image available for review only seconds after the exposure Some systems even provide a true real time radioscopic mode with display rates up to 30 images per second This almost instant image formation is the reason that some consider DR the only genuine true method of digital radiography This instant availability of results offers immediate feedback to the manufacturing process to quickly correct production errors 16 5 1 Detector types Many materials or combinations thereof are sensitive to the impact of ionising radiation Over the years a considerable number of them proved to be efficient and commercially viable to create radiation detectors for NDT applications As a result a wide variety of detector types are in use for formation of DR images Often the application dictates the selection of a particular
20. density difference at equal radiation dose the characteristic curve at any given point is equal to the slope of the tangent line at this and so a greater contrast resulting in better point This slope a b in figure 3 7 is called the film gradient Gp film contrast or the defect discernibility If one requires high film gamma contrast it is therefore necessary to use the highest possible density radiograph while remaining within the acceptable density range of the viewing screen so as not to impede film interpretation Most codes of good practice ask for densities between 2 0 and 3 0 in the relevant area of the image Table 1 7 shows the loss in contrast on Fig 5 7 Gradient density curves typical film as density values obtained fall for three types of film _ _ below 3 0 A low sensitivity very fine grain film D2 B average sensitivity fine grain film D4 C high sensitivity medium grain film D8 Density D Film contrast as a of the value at D 3 0 3 0 100 log rel exp log rel exp 2 9 85 Fig 3 7 Gradient of an X ray film Fig 4 7 Average gradient a b of an X ray film Average gradient The straight line connecting two points on a characteristic curve as figure 4 7 shows is Table 1 7 Contrast loss with reduced film density equal to the average gradient of the segment of the curve linking these two points This gradient Gp is the average of all gradients in the
21. different disciplines e g research and development industry universities and authorities Participation of authorities at an early stage can speed up lega lisation legislation and release of a standard Sometimes industrial economic and scientific interests do not fully match and hamper quick release of a new stan Major dard Even under the most favourable conditions making a Issul ng i standard is a process that takes many years at first it requi Organisations res a working document followed by a preliminary ver sion prior to official release Until 2005 this was the situa tion for CR and it still is for DR In addition to the many specialists involved there is also a large number of national and international normalisation organisations that issue such standards They often adopt the content of each other s documents to save time after careful judgment of the content and give them their own specific issue numbers and annotations 165 Status of CR standards For CR standard EN 14784 has been issued with EN 444 EN 584 1 and EN 462 5 in mind to achieve conformity with film radiography Part 1 of EN 14784 describes classification of systems and part 2 describes principles and applications not including welds Although a working group for the compilation of an EN standard for welds exists the issue of such a standard would still take several years Status of DR standards Application standards for DR do
22. enough space in the vicinity of the pipe weld for the scanner to move 17 5 Neutron radiography neutrography Neutrons which are atomic particles without an electric charge will penetrate most materi als are attenuated in passage and so can be used to produce radiographs There are vari ous kinds of neutron energies but only the thermal and cold neutrons are suitable for NDT applications Contrary to ionising radiation in the keV and MeV range neutron absorption is higher in light than in heavy materials Neutrons will be strongly influenced by hydroge nous materials plastics all types explosives oil water etc even when these materials are inside metal containments made of lead steel or aluminium There are many potential applications for neutrography but its practical use is limited to a large extent by the lack of suitable portable neutron sources A neutron window in an ato mic reactor is by far the best source but such facilities are not commonly available The only neutron emitting radioactive source is Californium252 which is extremely costly and has a half life of only 2 65 years An X ray film also reacts to neutron energy but useably results are not obtained until it is combined with gadolinium or cadmium intensifying screens The Agfa D3SC SC single coated film is frequently used for this purpose The secondary radi ation generated in the intensifying screens brings about the image formation Ano
23. h measured at 1 meter distance to 10 uSv h the required distance according the inverse square law is V 2560 10 16 metres To achieve the same by placing a shield the number of HVTs is calculated as follows Required intensity reduction is 2560 10 256 x Number of HVTs is log 256 log 2 8 The example above demonstrates that an intensity of 2 56 mSv h can be reduced to 10 uSv h by increasing the distance to 16 metres or place shielding material of 8 HVTs as close as possible to the source If either of these methods cannot be used on its own a combination of the two could be considered 213 214 Standards literature references acknowledgements and appendices European norms EN standards Ever since the introduction of industrial radiography there has been a growing need for stan dardisation of examination techniques and procedures At first these standards had mainly a national character e g ASTM and ASME DIN AFNOR BS JIS etc but as a result of industri al globalisation the need for international standards grew The national standards were and still are frequently used internationally in particular the ASTM and ASME standards International standards are largely based on existing national standards Organisations that engage in international standardisation are ISO and CEN These standards are developed by working groups of experts who present the newly adapted harmonised standards to the ISO CEN etc A
24. is they liberate electrons in matter 6 they can impair or destroy living cells 2 5 Radiation energy hardness Radiation hardness beam quality depends on wavelength Radiation is called hard when its wavelength is small and soft when its wavelength is long In industry the quality of the X ray tube ranges from very soft to ultra hard The beam quality is related to a tube voltage kV range or keV for isotopes The first two columns of table 2 2 below indicate the relationship hardness tube voltage range applied in NDT The third column gives the related qualification of the radiation effect i e half value thickness HVT described in detail in section 2 9 Radiation quality Tube voltage Global half value Hardness thickness for steel mm Very soft Less than 20 kV Soft 20 60 kV Fairly soft 60 150 kV 0 5 2 Hard 150 300 kV 2 7 Very hard 300 3000 kV 7 20 Ultra hard more than 3000 kV 20 Table 2 2 Comparative values of radiation quality hardness against tube voltage 22 2 6 Absorption and scattering The reduction in radiation intensity on penetrating a material is determined by the following reactions 1 Photoelectric effect 2 Compton effect 3 Pair production Which of these reactions will predominate depends on the energy of the incident radiation and the material irradiated Photoelectric effect When X rays of relatively low energy pass through a material and a photon collides with an atom
25. length between the control panel and the tube may be of influence 78 Therefore an exposure chart for each individual X ray set should be drawn up This is an excellent way to become familiar with the equipment while time and money put into this work will be amply repaid at a later stage Exposure charts for gamma ray examination are drawn up in a similar way as described above Figure 5 9 shows one for a Cobalt60 source A specially designed slide rule can also be used since there is no need to consider individual radiation spectra as for X ray tubes Figure 6 9 shows a similar exposure chart for an Iridium192 source GBq min 100 000 Cobalt60 Film type C3 Agfa D4 Steel screens Density 2 5 10 10 20 30 40 50 60 70 80 90 100 110 mm steel Fig 5 9 Exposure chart for examination of steel with a Cobalt60 source GBq min 100 000 lridium192 Film type C5 Agfa D7 Lead screens 0 027 mm Density 2 5 10 10 20 30 40 50 60 70 80 90 100 110 mm steel Fig 6 9 Exposure chart for examination of steel with an Iridium192 source FFD focus to film distance 79 9 6 Relative exposure factors Relative exposure factors can be used to convert an exposure chart for one type of film to another film although still for the same radiation energy These factors are not constant for different radiation energies and should therefore be used with some caution Some examples of relative exposure factors for Agfa films ar
26. maximum sharpness The dimensions of the focus are governed by 1 The size of the focal spot and 2 The value of angle a see figure 2 4 It should be noted that when in radiography we speak of the size of the focus without spe cifying this more exactly it is normally the effective focal spot size which is meant Conventional X ray tubes have effective focal spot sizes in the range 4 x 4mm to 1 x 1 mm There are fine focus tubes with focal spots from 0 5 x 0 5 mm ad microfocus tubes down to 50 um diameter or even much less known as nanofocus tubes The effective focal spot size can be determined in accordance with the procedures descri bed in EN 12543 replacing the old IEC 336 which however is still in use For more infor mation on focal spot measurement see section 18 1 Dimension of the electron beam Focal spot Effective focal spot size Anode target True focus size Fig 2 4 Effective focal spot size 34 4 3 Tube voltage and tube current The voltage across the X ray tube determines the energy spectrum and so the hardness of the radiation see figure 3 4 The intensity is proportional to the tube current see figure 4 4 This graph shows that contrary to a change in tube voltage a change in tube current does not shift the spectrum in other words the hardness does not change w ty D S YV wb amp 20 YV tx D relative intensi N pi 50 100 150 200 KeV Fig 3 4 Energy spectr
27. number of European norms EN relevant to radiography are listed in table 1 20 Norm number EN 444 EN 13068 3 Subject General principles for radiographic examination of metallic materials by X and gamma rays General principles of radioscopic testing of metallic materials by X and gamma rays EN 462 1 through 5 Image quality of radiographs IQIs EN 473 Qualification and certification of NDT personnel EN 584 1 Equivalents ASTM E 1815 ISO 11699 1 JIS K7627 Classification of film systems EN ISO 14096 Film digitisation EN 584 2 ISO 11699 2 Verification of film systems EN 1435 EN 12543 1 through 5 replaces IEC 336 EN 25580 EN 14784 1 and 2 Table 1 20 European norms for industrial radiography Radiographic examination of welded joints Characteristics of focal spots in industrial X ray systems for use in NDT EN 12544 1 through 3 Measurement and evaluation of the X ray tube voltage EN 13068 Fluoroscopic radioscopic testing Industrial radiographic illuminators minimum requirements Industrial CR with storage phosphor imaging plates Classification of systems and general principles of application 215 Literature and references 1 Industrial Radiology Theory and Practice English R Halmshaw Applied Science Publishers Ltd London and New Jersey 1982 2 Niet destructief onderzoek ISBN 90 407 1147 X Dutch W J P Vink Delftse Universitaire Pers 3 Die Rontgenpriifung Band 7 ISBN 3 9
28. of resolution sharpness contrast resolution and noise Conventional X ray films exhibit an extremely high intrinsic resolution due to the fine gra nularity of the radiation sensitive crystals a few microns in size The resolution of the resulting image is far better than the human eye can resolve Hence for film contrast IQI s provide an adequate measure of indicating resolution and image quality that meets the qua lification needs of industry thus there is no need for any additional resolving criteria for tra ditional film However digital radiography has a much coarser intrinsic resolution typically 50 microns or more so a different situation exists compared to film radiography To select or purchase the proper digital system information that quantifies the resolving power of a digital system is needed Although generic methods to measure optical resolu tion exist they have not yet been fully specified for digital radiographic systems gt lt To fulfil the need for quantifica tion of resolution prior to antici pated release of future stan dards suppliers of digital radio graphy systems already proacti vely use methods and definit gt ions that are common in other 1 mm 1 mm sciences 1 Lp mm 4 Lp mm Fig 20 16 Resolution line pairs per mm Resolution is defined as the smallest separation distance between two objects that the human eye can distinguish Because the human eye is not easily quantifiab
29. of this material the total energy of this photon can be used to eject an electron from the inner shells of the atom as figure 3 2 illustrates This phenomenon is called the photoelectric effect and occurs in the object in the film and in any filters used incident X rays ejected electron Compton effect With higher X ray energies 100 Fig 3 2 Photoelectric effect keV to 10 MeV the interaction of photons with free or weakly bonded electrons of the outer atom layers causes part of the energy to be transferred to these ee ne electrons which are then ejected A e as illustrated in figure 4 2 At the same time the photons will be deflected from the initial angle of incidence and emerge from the collision as radiation of redu ced energy scattered in all direc tions including backward known as backscatter see section 17 6 In this energy band the absorp tion of radiation is mainly due to the Compton effect and less so to the photoelectric effect Fig 4 2 Compton effect M scattered radiation Pair production The formation of ion pairs see figure 5 2 only occurs at very high energy levels above 1 MeV High energy photons can cause an interaction with the nucleus of the atom involved in the colli sion The energy of the photon is here converted tot an electron e and a positron e Total absorption attenuation The total linear absorption or attenuation of X rays is a combi nation of
30. on a crawler single wall panoramic image or outside at a 180 shifted position and rotates simultaneously with the line array double wall image Similar scanners exist not using a band wrapped around the pipe but using magnetic wheels instead For such applications the CMOS line array must have a fast response and erase time in order to frequently many times per second refresh the information CMOS detec tors are able do that and are thus fast enough for girth weld inspection in the field 170 Useful life of plate and panel CR plates by handling like DR detectors by radiation have a finite useful life which has to be included in an economic evaluation The working life of flat panel devices can range up to millions of images dependent on application specific details see paragraph 16 5 3 Thus cost per image should be considered in any return on investment financial analysis CR plates in flexible cassettes can be used up to one thousand times If used in a rigid cassette their life can considerably be prolonged In the field care must be taken that rigid cassettes are not too tightly strapped to a curved component pipe to prevent permanent bending causing problems with automated readers as shown in figure 4 16 Such readers refuse over bended cassettes This may result in undesirable manual handling of the plates causing plate damage and excessive wear 16 12 Work station Hardware and software A computer and extremely high
31. on stream radiography the majority of the work in 2008 using isotopes to detect general internal erosion or corrosion of non insulated piping see figure 28 16 and detection of internal and external corrosion under thermal insulation CUI see figure 29 16 For wall thickness determination of insulated pipes the so called projection shadow technique or tangential technique is applied see section 18 6 Fig 28 16 CR image of bare pipe with areas marked Fig 29 16 CR image of insulated pipe with WT values for WT measurements CR is also very suitable for detection and quantification of erosion corrosion in or adjacent to the root of welds and for detection and quantification of scaling or clogging concrete inspection and non critical castings In case of offshore work CR is attractive using low acti vity sources first of all because its smaller controlled area and secondly to avoid that level detectors using radiation are falsely activated or disturbed 168 Weld inspection Although conventional film is still superior compared to the CR technique standards permit CR in several cases because it can under circumstances provide sufficient image quality even for weld inspection see section 16 9 Figure 30 16 shows an image of a weld with a clear indication of a serious longitudinal defect 10 x1 1mm CS weld 160k 2mA CP Pan 5s No Po screens Medium Gain HT vy sep o i p whos Sr Se gt k 0 5 10 15 Defect Defect Defect
32. one After development the densities D are measured by means of a densitometer and then plotted against the logarithmic values of the corresponding exposures log E The points obtained are then joi ned together by a continuous line Itis not necessary to know the absolute exposure Fig 2 7 The characteristic curve for an values relative values can be used so at a industrial X ray film fixed X ray intensity only exposure time needs to be changed log E exposure relative units Density D of a photographic emulsion does not increase linearly with exposure E over the entire density range but has a shape as in figure 2 7 The lower part of the curve a b is called the toe the middle part b c is called the straight line linear portion and the upper part c d is called the shoulder Usually the characteristic curve of industrial X ray films shows an S like shape The shoulder of a characteristic curve relating to industrial X ray film corresponds to densi ties higher than 4 Since such densities are too high for normal film viewing the curve from density D 3 5 upwards is shown as a broken line It should be noted that the straight line portion b c is not truly straight but slightly continues the trend of the toe of the curve 61 Gradient of the density curve A steeper gradient means an increase in The density curve shows one of the most important characteristics of a film The slope of
33. packed in boxes with lengths up to 100 metres For large projects pre cut films at specified lengths with sealed ends exist For onshore application of X ray crawlers the required radiation safety is easily achieved by distance and all other related safety measures and warning signs On a lay barge crowded with people this is not as easy Here as a first measure radiation reduction is largely achieved by a lead tunnel as shown in figure 15 18 Moreover high speed film requiring a lower dose than traditional film thus requiring a lower source activity or lower mA value or shorter exposure times contribute to reduce radiation 205 On stream image of insulated pipe 206 CR image of a weld see acknowledgements Radiation hazards measuring and recording instruments 19 1 The effects of radiation on the human body The human body is constantly exposed to natural radiation e g from space the soil and buildings also known as background radiation All ionising radiation whether electro magnetic gamma y or corpuscular particles in the form of alpha a or beta 6 and neu trons are harmful to the human body The unit absorbed dose D defines the effect of radiation on various substances D is the absorbed dose in J kg or Gray Gy The biological damage done by the various types of ionising radiation a p y or fast neu trons differs and depends on the quality factor Q The unit to which the damage qualit
34. place an unexposed film on the workbench and cover part of it up with a sheet of cardboard which is then gradually removed so as to produce a series of different exposures By developing the film in the usual way it will then be possible to see how safe the light is and how long a film can be exposed to it before it exceeds the maximum acceptable difference in density of 0 1 Darkroom layout The darkroom should preferably be divided into a dry side and a wet side The dry side will be used for loading and emptying cassettes fitting films into developing frames and so on in short for all the work that does not allow dampness On the wet side the films will be processed in the various tanks of chemical solution For efficient working and to ensure uniform quality there should be automatic control of the temperature of the solutions Tanks In processing tanks used in the manual process films are held vertically in their frames These tanks can be made of stainless steel or plastic The dimensions of the tank must be suited to the size and number of films to be processed There must be a space of at least 1 5 cm between films The top edge of the films must be approx 2 cm below the surface of the solution The wet side of the darkroom will have five tanks arranged in the following sequence developer tank stopbath or rinse tank fixer tank final wash tank tank for wetting solution toes YT 10 2 Chemicals and film devel
35. radiation is then caught by the detector through a specially sha ped diaphragm see figure 20 17 The detector is made up of 20 or more detector elements marked A B C etc each of which measures the quantity of back scattered radiation from at a certain depth A B C in the object as figure 20 17 shows Each sensor element is say focussed at a certain depth The cylindrical scanner measures only 7 x 7 cm and scans the object in a grid By linking the scanning system with a data processor a comprehensive Compton image of the object develops and any possible defects in it The Compton backscatter technique is for instance frequently applied to honeycomb constructions and composite materials and has a penetra tion depth of approximately 50 mm The method is still fairly slow scanning a 50 cm surface takes approximately 5 minutes An added advantage is however that the depth position of defects becomes known immediately as a result of the quasi focussing of each individual detector element diaphragm X ray beam Fig 20 17 The Compton back scatter technique 191 X ray image of an X ray crawler in pipe at weld location 192 Special radiographic applications There are many special applications of radiography in NDT This chapter describes a limited number of different examples to illustrate this diversity Apart from the use of radiation in image forming radiography it is also used in for instan
36. so called collimator e the use of masks lead strips around the edges of the object 32 6 2 Radiation filters When a metal plate usually lead or copper is placed between the tube window and the object radiation hardening occurs leading to a lower image contrast This may be counter balanced by a metal filter placed immediately behind the object i e between object and film This filter will cause the softer scattered radiation passing through the object to be absorbed by the filter to a greater extent than the primary har der radiation This also improves the image quality If the edges of an object being radiographed are not close to the film as in the case of a cylindrical body in figure 3 6 considerable scatter of the primary radiation can occur leading to fogging This scatter can be prevented by positioning sheets of lead foil between the object and the film as illustrated in this figure Reducing the contrast by filtration is also desirable when a radiographic image of an object of widely varying thicknesses has to be obtained on a single film see section 18 2 Typical filter thicknesses are 0 1 0 25 mm lead for 300 kV X rays 0 25 1 0 mm lead for 400 kV X rays 6 3 Intensifying screens The radiographic image is formed by only approximately 1 of the amount of radiation energy exposed at the film The rest passes through the film and is consequently not used To utilise more of the available radiation
37. step 7 The reference values according to the corresponding certificate are S and C The following calculations are then made e Sensitivity index S D3 Do e Contrastindex C D D3 S S The system is acceptable if the following criteria are met a Dp lt 0 3 b S hasa value 10 of S c C has a value between C 15 and C 10 If one or more of these criteria are not met the development process must be adjusted Thiosulphate test to check film archiving properties The archival properties of a radiographic image must also be determined in accordance with the standards ISO and EN by analysing the quantity of residual thiosulphate in the film s emulsion layers This quantity depends on the thoroughness with which the fixing and rinsing processes have been carried out For storage over a period of 100 years 100 g m2 is allowed for a period of 10 years dou ble this figure is allowed see table 3 10 These values are difficult to measure however The so called Thio Test developed by Agfa is a very useful and quick method to quantify film keeping properties in practice 94 The unexposed area on the PMC strip shown in figure 2 10 apart from providing a reference for fog and base density also allows for the Thio Test to be carried out The components used for this test are 1 The Thio Test colour step wedge 2 A dropper bottle of Thio Test reagent The reagent consists of a 1 silver nitrate solution in demin
38. the film from time to time to prevent film faults such as lines or streaks Replenishing Up to 400 ml of liquid per square meter of film processed may be carried over to the next tank When developing frames used it is therefore preferable to hold the film 2 3 seconds over the developer tank to drip After each square meter of film developed 600 ml of replenisher must be added to the bath regardless of the quantity of developer lost from the tank Up to about 4 litres of replenisher can be added in this way for every litre of the original developer in the tank The solution must be discarded and replaced with fresh developer when the total quantity of replenisher added is three times the original total contents but in any case after eight weeks irrespective of the number of X ray films processed Stopbath Before transferring the developed film to the fixer tank it is placed in a stopbath consisting of 30 ml glacial acetic acid to 1 litre of water for 30 seconds to prevent the fixer solution from being neutralized too rapidly by the developer and stripes or dichroitic fog from forming on the film If a film is not passed through a stopbath it must be rinsed in running water for a few minutes immediately after leaving the developer Fixing Fixing renders the image formed during development permanent by removing unde veloped silver halide salts from the emulsion When the film is taken from the stopbath it still has a milky appear
39. times greater than through the 15 mm section The logarithm of this ratio equals 0 3 log 2 0 3 The characteristic curve fig 8 9 of the D7 film shows that density 2 corresponds to log relative exposu re 2 2 point C in fig 8 9 _ ae epee At 12 mm the log rel exposure is Fig 8 9 Characteristic density curve of the D7 film 22 03 2 5 The corresponding density is then 3 5 point D in fig 8 9 Example 2 Effect of exposure on contrast Assume that when an exposure of 15 mA min is used for a radiograph on D7 film both average density and contrast prove to be too low after pro cessing The highest and lowest density in the most relevant section of the image are only 1 5 and 0 5 The intention was to make a 3 5 radiograph with a maximum logrel exp density of 3 0 Fig 9 9 Characteristic density curve of the D7 film Questions What exposure time would be required for the same radiation intensity and what contrast increase would be achieved 82 Method and answer The characteristic curve fig 9 9 shows that at the measured densities of 1 5 and 0 5 respectively the corresponding logarithm of relative exposures are 2 15 and 1 65 Since density 3 0 should not be exceeded the area which is most important for inter pretation which showed density 1 5 on the first exposure must now display 3 0 Characteristic curve figure 9 9 shows that density 3 0 corresponds with log rel exp 2 45 and
40. value by increasing the source to film distance However in view of the inverse square law this distance cannot be increased without limitation as extremely long exposure times would result The formula also indicates that geometric unsharpness assumes more and more importance as the distan ce between defect and film increases A special case arises however when one uses a micro focus X ray tube with a focal spot size in the range 10 50 wm With such a small focus size the image can be deliberately magnified see section 17 1 by using a short source to specimen distance and a large specimen to film distance and still retain an acceptably small value of U The advan tage of this technique called the projective magnification method is that the grainin ess always present in a photographic image is less of a disturbing factor in the discerni bility of very small defects Figure 2 11 shows the effect of geometric unsharpness on the image of a defect smaller than the focus size A Point focus size s no geometric unsharpness defect image sharp Small focus size s geometric unsharpness U defect image blurred Increased focus size s still larger Ug defect image blurred and loss of contrast C is less than in A and B C contrast Fig 2 11 Geometric unsharpness effect on the image of a small defect 98 In this situation the unsharp images of each of the two edges of the defect may overlap as sho
41. wall thickness _ Iridium192 or Cobalt60 Focus to film distance minimum 3 x Dinsulation Irradiated thickness 2 x nominal wall thickness plus steel equivalent of the pipe content Film type minimum C5 EN584 1 Film density minimum 2 5 in the centre of the pipe projection The steel equivalent of the pipe content is determined as follows specific density in kg m of content specific density in kg m of steel x internal diameter mm of steel Density of steel 7 800 kg m Density of content oil and aqueous liquids 800 to 1 000 kg m Fig 8 18 Preparations for on stream radiography Fig 9 18 On stream radiography of pipe with corrosion 200 Notes e Inthe most commonly used insulation materials absorption is negligible e The long exposure times cause over irradiation at the edge of the pipe As a result the pipe wall shows up thinner Figure 8 18 shows preparations for on stream radiography being made The end piece for the gamma source is positioned above the pipe while the flat film cassette is placed below Figure 9 18 shows an on stream radiograph of a pipe with severe pitting corrosion Since the introduction of digital radiography the CR method using storage fosforplates is rapidly becoming an alternative for traditional film The main advantage being that it reduces the exposure time by a factor of up to 10 or if weaker sources can be applied a reduced con trolled area which is very attrac
42. weld defects in new constructions have been specified in standards However NDT is also used for purposes such as the checking of assembled parts the development of manufacturing processes the detection of corrosion or other forms of deterioration during maintenance inspections of process installations and in research There are many methods of NDT but only a few of these allow the full examination of a component Most only reveal surface breaking defects One of the longest established and widely used NDT methods for volumetric examination is radiography the use of X rays or gamma rays to produce a radiographic image of an object showing differences in thickness defects internal and surface changes in structure assembly details etc Presently a wide range of industrial radiographic equip ment image forming techniques and examination methods are available Skill and expe rience are needed to select the most appropriate method for a particular application The ultimate choice will be based on various factors such as the location of the object to be examined the size and manoeuvrability of the NDT equipment the existance of standards and procedures the image quality required the time available for inspection and last but not least financial considerations This book gives an overview to conventional industrial radiography as well as digital computer aided techniques and indicates the factors which need to be considered for sele
43. well Both instruments measure the electric current that is produced by ionisation The radiation level can be read instantly off a micro ampere meter with a uSv h or mSv h calibrated scale Some radiation monitors give an audible signal when 615 E a pre set dose is exceeded i hd navn ao Wy Ferran a ober The instruments are used by personnel working with tt He radioactive material or X ray equipment to determine the safe distance and the dose rate of for instance 10 wSv h at the safety barrier A GM counter has a Fig 1 19 Geiger Miiller counter measuring range from 1 wSv h to 2 mSv h Scintillation counter This is an accurate and multifunctional instrument to measure and analyse radiation The incidence of ionising radiation on a Sodium iodine crystal is converted into weak light flashes which are amplified into electric pulses by an integrated photo multiplier By measuring amplitude and number of these electric pulses energy and intensity dose rate of the radiation can be determined These instruments are predominantly used for scientific purposes 210 Personal protection equipment Pendosismeter PDM The PDM consists of a quartz fibre electrometer and a simple optic lens system housed in a fountain pen type holder see figure 2 19 A small charging unit is used to electrically charge the fibre which can then be viewed through the lens The fibre is set on the zero mark of the calibrated scale as
44. where n 5 so the number of radiographs must be at least 5 Example 2 When using a source placed against the pipe wall Operaning t De 10 200 0 05 and not being used De F 200 200 10 200 210 0 95 The intersection of the two co ordinates now lies in the area where n 4 So by using a radioactive sour ce which is located closer to the pipe surface one less exposure would still ensure compliance with EN 1435A Initially the code would howe ver have to allow the use of an i i Fig 4 12 Graph for the minimum number of exposures in accordance isotope instead of an X ray tube with EN 1435 A at maximum thickness increase of 20 This graph appears enlarged in the appendix 109 Digitised and enhanced image of a radiograph of the American Liberty Bell The image was used to monitor possible lengthening of the crack 110 Image quality 13 1 Factors influencing image quality With regard to image quality the term frequently used is sensitivity Sensitivity determines the extent to which a radiograph is able to clearly show anoma ly details of a certain size Sensitivity in this sense must not be confused with the sensitivity or speed of the film see section 7 5 Discernibility of defects on a radiograph depends in general on e the quality of the radiation e the properties of the film e the film viewing conditions Image quality is governed by contrast sharpness
45. with In codes useful film length is determined by the percentage of extra wall thickness which may be penetrated in relation to the nominal wall thickness t of the pipe Percentages of 10 20 and 30 are commonly applied For general use 20 is a practical value whereby the lightest section of the film shall have a density of at least 2 108 The number of radiographs necessary for 100 examination of a circumferential weld can through calculation also be obtained from the codes When large numbers of simi lar welds are involved this is an important figure because too many radiographs would be uneconomical and too few would lead to insufficient quality of the examination The minimum number of radiographs required for various pipe diameters and wall thicknesses at varying source positions can be derived from the graph in figure 4 12 The graph is applicable to single wall and double wall technique whereby the maximum increase in thickness to be penetrated is 20 in accordance with EN 1435 A Example 1 An X ray tube with an outside diameter of 300 mm is used to examine a circumferential weld in a pipe of a diameter De of 200 mm and a wall thickness t of 10 mm The distance between the focal spot and the outside of the X ray tube is 300 2 150 mm F half the X ray tube diameter De 150 200 350 mm t De 10 200 0 05 and De F 200 350 0 57 The intersection of the two co ordinates 0 05 and 0 57 is in the range
46. 0 30 200 60 150 Table 3 11 Thickness ranges in mm for examining steel with the usual types of gamma sources Note Standard sensitivity permits a slightly poorer image quality than high sensitivity Thus a larger thickness range can be inspected coping with the quality requirements 103 11 4 Radiation hardness and film contrast When radiation hardness increases the half value thickness HVT also increases Tables 2 2 and 3 2 for steel and lead respectively show this in figures This is why in an object with different thicknesses image contrast diminishes when radi ation hardness increases Figure 7 11 clearly illustrates this The left side of a step wedge is radiographed with 150 kV the right side with 80 kV The right side shows the greater contrast between two steps whereas on the left the contrast range is the biggest 150 kV 80 kV Fig 7 11 X rays of a step wedge with 150 kV left and 80 kV right 104 11 5 Summary of factors that influence image quality The factors that influence image quality are 1 Contrast 2 Unsharpness 3 Graininess 1 Contrast depends on e Radiation energy hardness e Variation in thickness e Backscatter e Front and back screen e Film screen combination e Film screen contact Defect location depth as well as orientation 2 Unsharpness depends on e Size of focus Thickness of the object e Source to film distance e Radiation energy hardnes
47. 1 IQI s Their origin and description can be 10 FE EN_ found in the following standards e EN 462 01 Europe e BS 3971 Great Britain ASTM 747 USA E e ASTM 1025 USA e AFNOR NF A 04 304 France ASTM 747 describes the wire penetrame ter quite similar to wire penetrameters of other origin ASTM 1025 describes the plaque penetrameter similar to AFNOR Both types of ASTM IQI s have been 1 A developed and standardised in the USA they now are used world wide Fig 2 13 Examples of image quality indicators 114 ASTM 1025 IQ s The plaques have markings showing their thickness in thousandths of an inch Ea ch plaque has three holes of dia meters 1T 2T and 4T T being the thickness of the plaque Thin plaques with T lt 0 01 form an exception to this rule Hole diameters for these plaques are always 0 01 0 02 and 0 04 so do not comply with the 1T 2T 4T rule These types of plaque are identifiable through notches cut in the edge by which they can Fig 3 13 ASTM plaque type IQI also be identified on the radiograph Originally it was standard practice to use a plate of 2 of the specimen thickness but at present 1 and 4 plates are used too If T is 2 of the specimen thickness and the 2T hole can be seen on the radiograph the attained sensitivity level is said to be 2 2T etc Equivalent sensitivity values in percen tages are shown in table 3 13 At least three sides of a pe
48. 1 y is 1 5 sensitivity Fig 30 16 CR image of a weld with a longitudinal defect see acknowledgements Dose reduction and controlled area Not only are CR 5x tol0x and DR techniques 20x with film quality to 200x with low quality much faster than standard X ray film exposures see figure 27 16 another attractive feature is their far greater dynamic range latitude gt 1000x These methods are therefore not over sensitive to variations in radiation dose and very tolerant to less than exact exposure times see figure 8 16 This can reduce so called re shoots retakes and can decrease the need for multiple expo sures of some parts with different thicknesses thus further improving inspection throughput The reduced exposure times in practice a factor 2 tol0 dependent on the type of plate or weaker sources that can be used are deciding efficiency and safety factors smaller controlled area The controlled area radiation exclusion zone reduces with the square root of source strength ratio V souree strength 1 source strength 2 Note For a given application the source activity strength Bq can be reduced but not its energy level MeV keV because it is the energy level that determines the penetration capability 169 Automated mechanised inspection The choice of DR flat panel detec tors depends on the image quality required and the number of parts to be inspected to make it cost effective return on in
49. 20th edition permits X ray imaging with other means 11 1 2 3 Other Imaging Media As a minimum the procedure for radiography using imaging media other than film shall include the following details A similar formulation is included in DNV OS F101 Radioscopic testing Specific requirements to radiography of installation of girth welds 213 Radioscopic testing techniques in accordance with 13068 may be used provided the equipment has been demonstrated in accordance with Subsection F to give sensitivity and detection equivalent to conventional X ray according to ISO 12096 Last but not least ASME V 2007 supports the use of the CR technique for weld inspection as well Although no EN standard for welds exists until yet an international working group is involved in its compilation It is the intention to split existing EN 1435 covering weld inspection into three parts respectively for film CR and DR For CR already many years on the market not even an EN working document for weld inspection exists so it will take several more years for it to appear Data exchange and tamper proof standard Similar to the medical world where digital radiography has been in use long before its introduction in the NDT industry standards for data recording have been developed For industry a standard practice exists ASTM E 2339 04 to facilitate interoperability between systems and third parties of digital NDT data acquisition and imaging
50. 34225 07 8 German The X ray Inspection Volume 7 ISBN 3 934255 22 1 English translation Both compiled by Dr Ing M Purschke Castell Verlag GmbH 4 Handbook of radiographic apparatus and techniques English Publication for the HW by the Welding Institute Abington Cambridge England 5 Radiographic film systems brochure issued by GE Inspection Technologies 6 Home page www geinspectiontechnologies com Acknowledgements Figures 9 5 and 4 17 as well as table 2 9 were copied with the publisher s consent from reference book 2 Niet destructief Onderzoek by W J P Vink Delftse Universitaire Pers Rontgen Technische Dienst bv Applus RTD NDT amp Inspection since 2006 Rotterdam consented to the use of a number of their illustrations and graphs Furthermore they supplied ample information for chapter 16 concerning application of the CR method and standards related to digital radiography Illustrations marked with are used by courtesy of Oceaneering Inspection Division Nanofocus is a registered trademark of Phoenix X ray Systems a division of GE inspection Technologies Phoenix also provided ample information and illustrations for chapter 17 concerning magnification X ray microscopy and CT techniques TomoCAR is the trademark of 3D CT equipment a mutual development of Applus RTD and BAM Berlin Applus RTD holds the patent 216 Appendices tables and graphs BS SI units Formerly used Conversion
51. 6 graphically illustrates the relative image quality of different films and digital techniques True real time Radiosco by DR Panels hy Q A 3 ao V Si L T 3 aa lt g Better Fig 27 16 Relative image quality and speed of the various radiographic methods This graph appears enlarged in the appendix This overview shows that the best image quality best IQI visibility of CR plates is similar to what can be achieved with medium to finer grain film compare point A with B but is appoximately five times faster At point C the quality is less than what can be achieved with coarse grain film but the speed is more than ten times faster compared to point B RCF films five tot ten times faster than D7 film are positioned in the same range as CR plates The graph for DR panels is based on the results obtained with common flat panel detectors with different numbers of pixels 25 to 400 microns The best quality that can be achieved with DR panels comes close to fine grain film D3 compare point D to point E 164 Guideline etc In general the issue of standards lags far behind the introduction of a new method Development of standards starts earliest once a new NDT method is almost mature and has shown certain market viability The creation of a standard usually is a time consuming and painstaking formal process Standards are compiled by international working groups con sisting of NDT specialists of
52. Designation of quantity Name Unit Name Unit Old to SI Designation Designation Activity A Becquerel 1 s Curie Ci 1 Gi 37 GBq Bq Ionization dose Coulomb C C kg Rontgen 1 R 2 58 x 10 C ke Ionization dose rate Coulomb C C kg s Ampere A or A kg Absorbed energy Gray J kg 1 Rad 0 01 Gy dose D Gy Equivalent dose H Sievert J kg 1 Rem 0 01 Sv H D x RBE Sv Table 1 3 Overview of new and old units disintegrations per second C Coulomb A s RBE Relative Biological Effect J Joule A Ampere See chapter 3 Material Steel 100 kV 8kV mm Aluminium 50 kV 2 kV mm Plastics 20 kV 0 2 kV mm Table 2 11 Rule of thumb values for the selection of X ray tube voltage See chapter 11 fmin minimum distance source to source side object mm S source size mm b distance source side object to film mm 5000 mm AQ So S g D 5 wa oe in 5 2 S E a distance fin for catagory A gt Fig 5 11 Nomogram for minimum source to film distance fmin according to EN 1435 criteria Catagory A less critical applications general techniques Catagory B techniques with high requirements of detail discernability Fig 5 11 Nomogram for minimum source to film distance Fmin according to EN 1435 criteria See chapter 11 218 distance b __ Fig 4 12 Operating range not being used Graph for the minimum numbe
53. GE Inspection Technologies picture this For intuitive intelligent and easy non destructive testing imaging focus on GE Our portfolio covers a full range of inspection modalities from ultrasonic and radiographic to remote visual and eddy current Our advanced technology solutions software and services can help you acquire the data you need to make informed decisions about productivity safety and quality Now part of the picture is phoenix x ray a leader in high resolution CT and x ray technology phoenix x ray joins AGFA NDT Wie Everest VIT Hocking NDT Krautkramer Nutronik and Seifert to become the company a with all your inspection solutions GE Sensing amp Inspection Technologies www geinspectiontechnologies com en GE imagination at work Issued by GE Inspection Technologies 2006 General Electric Company All Rights Reserved We reserve the right to technical modifications without prior notice GEIT 30158EN 06 08 GE Inspection Technologies Industrial Radiography Image forming techniques GE imagination at work m Industrial Radiography Image forming techniques 1 5 sensitivity 3 e ee oe a g gt 10 x11mm C S weld 160kY 2mA CP Pan 5s No Pb screens Medium Gain Digital radiography CR image of a weld see acknowledgements 1 m Introduction to the overview of Industrial Radiography Image forming techniques The first issue of
54. Industrial Radiography was published by Agfa in the sixties for educational and promotional purposes Some improved editions have been released since providing information on the latest image forming radiographic techniques The booklet has been published in a number of languages and has been very much in demand The latest edition was compiled in the seventies but is now obsolete because of the large number of computer aided NDT techniques which have entered the market in recent years In 2007 a new edition in the English language was published by GE Inspection Technologies That edition was compiled by Mr J A de Raad NDT expert and consultant who has a considerable number of publications on the subject of Non Destructive Testing to his name Mr A Kuiper an experienced specialist and tutor on industrial radiography assisted him Both had been involved in Non Destructive Testing during their professional careers at Applus RTD NDT amp Inspection headquartered in Rotterdam the Netherlands Apart from the developments in conventional radiography primarily regarding X ray equipment and films the 2007 issue describes the now mature methods of digital radiography using radiation sensitive plate and panel detectors including digitisation of traditional film Several other computer assisted methods such as the CT technique are also included as well as a separate chapter describing a variety of applications In this latest 2008 edition we c
55. Notation pico 0 000000000001 10 12 nano 0 000000001 10 micro 0 000001 10 6 0 001 1 1000 1000000 1000000000 Table 2 3 Prefixes 29 3 2 Definitions Radioactivity The activity of a radioactive source of radiation isotope is equal to the number of disinte grations per second The SI unit is the Becquerel Bq and is equal to 1 disintegration per second The Becquerel is too small a unit to be used in industrial radiography Source strengths are therefore quoted in Giga Becquerel GBq 1 Curie 37 GBq see table 2 3 lonisation dose rate The output of an X ray set or isotope per unit of time is generally quoted at one metre distance from the source and designated in C kg see table 1 3 lonisation dose The ionising effect of radiation in one kilogram of dry air is used to define the ionisation dose The dose of radiation delivered is equal to the ionisation dose rate multiplied by the amount of time during which radiation takes place The designation used is C kg sec The output of an X ray set however is quoted in Sievert per hour measured at 1 metre distance Absorbed energy dose The radiation energy that is absorbed is expressed in Joules per kilogram J kg The SI unit is called Gray Gy whereby 1 J kg 1 Gy Equivalent dose man dose The Sievert Sv is the SI unit for the biological effect of ionising radiation upon man It corresponds with the product of the absorbed energy dos
56. Radiography of welds in large diameter pipes Radiation hazards measuring and recording instruments The effects of radiation on the human body 179 Responsibilities The client The radiographer The effects of exposure to radiation Protection against radiation Permissible cumulative dose of radiation Radiation measurement and recording instruments Radiation measuring instruments Dose rate meters Scintillation counter Personal protection equipment Pendosismeter PDM Thermoluminescent dose meter TLD badge Film dose meter film badge Dose registration Radiation shielding Distance Absorbing barrier and distance 10 187 190 190 193 193 194 195 197 198 198 201 202 207 207 207 208 208 209 210 212 212 20 Standards literature references acknowledgements and appendices 215 European norms EN standards Literature and references Acknowledgements Appendices tables and graphs 11 12 m Preface To verify the quality of a product samples are taken for examination or a non destructive test NDT is carried out In particular with fabricated welded assemblies where a high degree of constructional skill is needed it is necessary that non destructive testing is carried out Most NDT systems are designed to reveal defects after which a decision is made as to whether the defect is significant from the point of view of operational safety and or reliability Acceptance criteria for
57. Result This example of a valve with a great variety of wall thicknesses also shows one of the strengths of a digital exposure If needed the same image can be used to study the thin and thick wall parts of the valve thanks to the large dynamic range contained in the image Fig 39 16 Detail of pipe wall of figure 38 16 with report Archiving and reliability of images Archiving can be done on almost all existing professional mass storage facilities e g CD ROM 700 MB double layer DVD 10 GB double layer HD DVD 30 GB double layer blu blue ray disk 50 GB or hard disk In the not too distant future other high capacity optical solutions such as holographic disk technology 300 GB will become available Such mass memories are needed to be able to store a number of high resolution digital images A single image of a 400 x 400 mm panel with a pixel size of 50 microns requires 120 Mb position and up to16 bit of density data A pixel size of 100 micron needs only 30 MB Integrity procedures should be applied to prohibit manipulation or even forgery of digital images To exclude such tampering with images it is part of the data handling protocol to always include the original unprocessed data with the processed data set images see sec tion 16 9 Although attractive to save memory space it is impossible to compress the original umpro cessed data However for reporting purposes algorithms such as JPEG
58. S oO WA S amp The two wires of a pair are resolved if the dip between the peaks is greater than 20 of the maximum intensity Analogue signal intensity Fig 18 16 Resolution criterion of duplex IQIs for digital X ray images To establish spatial resolution a density line scan across the X ray image at the location of theduplex IQI should be made to determine this resolution The resulting linear analogue response of this scan is then interpreted to determine achieved resolution as illustrated in figure 18 16 The criterion is that the dip between two peaks of the wire pair must be equal to or more than 20 of the peak heights To avoid line interference or Moir effects during the line scan process of the reader the IQI for that purpose should be rotated for 5 with respect to edge of the CR plate or DR panel as required by EN 14784 1 and illustrated in figure 19 16 Fig 19 16 CR image of a weld with rotated duplex IQI 159 16 6 3 Indicators of image quality MTF and DQE Factors influencing image quality In the process of making a radiograph three factors influence the ultimate image quality 1 exposure conditions 2 detector performance efficiency 3 performance of the processing equipment to form an image To enable quantification of the quality of digital radiographs and the hardware used to create them two notions are in use MTF and DQE Image quality definitions Image quality is the total result
59. V 200 kV 300 kV Ir192 EN 584 1 1 2 3 4 w a 5 0 C2 1 30 G3 ils C4 1 0 G5 D8 i 0 6 0 6 C6 1 1 3 F6 RCF 5 wE n Tn F8 RCF 5 0 035 0 040 ff Table 1 8 Listing of various Agfa films and their relative exposure factors and film system classification Note I It is common practice to compare relative exposure factors with those of D7 film which are shown bold as reference value 1 0 in the table Note II The numbers 1 to 5 used in the table indicate the use of the following screen types without lead screens with lead screen 0 027 mm thickness with lead screen 0 027 mm thickness with lead screen front 0 10 mm back 0 15 mm thickness with fluorometallic screen RCF mW KR UNK Note III Developing process for table 1 8 automatic 8 minute cycle 100 seconds immersion time in developer G135 at 28 C Note IV The relative exposure factor depends not only on radiation intensity but also on expo sure time and is therefore not a constant value 69 8 2 Film type selection Most procedures and codes of good practice for the performance of industrial radiography base the choice of type of film for a specific application on the EN or ASTM classification systems For weld inspection when one is attempting to detect small cracks a film of class C2 or C3 would be specified For the examination of castings or general radiography a film of class C4 or C5 would nor
60. a at varying tube voltages and con Fig 4 4 Energy spectra at varying values for tube current stant tube current here 10mA and constant high voltage here 200 kV 50 100 150 The energy spectrum is also influenced by the characteristics of the high voltage applied to the tube When the spectrum of one X ray tube on constant voltage is compared with that of another with a current of pulsating voltage of the same kV value both spectra will be slightly different With a current of pulsating voltage there are during each cycle moments of relatively low voltage during which there will be a greater proportion of soft X rays with their side effects This means that a set working on a constant voltage will provide a higher intensity of hard radiation than one on a pulsating voltage although both working at the same nominal kV value However even identical X ray tubes may also show differences in generated energy The energy generated by one 200 kV X ray tube will not be true identical to the energy genera ted by another X ray tube with the same applied voltage not even if they are the same type of tube This behaviour impedes individual calibration in kV of X ray sets Another reason why it is hard to calibrate an X ray tube within a small tolerance band is that the absolute level and wave characteristics of the supplied high voltage are difficult to measure It follows that it is difficult to standardise and calibrate X ray equipment as
61. able bracket 4 collimated radiation source 9 sliders rollers 5 miniature flat panel detector 10 corrosion spot Fig 10 17 Schematic arrangement of a portable real time radiographic corrosion detector 184 This way an image is obtained of the pipe horizon with possible presence of corrosion swelling or pitting The image is presented real time on a portable monitor The battery powered equipment uses soft radiation of low intensity so that it can manually be moved along the pipe The system can also be used to locate welds under insulation providing the weld crown has not been removed 17 3 Computer Tomography CT Unique features For medical diagnostic purposes techniques have been developed to obtain a radiographic quasi 3D picture a so called CT image with a high resolution of a few tenths of a mm Powerful computers are used to transform a large number of absorption variations that occur when irradiating a human body with a moving source around the stationary patient and their coordinates into a comprehensive 3D volumetric image This technique is now also used in industry e g for checking the integrity of components with complex geome tries high quality castings miniature electronic circuits as built into mobile telephones and even for 3D metrology a method to measure even internal inaccessible dimensions of components that otherwise cannot be measured at all CT systems with a resolution of only a few mic
62. acent to the indicator The material of the indicator must be identical to the material being examined 112 The image quality of a radiograph is for example defined as the number of the thinnest wire still visible and is generally said to have image quality number X The image quality can also be expressed as a percentage of the object thickness examin ed If for instance the diameter of the thinnest wire visible to the naked eye is 0 2 mm and material thickness at the point of exposure is 10 mm wire discernibility or wire recognizability is quoted as 2 As emphasised above the use of an IQI does not guarantee detection of defects of com parable size It would be incorrect to say that because a wire of 2 of the object thickness can be seen on the radiograph a crack of similar size can also be detected The orientation relative to the X ray beam of a defect plays an important role in its dis cernibility see section 12 1 There are various types of IQI but the four most commonly used are 1 the wire type used in most European countries 2 the step hole type still occasionally used in France but the wire type is generally accepted as well 3 small plates with drilled holes called penetrameters which are used for ASME work although the ASME code nowadays includes the wire type IQI 4 the duplex IQI In some countries e g Japan and France additional means such as step wedges are used to veri
63. actice however this pro ved not to be achievable In particular where small cracks and other two dimensional defects are concerned it can never be guaranteed that they are not in fact present when no indication of them can be found in the X ray image However it is reasonable to expect that at least the quality of the radiographs and of course the rest of the entire process the film undergoes meets certain requirements The probability is high that defects will be more easily detected when the image quality is high The exposure technique and required image quality described in the code depend on the purpose for which the object involved will be used In order to be able to assess and quantify the image quality of a radiograph it needs to be converted into a numerical value and to do this image quality indicators IQI are used known in the USA as penetrameters Image quality indicators typically consist of a series of wires of increasing diameters or a series of small plates of different thicknesses with holes drilled in them of increasing diameters Although codes describe their techniques differently they agree on the following points e An image quality indicator shall be placed at the source side of the object being examined e If it is not possible to place the indicator on the source side it may be located on the film side This exceptional situation must be indicated by a lead letter F on or directly adj
64. ade creature _ o The British Museum 2 digital radiography with rigid flat panel or flat bed detectors and instant computer processing referred to as Digital Radiography DR for short and considered as the genuine true DR method and sometimes in the field referred to as Direct Radiography Each method has differing strengths advantages and limitations that should be evaluated in terms of specific application inspection requirements and economics capital human investment and productivity number of exposures in a certain time 144 145 3 The major parameters to compare film to digital radiography are spatial resolution contrast sensitivity and optical density range The major merits of digital radiography compared to conventional film are Shorter exposure times and thus potentially safer e Faster processing e No chemicals thus no environmental pollution e No consumables thus low operational costs e Plates panels and flat beds can be used repeatedly A very wide dynamic exposure range latitude thus fewer retakes e Possibility of assisted defect recognition ADR Despite all these positive features the image resolution of even the most optimised digi tal method is still not as high as can be achieved with finest grain film A few other limitations are also explained in this chapter 16 2 Digital image formation In conventional film radiography the human eye is used to examine a physical
65. akes about one minute and the refore the CR plate is almost immediately available for the next exposure Fig 6 16 Opened CR cassette CR cassettes Bare CR plates are nearly as pliable as film They can be packed in paper or vinyl cassettes either with or without lead screens These packages are still pliable Technically the plates can be used many times up to 1000 x provided they are handled with utmost care while their surface despite a protective coating is very sensitive to touching and dirt A single scratch can make the plate unsuitable for further use Rigid cassettes developed especially for the NDT market have built in intensify ing lead screens at the source side and a second lead screen at the back to absorb radiation caused by backscatter These multi layer cassettes are not flexible anymore but can be re used more often than the flexible cassettes even a few 1000 times Plastic front of cassette Magnetic sheet Lead intensifying foil 250 um Storage phosphor plate Lead screen 150 um Steel sheet Plastic back of cassette Fig 7 16 Structure of a CR cassette with storage phosphor Figure 7 16 shows a cross section of the CR imaging plate in a rigid cassette The steel and magnetic plates ensure that the various active layers are evenly and closely pressed together For low energy exposures clip type cassettes exist to replace the steel and magnetic plate which also ensure intimate contact between the la
66. al to the square of the distance better known as the inverse square law Both X and Gamma radiation follows that law The intensity of radiation per unit area of film is inversely proportional to the square of the sour ce to film distance s f d As figure 6 11 shows at a distance 2F from the source a beam of rays will cover an area b four times greater than area a at distance F Consequently the intensity per unit of surface area for b will be only 1 4 of the value for area a This means that all other parameters being equal at distance 2F exposure time must be multiplied by four to obtain the same film density Fig 6 11 Inverse square law for distances This principle obviously has its economical and practical limitations beyond which a further increase in s f d is just not feasible Selection of radiation energy kV Once the appropriate source to film distance is chosen the correct kilo voltage can be determined from an exposure chart see chapter 9 The importance of choosing the exact kilo voltage varies considerably with the kilo vol tage range being considered For X rays below 150 kV the choice is reasonably critical and gets more critical still at lower kilo voltages The kilo voltage to be applied is specified in EN standards see chapter 20 Table 2 11 gives useful empirical rule of thumb values for radiographs of aluminium steel or plastic objects TOO kV 8 kV mm 50 kV 2kV mm
67. al measurements These maps can be linear or non linear for example a logarithmic map is sometimes used to more closely mimic the response of conventional films 16 3 Digitisation of traditional radiographs Although the image forming of traditional film has nothing to do with digital radiogra phy digitisation of such films makes use of a major part of the technology and hardware also used for CR and DR and as such is part of this chapter of the book Storing and archi ving of chemically processed X ray films not only demands special storage conditions see section 10 7 but also takes up quite a bit of space 146 Digitisation of these films provides an excellent alternative that also prevents degrading Special equipment has been developed for this purpose Current digitisation equipment actually consists of a fast computer controlled flat bed scanner that scans the film spot wise in a linear pattern identical to the formation of a TV image measuring densities while digitising and storing the results The spot of the laser beam can be as small as 50 um in diameter 1 um 1 micron equi valent to one thousand s of a millimetre but the equipment can be adjusted for a coarser scan for example 500 microns to achieve shorter scanning times The values measured are compared to a calibrated density scale and processed digitally Density variations bet ween 0 05 up to 4 7 can be measured The scanner has part of its technology in commo
68. amic exposure of the weld Usually the crawlers are self contained and electrically driven Most of them are powered by heavy duty batteries or sometimes by a motor generator for the larger diameters see figure 12 18 Such crawlers must be very reliable to limit cut outs in case of brake down onshore or avoid costly loss of time on lay barges Such crawlers are equipped with X ray tubes with true panoramic conical beam to create a full circumferential exposure in one shot Although the resulting image quality is less than with X ray sometimes radioactive sources are allowed Figure 13 18 shows a battery powered gamma ray crawler Usually crawlers are built according to a fail safe design to avoid spontaneous radiation not triggered by the operator Fig 12 18 Large diameter X ray crawler with motor generator 203 da fji pes Control Battery r Isotope electronics pack container Fig 13 18 Small diameter gamma ray crawler Positioning devices using different physical methods are used to stop the crawler and thus the source of radiation at the correct position at the weld plane These positioning devices have to transmit or receive a position signal through the often thick pipe wall Despite its radiation and related risks low activity radioactive sources are often used either in or outside the pipe in combination with collimated detectors to achieve the required positioning accuracy Most attractive for obvious r
69. amtvcace For examinations using a tube voltage of less than 175 kV the thickness of the wedge might increase by 0 5 or 1 0 mm at each step while for radiographs using a higher tube voltage the increase could be in the order of 2 3 mm In addition several flat plates made from the same material and of a specified thickness e g 10 mm should be available If the thickness range of a step wedge runs from say 0 5 to 10 mm the step wedge and flat plate together would give a thickness range of 10 5 20 mm 15 Preliminary charts Before producing an exposure chart it is useful to first draw up preliminary charts the so called density thickness chart for the voltage range of the specific X ray set and a kV thickness chart The two preliminary charts are produced on the basis of the following data X ray set tube voltage 60 200 kV tube current 5 10 mA Filter none Source to film distance 80 cm Material steel Intensifying screens none Type of film D7 Density 2 0 Development automatic 8 minutes at 28 C in G135 developer O So pr pa o a Exposures Using a tube current of say 8 mA and an exposure time of 1 minute i e 8 mA min radio graphs of the step wedge are made at voltages of for example 75 90 105 120 135 150 165 180 and 195 kV Only a narrow strip of the film is used for each exposure The same process is repeated at say 10 mA with an exposure of 20 minutes i e 200 mA min Measuring the
70. ance this changes in the fixer and the light areas of the film become transparent As a rule the film is left in the fixer twice as long as it takes to clear or become trans parent Fixing time in a fresh solution approx 3 minutes is twice the clearing time 1 5 min As soon as it takes double that time to clear a film in G328 fixer solution at 20 C it must be replaced For every liter of solution in the fixing tank a square meter of film can be treated Films have to be kept moving during the first 30 seconds in the fixing bath 88 Final wash The final wash is intended to remove the residual fixer and the soluble silver com pounds left behind in the emulsion which if not flushed out would reduce film shelf life Washing should preferably be done with running water ensuring that all parts of the film are reached by fresh water The duration of the final wash depends on the tem perature of the water See table 2 10 Temperatures over 25 C must be avoided Temperature range C Washing time minutes Table 2 10 Relationship between water temperature and washing time Drying in the drying cabinet When the film is taken out of the water the water on the film as a result of its surface tension runs together to form droplets of varying size The film will therefore dry une venly causing drying marks For this reason it is advisable to immerse the films in a solution of 5 10 ml wetting agent to ea
71. and a disintegration graph The radiation material proper also called the source or pellet ranges in size from 1 to 4 mm The size is dictated by the specific radiation activity of the source material The outside dimensions of the cylindrical capsule are approximately 5 5 x 15 mm as shown in figure 9 5 Fig 9 5 Cross section of a capsule for a radioactive source Fig 10 5 Sealed capsule 5 6 Transport and exposure containers Transportation and handling of sealed sources are subject to strict international safety regulations as a source is continuously emitting radiation in all directions in contrast to an X ray tube which can be switched off During transportation and use the source must be surrounded by a volume of radiation absorbing material which in turn is encapsulated in a container The level of radioactivity at the outside surface of the container shall not exceed the legally established maximum limit Like the transport container the exposure container needs to be robust and must function safely at all times The exposure container also called camera must be fail safe and water and dirt proof It must also not be effected by impact Moreover if the radiation absorbing material for example lead melts in a fire the radia tion absorbing qualities must not be lost This requires a casing made of a material with a high melting point for example steel Besides lead increasingly a new sintered material with very high t
72. and film graininess Image contrast is affected by e differences in thickness of the specimen e the radio opacity radiation transparency of the specimen and its defects e the shape and depth location of the defects e the quality hardness of the radiation e the amount and effects of scattered radiation e the effect of filters used Film contrast depends on e the type of film e the density level Sharpness of an image is governed by e the effective size of the focal spot or radiation source e the source to object distance e the object to film distance e the contact between film and intensifying screens e the type of intensifying screens used e the radiation energy used 111 The last factor graininess depends on e the thickness of the emulsion layer e the concentration of silver crystals in the emulsion silver gelatine ratio e the size of the silver crystals e the radiation energy used e the developing process employed The radiation energy level is the only factor that can be influenced by the radiographer the other factors are determined by the film making process 13 2 Image quality indicators IQI s In the past it was thought possible to assess the smallest defect detectable by fixing a simple type of indicator on the test object during exposure This would supposedly guarantee that defects of a certain minimum size expressed as a percentage of the material thickness could be detected In pr
73. are in use to redu ce file sizes of processed images and for printing Exchange of data The workstation can also transfer images electronically over great distances through internet intranet or wireless which can be viewed interpreted or stored by remote users on identical satellite workstations 174 This way information is sent to the experts rather than sending the experts to the information Because the images are digital multiple copies of the images are always identical These capabilities are driving the latest trends of enhanced database capabilities and common workstation standards for digital radiography software Figure 40 16 shows a block diagram of the various components that make up a complete system for digital radiography v reres DR panel CR plate Printer Main CR tower in cassette Storage Film Archive digitiser Satellite workstation s Fig 40 16 Block diagram for digital radiography with workstation and supporting equipment 175 176 17 Special radiographic techniques The previous chapter 16 dealt with techniques that would be impossible without the aid of computers These techniques share a common feature whereby the processing interpre tation and storage of data is done by a central computer and monitor also called the work station In the current chapter 17 computers also play an everincreasing important role in some of the techniques discussed Computer tomography CT
74. are made together resulting in a large number of voxels On the basis of these many voxels the image of the entire defect can be reconstructed including its position orientation and depth location in the component This can be done with considerable accuracy typically 1mm or better accurate enough for calculation purpo ses Figure 18 17 shows a crack in an austenitic weld and next to it its CT reconstruction Figure 19 17 shows another example of a CT image obtained with this system The image shows two planar defects in a V shaped weld with a clear indication of their orientation and size 0 Macro of crack in a weld CT reconstruction of the crack Fig 19 17 CT reconstruction of two planar Fig 18 17 Crack in austenitic weld defects in a weld 189 Such 3D CT systems are primarily intended for defect analysis Scanning of a girth weld typically takes about one hour Inspection and reconstruction of one cross section takes less than 10 minutes An example of such a system is the so called TomoCAR see acknow ledgments at the end of this book This system uses a CMOS line array and is capable to inspect analyse pipe diameters of up to 500 mm with a total irradiated thickness of up to 50 mm 2 x 25 mm To analyse a defect in a circumferential weld the X ray tube is at one side of the pipe and the detector is located diametrically at 180 at the other side Both are moving synchronously Condition for use is that there is
75. ase rectifying If there is no smoothing applied considerable changes in voltage per cycle of alternating current will occur This periodic and greatly varying high voltage affects the intensity and spectrum of the radiation generated see section 4 3 41 The intensity of radiation is increased by double phased rectifying and varying degrees of smoothing At very low voltage ripple these sets are considered constant potential CP equipment In the latest types of tank sets the mains frequency is first converted to a high frequency alternating current and only then transformed upward which makes it easier still to smooth the ripple At very high frequencies up to 50 kHz smoothing is hardly necessary anymore and such X ray sets can be called CP systems Additional features may be built in for example an automatic warm up facility see note below This type of circuitry with advanced electronics leads to a higher degree of reliability and significant space and weight reduction compared with earlier power supply systems As a result of the various improve ments that have gradually been implemented present day high frequency AC X ray sets perform as well as true CP sets Note Because of the high vacuum prevailing inside the X ray tube it carefully has to be warmed up after a period of rest During rest the vacuum deteriorates This warm up procedure has to be done in accordance with the supplier s instructions to prevent high voltage shor
76. ce measuring instruments such as metal alloy analysing instruments Positive Material Identification PMI and humidity detection in insulation of thermally insulated pipping This type of non image forming instruments and applications are outside the scope of this book 18 1 Measuring the effective focal spot The effective focal spot size is an important feature of an X ray tube and is specified by the manufacturer In general it can be said the smaller the better As focal spot size is a criti cal exposure parameter see section 11 1 for a particular application the accuracy of the manufacturer s information is of vital importance Since 1999 EN 12543 1 requires a standardised method which however does not have the general support of suppliers as it requires expensive instrumentation and is time consuming The EN method suitable for effective foci gt 0 2 mm involves scanning the X ray tube radiation beam with a scintillation counter through a double collimator with an extremely small opening of 10 gm The resulting intensity values are then represented in a three dimensional isometric diagram from which the effective focal spot can be deduced EN 12543 1 replaces the older less accurate IEC 336 procedure The values based on EN 12543 are often still reported together with data based on the IEC 336 procedure for example 3 5 mm EN 12543 2 2 IEC 336 The numbers based on the IEC refer to a look up table from which th
77. ch litre of water Wetting agent reduces the sur face tension of the water so that after the film has drained the surface will be evenly wetted and will dry evenly with no risk of marks Films should be hung to drain for about 2 minutes before they are placed in the drying cabinet Drying should preferably be done in a drying cabinet or alternatively in a dry and dust free room No drops of water must be allowed to fall on films that are already drying as this will cause marks Wet films should therefore always be hung below already drying films Drying time will depend on temperature air circulation and relative humidity of the air in the cabinet Films will dry more quickly when they have first been put into a wetting agent Before a film is taken out of the drying cabinet it must be checked that the corners and edges of the film are thoroughly dry Air temperatures above 40 C should be avoided as this may cause ugly drying marks There must be free circulation of air between the films in a cabinet if they cannot dry evenly on both sides they may curl or distort Roller dryers Industrial dryers can be used to dry films quickly and uniformly after washing This mechanised drying process only takes minutes Dryers and chemicals should preferably be matched There are compact roller dryers on the market which are capable of developing approx 15 cm of film per minute and take up far less space than a drying cabinet 89 10 3 Recomm
78. ction of the most suitable system and procedures to be followed At the end of the book a chapter is added describing aspects of radiation safety 13 Fig 1 1 Basic set up for film radiography source homogeneous radiation 14 NA screens X ray film projection of defect on film i Introduction to industrial radiography Image forming techniques In industrial radiography the usual procedure for producing a radiograph is to have a source of penetrating ionising radiation X rays or gamma rays on one side of the object to be examined and a detector of the radiation the film on the other side as shown in figure 1 1 The energy level of the radiation must be well chosen so that sufficient radi ation is transmitted through the object onto the detector The detector is usually a sheet of photographic film held in a light tight envelope or cas sette having a very thin front surface that allows the X rays to pass through easily Chemicals are needed to develop the image on film which is why this process is called the classic or wet process Nowadays different kinds of radiation sensitive films and detectors not requiring the use of chemicals to produce images the so called dry process are used increasingly These techniques make use of computers hence the expressions digital or computer aided radiography CR or genuine true digital radiography DR see chapter 16 ADR related technique that has been ava
79. ctromagnetic radiation just like light heat and radiowaves 2 1 Wavelengths of electromagnetic radiation The wavelength lambda A of electromagnetic radiation is expressed in m cm mm micrometer um nanometer nm and Angstrom 1 A 0 1 nm Electromagnetic radiation Wavelength m Type 10 km 104 1km 10 100 m 102 10m 10 lm 1 10 cm 1071 Lem 102 1mm 10 100 um 104 10 um 10 gt Heat rays Infra red rays lum 10 6 microwaves X ray energy 100 nm 10 7 Visible light and Ultraviolet UV 100 eV 10 nm 10 8 1 keV lnm 10 0 1 nm 10 10 0 01 nm 10 4 X rays and Gamma rays 1pm 19 12 Radiography 10 MeV 0 1 pm 10 18 100 MeV 0 01 pm 10 14 Table 1 2 Overview of wavelength energy and type of electromagnetic radiation 9 2 2 X rays The radiation which is emitted by an X ray Fig 1 2 X ray spectrum intensity wavelength tube is heterogeneous that is it contains distribution oe X rays of a number of wavelengths in the se ee are the characteristic radiation of the form of a continuous spectrum with some superimposed spectrum lines See fig 1 2 The shortest wavelength of the spectrum is given by the Duane Hunt formula 1 234 ein kV wavelength In which wavelength in nanometers 10 m kV voltage in kilovolts The average shape of the X ray spectrum is generally the same however not truely identical for different X ray sets it dep
80. d films 95 11 Defect discernibility and image quality 97 11 1 Unsharpness 97 Geometric unsharpness Inherent film unsharpness Total unsharpness 11 2 Selection source to film distance 101 11 3 Other considerations with regard to the source to film distance 102 Inverse square law Selection of radiation energy kV Selection of gamma source 11 4 Radiation hardness and film contrast 104 11 5 Summary of factors that influence the image quality 105 12 Defect orientation image distortion and useful film length 107 12 1 Defect detectability and image distortion 107 12 2 Useful film length 108 13 Image quality 111 13 1 Factors influencing image quality 111 13 2 Image quality indicators IQI 112 Wire type IQI according to EN 462 1 13 3 List of common IQI s 114 ASTM IQI s AFNOR IQ s Duplex IQI s 13 4 Position of the IQI 117 13 5 IQI sensitivity values 117 14 Film exposure and handling errors 119 7 15 Film interpretation and reference radiographs 123 15 1 Film interpretation Pe 15 2 The film interpreter 124 15 3 Reference radiographs 124 Weld inspection Casting radiography Examination of assembled objects 16 Digital Radiography DR 145 16 1 Introduction to DR 145 16 2 Digital image formation 146 16 3 Digitisation of traditional radiographs 146 16 4 Computed Radiography CR 148 Two step digital radiography The CR imaging plate Image development Scanners Readers CR cassettes Dynamic range Exposure latit
81. d screens is polished to allow as close a contact as possible with the sur face of the film Flaws such as scratches or cracks on the surface of the metal will be visible in the radiograph and must therefore be avoided There are also X ray film cassettes on the market with built in lead screens and vacuum packed to ensure perfect contact bet ween emulsion and lead foil surface Figure 4a 6 and figure 4b 6 clearly show the positive effect of the use of lead screens Summarizing the effects of the use of lead screens are e improvement in contrast and image detail as a result of reduced scatter e decrease in exposure time Fig 4a 6 Radiograph of a casting without lead Fig 4b 6 Radiograph of a casting with lead intensi intensifying screens fying screens 54 Steel and copper screens For high energy radiation lead is not the best material for intensifying screens With Cobalt60 gamma rays copper or steel have been shown to produce better quality radio graphs than lead screens With megavoltage X rays in the energy range 5 8 MeV linac thick copper screens produce better radiographs than lead screens of any thickness Fluorescent screens The term fluorescence often mistaken for phosphorescence is used to indicate the cha racteristic of a substance to instantly instantly emit light under the influence of electro magnetic radiation The moment radiation stops so does the lighting effect This pheno menon is made good use
82. dards Tubes that produce a real perpendicular beam are known as true panoramics Fig 2 5 Anode configuration for an annular panoramic tube There are also panoramic tubes in which the electron beam is focused over an extended length by means of a magnetic lens or an electrostatic lens Wehnelt cylinder to produce a very small focal spot size These sets are called microfocus rod anode tubes with which a very small focal spot size of less than 10 micrometers can be achieved Since the anode can be damaged relatively easy through overheating the anode is usually interchangeable This requires a separate vacuum unit in order to restore the vacuum after replacement The advantage of this construction is that with different types of anodes different radiation patterns can be obtained for special applications The maximum energy level is usually below 150 kV However there are 150 kV microfocus tubes with a fixed anode for enlarging or scanning purposes see section 17 1 With these tubes the tube current has to be kept low because of heat dissipation limitations of the non interchangeable anode Some X ray tubes used in the radiography of plastics and aluminium are equipped with a beryllium window to allow the softer radiation generated at the lower tube voltages of 5 to 45 kV to pass 40 5 2 High voltage generators Conventional trans portable X ray equipment for use up to approximately 300 kV are pro vided with step up HT transforme
83. de For linear arrays smaller pixel sizes of 50 microns exist this size can technically be redu ced further Probably negative side effects cross talk and noise would then eliminate the advantages of such smaller pixels Limitations In practice DR flat panel detectors have proven to be excellent tools for the NDT industry however some limitations apply as well Both true DR and flat bed CMOS scanners have a restricted lifetime cuased by the accumulated radiation Flat panel detectors can be used continuously for years in mass production processes The ultimate lifetime is determined by a combination of total dose the dose rate and radiation energy Flat panel detectors are less tolerant for high than for low energy radiation hence extremely high energies should be avoided 157 With millions of pixels it is normal that over time a few pixels become less responsive similar to pixels of flat panel displays as used with notebook computers Usually the un acceptable number and pattern of dead pixels is specified by the manufacturer Fortunately in cases a small area of the panel is out of order an experienced interpreter of DR images is able to differentiate by pattern recogni tion and known position on the panel real component defects from less responsive pixels Also interpolation software is used to reduce or eliminate the effect of bad pixels So when a small portion of the pixels respond typically the corresponding
84. density After development of the radiographs the density of all steps is measured by a densitome ter see section 9 2 Drawing up the preliminary charts The densities measured are plotted graphically against the material thickness for which they were obtained A smoothly curved line then joins the points relating to one particular voltage The result is two preliminary charts figure 2 9 made at 8 mA min and e ATAW e a o amp AJAA B gt A 2 Zz Z 5 30 200 mA min 8mA min 200 mA min 1 0 mm 5 10 15 2 Fig 2 9 Density thickness preliminary charts 76 The density thickness preliminary charts as described provide the data needed to prepare the final exposure chart In order to eliminate any inaccuracies an intermediate chart based on the preliminary charts is prepared for density 2 using the data already recorded in the first charts This is how the thickness tube voltage chart of figure 3 9 is arrived at Points relating to the same series of exposures are then joined in a smooth line producing the intermediate curves for 8 mA min and 200 mA min In this way deviations in the results of any of the radiographs can be compensated for 2 4 6 8 12 14 16 18 20 22 24 Fig 3 9 Thickness versus tube voltage preliminary chart 17 9 4 The exposure chart The exposure chart should be drawn on uni directional logarithmic paper The material thickness in mm is plotted on the horizontal linea
85. detection sensor system dependent on pros and cons of such a system The detectors can be characterised by detection method direct versus indirect and by geometry linear versus two dimensional 2D All the different detector types that are useful in industri al inspection applications have a wide dynamic range similar to CR plates see figure 8 16 Direct versus indirect detection Radiation All X ray detection methods rely on the ioni photons sing properties of X ray photons when they Amorphous silicon array detector interact with matter In direct detection one step devices the amount of electric dy light vy charge created by the incident X rays is CsI Scintillator TFT photodiode array directly detected in semiconductor materials In indirect two step devices the X ray ty electrons ty energy is absorbed by phosphorescent Read out electronics digitisation materials known as scintillators which er Digital data to workstation emit visible light photons and these P ho Fig 11 16 Schematic of an indirect two step tons are then detected by a photo detector flat panel detector being the second layer thus being an indirect process The different active layers are illustrated in figure 11 16 Because many thousands of ioni sed charges can be created by a single X ray direct photo detectors must be both very sensi tive and able to measure large amounts of charge to produce good image quality The t
86. e Gamma rays arise from the disintegration of atomic nuclei within some radioactive sub stances also known as isotopes The energy of gamma radiation cannot be controlled it depends upon the nature of the radioactive substance Nor is it possible to control its intensity since it is impossible to alter the rate of disintegration of a radioactive substance Unlike X rays generated to a continuous spectrum Gamma rays are emitted in an isola ted line spectrum i e with one or more discrete energies of different intensities Figure 2 2 shows the energy spectrum lines for Selenium75 Cobalt60 and Iridium192 In practical NDT applications sources radio active isotopes are allocated an average nominal energy value for calculation purposes see section 5 4 Spectrum components with the highest energy levels keV values influence radiographic quality the most Se 5 Ir192 0 200 400 600 relative intensity 1200 1400 energy keV Fig 2 2 Energy spectrum lines for Se75 Ir192 and Co60 21 2 4 Main properties of X rays and y rays X rays and y rays have the following properties in common 1 invisibility they cannot be perceived by the senses 2 they travel in straight lines and at the speed of light 3 they cannot be deflected by means of a lens or prism although their path can be bent diffracted by a crystalline grid 4 they can pass through matter and are partly absorbed in transmission 5 they are ionising that
87. e X ray beam Vv Angle L Relative radiation angle A i I eZ thickness Defect y Weld Good image Poor image No image Fig 13 17 Effect of position of X ray source Fig 14 17 Image formation versus relative radiation angle versus defect orientation 187 PEET OONIRAST In practice the following rule of thumb is GOOD applied to the detection of planar defects with a high probability defects Open e a defect is detectable if the angle between the X ray beam and the defect is approximately 10 or less Eo n i The value of 10 is based on decades of l practical experience but does not gua rantee detection The rule is visualised in the graph of figure 15 17 To detect 10 Angle 2D defects with unknown orientations and exceeding 10 a multi angle tech nique would be required which in general is impractical As a measure to Fig 15 17 Graphical presentation of rule of thumb enhance detection of lack of fusion defects in critical welds sometimes two additional shots are made in the direction of the weld preparation Apart from orientation the detectability of 2D defects is also dependent on the type of defect its height and its ope ning width Lack of side wall fusion LOF dependent on the welding method will in general be easier to detect than a crack because LOFis often accompanied by small 3D slag type inclusions Rule of thumb A In fact LOF def
88. e shown in table 1 9 These are the factors by which to increase or decrease the exposure time when using the types of film other than those for which the exposure charts have been prepared In view of the widely varying quality of the radiation emitted by different types of X ray equip ment and the appreciably different characteristics of the various types of X ray films made for industrial use caution should be exercised in applying these relative exposure factors generally Type of film Relative exposure factors 200 kV 300 kV Ir192 2 3 4 RCF F6 5 RCF F8 5 Table 1 9 Relative exposure factors For 1 to 5 refer table 1 8 Darkroom technique too plays an important role and a uniform manual or automatic development process is therefore essential With radioactive sources which give a constant quality hardness energy of radiation the relative exposure factors listed can be used quite safely 9 7 Absolute exposure times Table 2 9 derived from reference 2 lists the widely varying absolute exposure times when different radiation sources are used for radiography on steel of varying thickness The relative exposure factors from table 1 9 for both types of film can be recognised in this table 80 yy X ray tube Gamma source Linac 1 25 MeV 4 7 100 20 70 15 1080 210 660 210 6 Table 2 9 Absolute exposure times for steel of varying t
89. e focal dimensions expressed in mm can be derived Those dimensions are given with a wide tolerance In fact the IEC values were too inaccurate to calculate image sharpness being the main reason to develop the EN procedure Manufacturers of X ray tubes usually apply the so called St nop pinhole tech nique camera obscura to determine focal spot size The X ray tube projects its focus through a very small hole pinhole in a lead plate onto a film The lead plate is positioned exactly halfway between focus and film To prevent scattered radiation the hole is sometimes made in a tungsten plug which forms part of the lead plate After development the effective focal spot size can be measured on the film with the aid of a magnifying glass The latter method still allowed and accepted by EN results in marginally smaller effective focal spot sizes Establishing the effective focal spot size of a panoramic X ray tube is considerably more complicated To circumvent this it is therefore recommended to just make a radiograph of the object pipe or vessel weld with the right IQI s and check the results for compliance with the quality requirements specified 193 18 2 Radiographs of objects of varying wall thickness For radiographs of an object with limited differences in wall thickness it is common to base exposure time on the average thickness to obtain the required film density of at least 2 It is possible that parts of the film are eithe
90. e gray Gy with a factor that has been experimentally determined and that indicates the relative biological effect RBE of the ionising radiation For X and y radiation this factor is equal to one so that the Sievert is equal to the Gray 30 31 O Radiation sources 4 1 X Ray tube The X ray tube see figure 1 4 consists of a glass or ceramic envelope containing a positive electrode the anode and a negative electrode the cathode evacuated to an ultra high vacuum 10 hPa hectoPascal The cathode comprises a filament that generates electrons Under the effect of the electri cal tension set up between the anode and the cathode the tube voltage the electrons from the cathode are attracted to the anode which accelerates their speed This stream of electrons is concentrated into a beam by a cylinder or focusing cup When the accelerated electrons collide with a target on the anode part of their energy is cathode converted to X radiation know as X rays Ue anode cylinder i glass 4 2 The anode ilamen arge l t target The target is generally made of tungsten Not only because it has a high atomic number but also because of its high melting point approx 3400 C It is essential to use a material with a high melting point because of the substantial amount of heat dissipated as the electron bombardment is concentrated focused on a very small surface Only a part approx 0 1 at 30 keV
91. e times can be dedu ced when using different films and screens at 200 kV for film density 2 The graph shows that an F8 film with RCF screen point C is approximately 8 times faster than a D8 film with lead point B and approximately 15 times faster than a D7 film with lead point A Since on stream examination as well as examination of concrete and also flash radiogra phy see section 18 7 allow concessions to image quality a special fluorometallic screen NDT1200 has been developed with extremely high light emission In combination with an F8 film it may result in a reduction in exposure time at a factor 100 at 200 kV against a D7 film with lead point D as opposed to point A in figure 6 6 or even a factor 140 to 165 depending on source selection see table 2 6 The intensification factor of the NDT1200 screens increases significantly at lower temperatures Table 2 6 shows the effect of radiation hardness on relative exposure times for the various film screen combinations compared with D7 film with lead screen Noticeably for the NDT1200 screen and F 8 film the factor increases with the increase in energy but for the F6 film the factor decreases at energy levels exceeding 300 keV Relative exposure times Energy level FilmF8 Factor FilmF6 Factor FilmD7 NDT1200 0 01 100 0 05 20 RCF 0 03 33 0 17 none 25 NDT1200 Pb RGF Lead Ir192 NDT1200 Pb 450 keV RCF Lead Co60 NDT1200 1 25 MeV RCF Lead Table 2 6
92. ea where radiographs are made or concrete bunkers with doors which automatically switch off the X ray equipment as soon as they are opened Both methods have the same objective i e to prevent unauthorised people entering the area of radiation An area of radiation can be defined as an area in which the radiation level exceeds the per mitted value of 10 uSv h There are three ways to achieve a reduction in intensity 1 by erecting a demarcation barrier at an appropriate distance 2 by erecting an absorbing barrier 3 bya combination of methods 1 and 2 212 Distance Since radiation is subjected to the inverse square law its intensity is reduced with the incre ase in distance to the square Absorbing barrier and distance Whenever radiation penetrates a material the absorption process reduces its intensity By placing a high density material such as lead around the source of radiation the quantity of transmitted radiation will decrease To determine the material thickness required for a certain reduction in radiation a factor known as the half value thickness HVT is used Table 3 19 shows the HVT values for lead for various types of gamma sources Symbol Average energy in Half value thickness MeV in mm lead Cesium137 0 66 8 4 Cobalt60 Iridium192 Selenium75 Ytterbium169 Thulium170 Table 3 19 Half value thicknesses for lead using different types of gamma sources Example To reduce 2 56 mSv
93. easons are positioning devices not using radioactive isotopes With these control positioning devices also the commands to the crawler are given for exposure forward to the next weld or reverse Prior to arrival of the crawler at the next weld the positioning device is accurately positioned on the pipe near this next weld to stop the crawler To increase its stop accuracy at first the crawler is slowed down at reduced speed the crawler is stopped in order to achieve the highest possible posi tion accuracy of the radiation source Fig 14 18 Preparation of strip film on girth weld 204 Crawler and control technologies for on and offshore application are almost the same The choice of film can be different For onshore application traditional film is ek Sw used in general type D4film or equivalent H Girth weld with a common developing process 4 location For lay barges requiring the shortest pos a sible cycle time often high speed film RCF rapid cycle film see chapter 8 in combination with special developer and fixer is applied Instead of using standard film lengths it is much more efficient to use a long strip of film to be wrapped around the pipe as Radiation illustrated in figure 14 18 e g Agfa pein Rollpac with a length covering the cir cumference including a small overlap Fig 15 18 Lead radiation protection tunnel Strip film with and without lead screens exists in different widths They are
94. ech nologies for CMOS scintillator materials and amorphous photo detectors are relatively mature and used in many commercially available DR detector products 153 Linear detectors Linear detector arrays LDAs based on CMOS tech nology as shown in figure 12 16 are commonly used in applications where a mechanical means pro vides a relative motion between the object being inspected and the X ray beam LDAs can be made in virtually any length In practice active lengths are available up to over 1 metre and energy ranges from 100 kV to several MeV F d Length 640 mm S A 7500 pixels Fig 12 16 CMOS linear detector array courtesy Envision A common well known application is airport luggage inspection where a conveyor belt carries objects through a fan shaped collimated X ray beam and past a linear detector array as illustrated in figure 13 16 A series of row like subimages from successive locations is then assembled to form a two dimensional radiograph for interpretation In NDT similar linear arrays MS with small sensor elements a f Fae kal are typically used in high speed testing machines for production inspection appli cations that incorporate either a manipulator or a conveyor to move parts past a stationary X ray tube de tector arrangement similar Workstation i to figure 13 16 Moving direction X ray tube Fig 13 16 Inspection set up using a linear array A relatively new application is the inspect
95. ects are in practice often Xray beam detected because of the presence of these small inclusions rather than because of the LOF itself Such secondary small defects do usually not occur along with cracks although their character might be A Unfovourable erratic and their small facets under diffe meme P i rent angles some of them just in line with the X ray beam might help to detect them L as illustrated in figure 16 17 From experi Crack image ments it is known that if more than 1 to 2 of the material in the line of radiation Fig 16 17 Crack facets creating the defect image is missing a defect is detectable 3D CT for sizing of defects in welded components To know the through thickness size of a defect can be of paramount importance to calcula te the strength of a cracked component its remaining strength or its fitness for purpose Traditional single shot and even multi shot radiography is unable to measure through thickness height of planar defects Even detection itself is not always easy as the previous paragraph describes Therefore sizing of defects once detected with radiography is often done by ultrasonics with accepta ble sizing accuracies for engineering critical assessment ECA calculations The application of ultrasonics on welds requires that both the material and the weld are acoustically transparent This requirement is often not met in welded or cast austenitic materials as used in nuclear power plant
96. ed as 0 4 mSv per week and 10 uSv h at a 40 hr working week These levels are acceptable but it is not to be automatically assumed that people working with radiation actually should receive these doses When radiography is carried out in factories and on construction yards etc special conside ration is required for non radiological workers and demarcation of the area in which radia tion is used and a maximum dose rate of 10 uSv h applies is essential This is also the maximum value to be measured at the outside surface of a charged isotope container 19 6 Radiation measurement and recording instruments From what has been said before it follows that establishing the presence of ionising radia tion and measuring its level is of paramount importance Since ionising radiation cannot be detected by the senses detectors and measuring equipment are used There are various instruments with which the radiographer can measure or register radiation The most common measuring instruments are 1 Dose rate meters 2 Scintillation counters The most common instruments for personal protection are 3 Pendosismeter PDM 4 Thermoluminescent badge TLD 5 Film badge Radiation measuring instruments Dose rate meters A portable Geiger Miuller counter of 7 x 15 x 4 cm see EI P figure 1 19 is the most commonly used instrument for measuring dose rate but the more accurate and more expensive ionisation chamber is used as radiation monitor as
97. ed down the surface of the emulsion prior to development Dark shapes 1 dark crescent shapes see clear shapes above these are darker than the surrounding area if the bending occurred after exposure 2 fingerprints the film has been touched with dirty fingers 3 electrical discharge see dark patches 121 Neutron radiograph of an iris flower made on Agfa D3 s c single coated Organic substances are well suited to examination by this type of radiation 122 Film interpretation and reference radiographs 15 1 Film interpretation The common term for film interpretation is film viewing Film viewing in fact means the evaluation of the image quality of a radiograph for compliance with the code require ments and the interpretation of details of any possible defect visible on the film For this purpose the film is placed in front of an illuminated screen of appropriate brightness luminance The edges of the film and areas of low density need to be masked to avoid glare The following conditions are important for good film interpretation brightness of the illuminated screen luminance e density of the radiograph e diffusion and evenness of the illuminated screen e ambient light in the viewing room e film viewer s eye sight Poor viewing conditions may cause important defect information on a radiograph to go unseen EN 25880 provides detailed recommendations for good film viewing conditions The luminance o
98. ed radiation is ten times higher than the image forming radiation The ratio I 1 Ip 1 1 I is called the build up factor and is of considerable importance for the detectability of defects It usually has a value between 2 and 20 depending on radiation energy and object thickness It must also be appreciated that any object in the neighbourhood of the object being exam ined table walls ground and so on which is struck by the gamma or X rays will partial ly reflect these rays in the form of backscatter which is liable to fog the film Backscatter coming from the object under examination is less hard than the primary radi ation that has caused it and can be intercepted by a metal filter between object and film Radiation scattered by objects nearby the film can be intercepted by means of a protective sheet of lead at the rear face of the film cassette 51 Scattered radiation also occurs in radiographic examination of cylindrical objects as shown in figure 3 6 Fig 3 6 Scattered radiation in radiogra phy of cylindrical objects Scattered radiation from object 1 causes a spurious band at B object 2 at A etc unless lead strips are used as shown in the lower part of this figure OOO lead strips 000 The effects of scattered radiation can be further reduced by e limiting the size of the radiation beam to a minimum with a diaphragm in front of the tube window e using a cone to localise the beam a
99. egularities in the object cause variations in film density brightness or transparency The parts of the films which have received more radiation during exposure the regions under cavities for example appear darker that is the film density is higher Digital radiography gives the same shades of black and white images but viewing and interpretation is done on a computer screen VDU The quality of the image on the film can be assessed by three factors namely 1 Contrast 2 Sharpness 3 Graininess As an example consider a specimen having a series of grooves of different depths machi ned in the surface The density difference between the image of a groove and the back ground density on the radiograph is called the image contrast A certain minimum image contrast is required for the groove to become discernible With increased contrast a the image of a groove becomes more easily visible b the image of shallower grooves will gradually also become discernible 16 Assuming the grooves have sharp machined edges the images of the grooves could still be either sharp or blurred this is the second factor image blurring called image unsharpness At the limits of image detection it can be shown that contrast and unsharpness are inter related and detectability depends on both factors As an image on a photographic film is made up of grains of silver it has a grainy appea rance dependent on the size and distribution of th
100. el container for Selenium75 with pigtail at right and operating hoses at left To enable radiography on work sites with many people in the vicinity for example on offshore installations or in assembly halls containers with rotating cylinders and collimators were developed so that only the beam of radiation required for the radiograph is emitted The remainder of radiation is absorbed by the collimator material which allows people to work safely at a distance of a few metres while radiography is in progress Such containers with collimators are known by the name of CARE Confined Area Radiation Equipment or LORA Low Radiation equipment 48 handle tungsten container rotating cylinder source Collimator base pipe and weld film lead shielding Ay boundaries of the beam of radiation Fig 16a 5 Gamma container with collimator Fig 16b 5 Cross section of CARE LORA container on the pipe on a circumferential weld in a pipe Without collimating the minimum safety distance is considerably more than 10 metres in all directions Such containers with collimators are particularly suitable for frequent and identical repetitive NDT work for example radiographic testing of welds in pipes of lt 300 mm diameter Figure 16a 5 shows such a special container with collimator set up for a double wall radiograph The cross section drawing of figure 16b 5 shows the boundaries of the beam of radiation For bigger focus
101. eld image and along the edge of the root pass image being welded Internal concavity suck back Burn through An elongated irregular darker density with fuzzy edges in the Localized darker density with fuzzy edges in the centre of the A darker density band with very straight parallel edges in the centre of the width of the weld image width of the weld image It may be wider than the width of the center of the width of the weld image root pass image Incomplete or Lack of Penetration LoP Interpass slag inclusions Irregularly shaped darker density spot usually slightly elongated and randomly spaced Elongated slag lines wagon tracks Elongated parallel or single darker density lines irregular in width and slightly winding lengthwise 130 Lack of side wall fusion LOF Elongated parallel or single darker density lines sometimes with darker density spots dispersed along the LOF lines which are very straight in the lengthwise direction and not winding like elongated slag lines Interpass cold lap Small spots of darker densities some with slightly elongated tails in the welding direction 131 Scattered porosity Rounded spots of darker densities random in size and location boo o0 oO oY o AD 0 o Cluster porosity Rounded or slightly elongated darker density spots in clusters with the clusters randomly spaced 132 Root pass aligned porosity Rounded and elongated darker densi
102. elopment 2 exhausted developer 3 unsuitable or wrongly mixed developer Excessive contrast i e lack of intermediate tones 1 radiation too soft 2 umnder exposure compensated by prolonged developing 3 unsuitable or wrongly mixed developer General lack of density 1 radiation too soft 2 umnder exposure compensated by prolonged developing 3 unsuitable or wrongly mixed developer General excessive density 1 over exposure 2 prolonged development or developing temperature too high 3 unsuitable or wrongly mixed developer Insufficient sharpness source to focus distance too short source or object moved during exposure film to object distance too great dimensions of source or focus too big poor contact between film and screens wrong type of foil used 119 oa he Grey fog local or overall unsuitable dark room safelighting excessive exposure to safelight i e too long or too close film accidentally exposed to X ray or Gamma ray or to white light heavy scatter film out of date or stored under unsuitable conditions ground fog extreme under exposure compensated by excessive developing exhausted or wrongly mixed developer film cassette with film exposed to heat e g sunlight heat from radiators etc cassette not properly closed edge fog O A ae a a Yellow fog 1 prolonged development in badly oxidised developer 2 exhausted fixing bath 3 insufficient rinsing between developing and fixing Note It ma
103. ements regarding viewing conditions and viewing equipment the film interpreter film viewer shall have thorough knowledge of the manufacturing process of the object being examined and of any defects it may contain The type of defects that may occur in castings obviously differs from those in welded constructions Different welding processes have their own characteristic defects which the film inter preter must know to be able to interpret the radiograph To become a qualified NDT operator various training courses course materials and leaf lets specifying the requirements they need to comply with exist The European NDT industry conforms to the qualification standards of the American ASNT organisation So far a training programme for film interpreter has not been established in similar fashion Textbooks for example are not uniform Sometimes the I W weld defect refe rence collection is used beside which the instructor usually has his own collection of typical examples supplemented with process specific radiographs ASTM has a reference set of defects in castings available There are incidental initiatives to introduce classification of film interpreters by level in a system comparable to the qualification of NDT personnel Some countries have alrea dy implemented such a system 15 3 Reference radiographs The two main areas for the application of radiography are weld examination and examination of castings Radiography i
104. endations for the darkroom Cleaning of tanks Whenever the processing solution is renewed the tank must be cleaned preferably with hot water and soap If this proves inadequate polyester tanks can be cleaned using a bleach solution 100 200 ml litre of water hydrochloric acid 10 ml litre of water or acetic acid 50 ml litre of water Stainless steel tanks may be cleaned with a solution of nitric acid 10 ml litre of water or acetic acid 50 ml litre of water Hydrochloric acid must never be used for stainless steel tanks There are industrial cleaning agents on the market for example Devclean and the envi ronment friendly Fixclean specially developed for cleaning of darkrooms Stained fingers Brown stains on the fingers can be avoided by rinsing the hands in water whenever they come into contact with developer If fingers do become stained they should be immersed in a solution of 1 litre of water 2 gr of Potassium permanganate 10 ml of concentrated sulphuric acid Next the hands should be rinsed in an acid fixer solution and finally washed with soap and water ae op Chalky water If hard chalky water is used for mixing the solutions troublesome processing faults may occur Calcium salts may in the presence of carbonates and sulphites result in a whitish deposit on the films which is insoluble in water To prevent this the diluant can be softened by using a special filter or by boiling it first and letting it cool d
105. ends chiefly on the energy range of the electrons striking the X ray tube target and therefore on the voltage waveform of the high voltage generator A constant potential CP X ray set will not have the same spectrum as a self rectified set operating at the same nominal kV and current The spectrum shape also depends on the inherent filtration in the X ray tube window glass aluminium steel or beryllium The energy imparted to an electron having a charge e accelerated by an electrical poten tial V is eV so the energy of the electrons can be quoted in eV keV MeV These same units are used to denote the energy of an X ray spectrum line The energy of a single wavelength is E hv A v c In which E the energy in electronVolt eV h Planck s constant v frequency c the velocity of electromagnetic radiation such as light 300 000 km s The heterogeneous X rays emitted by an X ray tube do not however have a single wavelength but a spectrum so it would be misleading to describe the X rays as say 120 keV X rays By convention therefore the e in keV is omitted and the X rays described as 120 kV which is the peak value of the spectrum 20 2 3 Gamma rays y rays Radioactivity is the characteristic of certain elements to emit alpha a beta 6 or gamma y rays or a combination thereof Alpha and beta rays consist of electrically char ged particles whereas gamma rays are of an electromagnetic natur
106. energy the film is sandwiched between two inten sifying screens Different types of material are being used for this purpose Lead screens Under the impact of X rays and gamma rays lead screens emit electrons to which the film is sensitive In industrial radiography this effect is made use of the film is placed between two layers of lead to achieve the intensifying effect and intensity improvement of approxi mately factor 4 can be realised This method of intensification is used within the energy range of 80 keV to 420 keV and applies equally to X ray or gamma radiation such as pro duced by Iridium192 Intensifying screens are made up of two homogeneous sheets of lead foil stuck on to a thin base such as a sheet of paper or cardboard between which the film is placed the so called front and back screens The thickness of the front screen source side must match the hardness of the radiation being used so that it will pass the primary radiation while stopping as much as possible of the secondary radiation which has a longer wavelength and is consequently less penetrating 53 The lead foil of the front screen is usually 0 02 to 0 15 mm thick The front screen acts not only as an intensifier of the primary radiation but also as an absorbing filter of the softer scatter which enters in part at an oblique angle see figure 2 6 The thickness of the back screen is not critical and is usually approx 0 25 mm The surface of lea
107. equently applied It will therefore be useful to describe manual processing in this chapter and so become familiar with the developing process 10 1 The darkroom Entrance and colour For practical reasons the darkroom needs to be as close as possible to the place where the exposures are made although naturally out of reach of radiation The darkroom needs to be completely lightproof so the entrance must be a light trap usually in the form of two doors one after the other a revolving door or a labyrinth In practice the labyrinth is found to be the best arrangement although it does take up a comparatively large space The walls of the passage are painted matt black and a white stripe about 10 cm wide running along its walls at eye level is enough as a guide Inside the darkroom itself the walls should preferably be painted in a light colour light walls reflect the little light there is and are easier to keep clean Darkroom lighting X ray films are best processed in normal orange red R1 or green D7 darkroom lights The distance between film and darkroom lighting needs to be considered depen ding on the sensitivity of the film and the duration of the development process The light safety of the darkroom lighting can be tested by covering half of a pre expo sed film density 2 lengthways leave it for 5 minutes and then process it as usual The difference in density may not exceed 0 1 85 Another method is to
108. equired A three dimensional representation 3D CT of the radiographic image requires vast computing capacity With present day com puters depending on resolution required the total acquisition and reconstruction time nee ded for a 3D image is between a few seconds and 20 minutes Reverse engineering CT offers an effective method of mapping the internal structure of components in three dimensions With this technique any internal anomaly often a defect that results in a dif ference of density can be visualized and the image interpreted These properties allow the use of CT as an NDT tool permitting examination of samples for internal porosity cracks de laminations inclusions and mechanical fit It shows the exact location of the anomaly in the sample providing information on size volume and density Due to the fact that CT images are rich in contrast even small defects become detectable CT widely expands the spectrum of X ray detectable defects in process control and failure analysis increasing reliability and safety of components for e g automotive electronics aerospace and military applications It opens a new dimension for quality assurance and can even partially replace destructive methods like cross sectioning saving costs and time CT is increasingly used as a reverse engineering tool to optimise products and for failure analysis which otherwise would require destructive examination CT metrology CT systems can also be
109. eralised water The working method which only takes a few minutes is as follows 1 apply the test liquid reagent to an unexposed part of undeveloped dry film allow to soak for 2 minutes 15 seconds remove excess liquid with absorbent paper leave to dry for 1 minute before treating the reverse side repeat the above procedure on the other side of the film in exactly the same spot uw A W N Evaluate as follows within 30 minutes 6 the test zone of the film is put against a white background 7 the Thio Test colour strip is put on the film as close to the spot as possible 8 the colour step of the wedge that resembles the colour of the test zone closest is regarded definitive for life expectancy Colour wedge Thiosulphate Archival quality from dark to light g m L E Life expectancy Darkest Min 0 35 Film needs repeat treatment Dark Max 0 20 L E 10 years Light Max 0 10 L E 100 years Lightest L E 500 years Table 3 10 Colour steps for the Thio Test These values apply to films with double sided emulsion layers A regular Thio Test provides early detection of deficiencies in the development process for example exhausted fixer solutions irregular water supply or insufficient rinsing and so prevents poorly processed films being archived 10 7 Storage of exposed films The way in which radiographs are handled and stored plays a very important role in their keeping properties Films that must be kept for lon
110. ese silver particles This granular appe arance of the image called film graininess can also mask fine details in the image Similarly in all other image forming systems these three factors are fundamental para meters In electronic image formation e g digital radiography or scanning systems with CCTV and screens the factors contrast sharpness and noise are a measure for the image quality pixel size and noise being the electronic equivalent of graininess The three factors contrast sharpness and graininess or noise are the fundamental para meters that determine the radiographic image quality Much of the technique in making a satisfactory radiograph is related to them and they have an effect on the detectability of defects in a specimen The ability of a radiograph to show detail in the image is called radiographic sensitivity If very small defects can be shown the radiographic image is said to have a high good sensitivity Usually this sensitivity is measured with artificial defects such as wires or drilled holes These image quality indicators IQIs are described in chapter 13 17 Basic properties of ionising radiation In 1895 the physicist Wilhelm Conrad Rontgen discovered a new kind of radiation which he called X rays The rays were generated when high energy electrons were suddenly stopped by striking a metal target inside a vacuum tube the X ray tube It was subsequently shown that X rays are an ele
111. f silver halide crystals suspended in gelatine Note In the past radiography on paper was not unusual In this instant cycle process results became available within 60 seconds The quality of the images however was extre mely poor and the life of the film limited to a few months The availability of better and faster instant cycle techniques such as digital radiography see chapter 16 has made radiography on paper obsolete 7 2 Radiographic image Latent image When light or X radiation strikes a sensitive emulsion the portions receiving a suffi cient quantity of radiation undergo a change extremely small particles of silver halide crystals are converted into metallic silver These traces of silver are so minute that the sensitive layer remains to all appearances unchanged The number of silver particles produced is higher in the portions struck by a greater quantity of radiation and less high where struck by a lesser quantity In this manner a complete though as yet invisible image is formed in the light sensiti ve layer when exposure takes place and this image is called the latent image Before and after exposure but prior to development of the film the latent image has a shiny pale green appearance 59 Developing the latent image Development is the process by which a latent image is converted into a visible image This result is obtained by selective reduction into black metallic silver of the silver hal
112. f the container to the most favourable exposure position casing container handling operation side shielding exposure position of the source storage position of the source Fig 13 5 Exposure container with S channel and flexible flexible connection operating hose ad cable Figure 14 5 shows an S chan nel container with a flexible metal hose and cable in rol led up transport position Figure 15 5 shows a more recent 2006 S channel Selenium75 container with operation hoses and pigtail Selenium75 radio isotope is becoming popular since new production enrichment methods resulted in a much better k factor Thus for a cer tain activity source strength a much smaller source size focus is achieved This results in a better sharper image qua lity than could be achieved with the old Selenium75 production method Fig 14 5 S channel container with the flexible cable and deployment mechanism Due to its average energy level of 320 kV Selenium75 increasingly replaces X ray equipment for a thickness range from 5 mm to 30 mm of steel This eliminates the need for electric power very attractive in the field for reasons of electrical safety and more convenient at remote or work locations with difficult access high deep offshore refineries etc Last but not least a Selenium container is of much lower weigth than needed for an Iridium192 container with the same source strength Fig 15 5 S chann
113. f the light passing through a radiograph shall not be less than 30 cd m and whenever possible not less than 100 cd m cd candela These minimum valu es require a viewing box luminance of 3000 cd m for a film density of 2 0 The practical difficulties of providing the required luminance for a film density of 4 0 are considerable The main problem with constructing a film viewing box for these higher densities is the dissipation of heat from the lamps However by limiting the film area requiring such high power lighting it becomes possible to view radiographs of a film density of 4 The light of the viewing box must be diffuse and preferably white Radiographs should be viewed in a darkened room although total darkness is not necessary Care must be taken that as little light as possible is reflected off the film surface towards the film viewer If the film viewer enters a viewing room from full daylight some time must be allowed for the eyes to adapt to the dark A yearly eye test according to EN473 for general visual acuity is required while especial ly sight at close range needs to be checked The film viewer must be able to read a Jaeger number 1 letter at 300 mm distance with one eye with or without corrective aids The trained eye is capable of discerning an abrupt density change step of 1 While interpreting a magnifying glass of power 3 to 4 can be advantageous 123 15 2 The film interpreter Apart from the requir
114. far as spectra and k V settings is concerned which precludes the exchange of exposure charts see secti on 9 1 Each X ray set therefore requires its own specific exposure chart Even the exchan ge of a similar control panel or another length of cable between control panel and X ray tube can influence the level of energy and its spectrum Usually after exchange of parts or repair the exposure chart for that particular type of X ray set is normalised curve fitting for the new combination of components In practice adjusting the zero point of the expo sure graph is sufficient 35 4 4 Radioactive sources isotopes Natural radioactive sources The elements from this group which have been used for the purposes of industrial radio graphy are radium and mesothorium These give a very hard radiation making them par ticularly suitable for examining very thick objects A disadvantage of natural sources next to their high cost is that it is not possible to make them in dimensions small enough for good quality images and still give sufficient activity Artificial radioactive sources Artificial radioactive sources for NDT are obtained by irradiation in a nuclear reactor Since 1947 it has been possible to produce radioactive isotopes this way in relatively large quan tities and in a reasonably pure state and particularly of sufficiently high concentration the latter being extremely important in NDT because the size of the source has to be
115. ferred technique but should only be applied if the following conditions are met e external diameter D is lt 100 mm in practice 75 mm e wall thickness t is lt 8 mm e weld width lt D 4 The number of exposures is determined by the relation between wall thickness t and diameter D If t D is lt 0 12 two images rotated 90 in relation to each other are sufficient for 100 coverage If t D is equal to or bigger than 0 12 three exposures rotated 60 or 120 in relation to each other i e equally divided over the circumference is considered to be a 100 examination Film Fig 2 18 Elliptical double wall double image technique 195 When using the elliptical exposure technique the images of the weld on the source side and on the film side are shown separately next to each other The distance between two weld images has to be approximately one weld width This requires a certain amount of source offset O relative to the perpendicular through the weld The offset can be calcu lated with the following formula O 12 w f D In which w width of the weld f distance from source to the source side of the object measured perpendicularly D external pipe diameter O Offset distance Perpendicular technique Alternatively the perpendicular technique can be used if the elliptical technique is not practical see fig 3 18 This is the case when for instance pipes of different wall thick nes
116. ffer the possibility of comparing the image obtained with a reference image and of automatic defect interpretation see chapter 16 Selection of the most suitable expensive system is made even more difficult because of the rapid development in sensor and electronic technology Fluoroscopy image intensifiers and magnifiers are more elaborately described in the booklets Die Rontgenprtifung and its translation The X ray Inspection 3 Portable real time equipment A portable version of real time equipment is used to detect external corrosion under thermal insulation It is generally very difficult to detect corrosion on piping with insulation still in place whereas removing and re installing the insulation is a costly and time consuming operation Sometimes the likely presence of corrosion is indicated by moisture water detected in the insulation see section 17 4 External corrosion in a low alloy steel pipe becomes apparent by local swelling of the pipe surface as a result of volume increase of the corrosion layer Figure 10 17 illustrates a system by which the swelling and even severe pitting can be detected On one side a strongly collimated source or X ray tube is located that must be aligned in a way as to direct a narrow beam of radiation along the tangent of the pipe towards a small flat panel detector behind 1 pipe 6 tangent pipe horizon 2 sheet metal AD cladding 7 portable monitor 3 insulation material 8 adjust
117. fy contrast and check the kV value used At the location of the step wedge there must be a minimum specified difference in density compared to the density at a location on the film where penetrated material thickness equals nominal wall thickness Wire type IQI according to EN 462 1 EN 462 1 standardises four wire type IQI s Each 1 FE EN 10 FE EN one is made up of seven i aie i oa equidistant parallel wires of various diameters as shown in figure 1 13 R In the USA IQ s are known 3 as penetrameters Batch number Batch number Fig 1 13 Wire type IQIs with different wire diameters 113 Table 1 13 shows the wire combinations for the four IQI s according to EN 462 01 The diameters of the wires are given in table 2 13 i Wire numbers Wire diameter from to mm 1 to 7 inclusive 3 2 to 0 80 inclusive 6 to 12 inclusive 1 to 0 25 inclusive 0 40 to 0 10 inclusive 0 2 to 0 05 inclusive 10 to 16 inclusive 13 to 19 inclusive Table 1 13 Wire IQIs according to EN 462 01 EN type IQI s are manufactured with wires of steel aluminium titanium or copper depending on the type of material to be examined On each IQI the wire material is indi cated Fe for steel Al for aluminium Ti for titanium and Cu for copper Diameter mm 0 63 0 50 0 40 Diameter mm 0 063 0 05 Wire no 13 3 List of common IQ s Figure 2 13 shows the five most common BS 397
118. ger periods of time require the same ambient conditions as new unexposed films i e e ambient temperatures below 24 C e relative humidity of less than 60 e preferably stacked on edge anode source density across film Fig 1 11 Geometric unsharpness U g The source diameter S is shown very large for clarity 96 Defect discernibility and image quality Three factors govern the discernibility of defects in a radiograph 1 Geometrical effects e Size of the source e Source to object distance Defect to film distance 2 Film properties governing image quality e Graininess e Contrast Fog e Inherent unsharpness 3 Quality of radiation applied 11 1 Unsharpness Geometric unsharpness X ray tubes and radioactive sources always produce radiographs with a certain amount of blurring the geometric unsharpness U in fig 1 11 because of the finite dimen sions of the focal spot or source size The magnitude of this unsharpness U is given in the following equation a U 5 F a In which s is the effective focus or source size F is the focus to film or source to film distance a is the defect to film distance The maximum value of U related to a defect situated at a maximum distance from the film and for which a t can be calculated from the formula S t U max T Inwhich t the thickness of the object Consequently U can be reduced to any required
119. ght portable linacs of 3 MeV capacity can have outputs of 1 5 Sv minute at 1 metre distance 43 magnetron vacuum pump focus coils electron gun wave guide Fig 6 5 Linear electron accelerator linac The main properties of a linear accelerator are 1 very high output of radiation 2 very small focal spot dimensions lt 2 mm 3 considerable weight approx 1200 kg for an 8 MeV stationary installation Figure 7 5 shows an 8 MeV linac in a radiation bunker examining a pump housing Fig 7 5 Linac and pump house in a radiation bunker 44 5 4 Radioactive sources Table 1 5 shows various radioactive sources for industrial NDT The most commonly used ones are Cobalt Iridium and increasingly Selenium Selenium is very attractive while it permits lighter containers than Iridium Due to its average energy level it often is a good alternative for an X ray tube also attractive while no electricity is needed Element Mass Specific gamma Average energy Number constant level k factor in MeV Cobalt60 0 35 1 25 Caesium137 Iridium192 Table 1 5 Radioactive sources used in industrial radiography in sequence of nominal average energy level Average energy level nominal value The spectrum of a source has one or more energy lines as shown in figure 2 2 For sources with multiple energy lines an average energy level is assumed the so called nominal value Source Number of Main energy No
120. ght unit expressed in Bq g Specific gamma ray emission factor k factor The k factor is the generally used unit for radiation output of a source and is defined as the activity measured at a fixed distance It indicates the specific gamma emission gamma constant measured at 1 metre distance The higher the k factor the smaller the source can be for a particular source strength A source of small dimensions will improve the sharpness of a radiograph Table 1 4 shows the various k factors and half life values Isotope Half life Specific gamma constant or k factor Ytterbium169 31 days 0 05 Iridium192 74 days Selenium75 120 days Cobalt60 5 3 years Caesium137 30 years Table 1 4 Various k factors and half life values Half life of a radioactive source Of an Iridium192 source with an activity of 40 GBq for example 10 GBq will remain after two half lives 148 days 5 GBq after three half lives 222 days etc 37 Directional X ray beam Fig 1 5 X ray tubes A position of target 1a 5 Bipolar tube Directional X ray beam 1b 5 Unipolar tube l l l l Focus l l Directiona X ray beam Panoramic X ray beam 38 E NDT equipment 5 1 X ray equipment X ray sets are generally divided in three voltage categories namely 1 Up to 320 kV mainly for use on intermittent ambulatory work Tubes are generally of the unipolar alternating current type Higher voltages are hardly possible
121. he density curve Average gradient Effect of developing conditions on the density curve Film speed sensitivity Graininess Film types and storage of films The Agfa assortment of film types Film type selection Film sizes Handling and storage of unexposed films Exposure chart Exposure chart parameters Type of X ray equipment The radioactive source Source to film distance Intensifying screens Type of film Density Developing process Densitometer Producing an exposure chart for X rays The exposure chart Use of the exposure chart Relative exposure factors Absolute exposure times Use of the characteristic density curve with an exposure chart Processing and storage of X ray films The darkroom Entrance and colour Darkroom lighting Darkroom layout Tanks Chemicals and film development Making up processing solutions Developer Fixer Developing time and bath temperatures Film agitation Replenishing 65 65 67 67 70 70 70 73 73 79 79 78 78 80 80 81 85 85 86 Stopbath Fixing Final wash Drying in the drying cabinet Roller dryers 10 3 Recommendations for the darkroom 90 10 4 Silver recovery 90 10 5 Automatic film processing 91 NDT U universal film processor NDT E economy film processor 10 6 Checking the developing process and film archiving properties 93 PMC strips to check the developing process Thiosulphate test to check the film archival properties 10 7 Storage of expose
122. hicknesses derived from ref 2 9 8 Use of the characteristic density curve with an exposure chart In the following examples the tube voltage and focus to film distance FFD are assumed to be constant and automatic development is for 8 minutes in G135 developer at 28 C Example 1 Effect of the thickness of the ceeds ni object on the density of the radio amp graphic image It is required to radiograph on D7 film a steel object comprising two sections of different thick Gs ness of 12 mm and 15 mm A The exposure chart figure 7 9 shows that at 160 kV and an B FFD of 70 cm using 10mA min a density of 2 behind the section measuring 15 mm in thickness will be obtained FLM D7 Question What image density Fig 7 9 Exposure chart for D7 Material steel density 2 ffd 70 cm will be obtained behind the Sec screens 0 02 mm lead tion measuring 12 mm under Automatic processing with developer G135 at 28 C i i 8 min cycle these given conditions Method and answer The exposure chart fig 7 9 shows that under the conditions mentioned above density D 2 is obtained on D7 film through the 15 mm thick section using an exposure of 10 mA min point A on the chart Under the same conditions the 12 mm section would require an exposure of 5 mA min point B in the chart which means an expo sure ratio of 10 5 The exposure through the 12 mm section is two
123. higher voltages and currents Replacement of target or filament once damaged requires less than half an hour avery acceptable down time Using micro or nanofocus X ray tubes has the following advantages e Very small defects are discernible Low backscatter because a small part of the object is being irradiated e High resolution Disadvantages are e Costly if separate high vacuum equipment is required Time consuming as for each high resolution exposure only a small part of the object is being irradiated Tube heads There are two types of tube heads for small focus X ray tubes Figure 2 17 illustrates the two types The transmission tube provides the highest magnification smallest focus The directional tube as common in standard X ray tubes provides the highest energy This figure also shows the magnetic lenses that create the essential focussing of the electron beam Magnetic lens Target Fig 2 17 Transmission and directional small focus tube heads 179 System set up Figure 3 17 shows the concept of a two dimensional 2D X ray microscopy system to inspect small components consisting of a micro or nanofocus X ray tube an XY Z manipulator and detector The manipulator can be joystick or CNC controlled Full automation is possible The geometric magnification can be controlled by the Z axis Closer to the tube results in a larger magnification factor X ray tube Component Manipulator Detect
124. hort for Processing Monitoring Control The purpose is to demonstrate conformance with the standard film system as described in the standards ISO 11699 or EN 584 e demonstrate the consistency of the development system e monitor and promote uniformity of the various development systems in different locations e initiate timely corrective action if deviations occur PMC strips are film strips that have been pre exposed in a regular step pattern by the supplier under special conditions and within narrow tolerances but have not as yet been developed They are supplied with a certificate of compliance with EN 584 2 and ISO 11699 2 In the development system to be checked a PMC strip is processed routinely in a way identical to a normal radiograph Finally the various densities are measured with a densitometer 93 Do Unexposed area for the Thio Test fog base density Fig 2 10 PMC strip with an unexposed area for the Thio Test Reference step for film contrast Reference step for film sensitivity A PMC strip as shown in figure 2 10 has to be used whenever the chemicals in an auto matic or manual processing system are replenished or changed It is also advisable to use a PMC strip regularly but at least once a month for a routine check of the development system A calibrated densitometer measures the following steps Dp fog and base density lt 0 3 D density of step 3 D density of
125. hown in figure 8 17 can be used In that cas the X ray shadow image is still converted by a scintillator foil to visible light which is then directly detected by the photo diode array This option is more expensive than the tra ditional image intensifier of figure 7 17 Digital flat panel detectors provide better images with far superior contrast resolutions of 0 5 compared to 2 of image intensifiers This can be a decisive factor for low contrast objects and for high quality computed tomography CT see section 17 3 17 2 Fluoroscopy real time image intensifiers Fluoroscopy also known as radioscopy is a technique whereby real time detection of defects is achieved by the use of specialised fluorescent screen technology At present there are many alternatives to photographic film for making an X ray image visible In addition to the CR and DR techniques described in chapter 16 a wide range of real time image forming techniques using display monitors are available It can generally be said that the image quality of conventional X ray film is superior to either true digital direct radiography DR or computer aided radiography CR Therefore these new techniques cannot be considered acceptable alternatives at all times However when the installation is adjusted to optimal refinement for a single application for example weld inspection in a pipe mill a filmequivalent image quality can be obtained which would only just comply w
126. i i a p i S a 2 w w w w a e a Pn Tei E o FP Bowe rati a bon zri F ae Fe T Arigi iJ p ama Fep EREN i W ae el i In cases of stationary DR systems in use for large production quantities so called Assisted or Automated Defect Recognition ADR programs software algorithms can be applied with no human interference to speed up uniform interpretation of images Apart from the original image and its imprinted exposure parameters on a true copy comments and display characteristics e g zoom contrast filters can be superimposed and archived as well This enables inspection professionals to streamline the process and improve the quality of distributed inspection information Figure 38 16 shows a screen peme shot made of the worksttions E o lt amp amp aa 4fa4 display The screen shows wri the results of an on stream aii Ga exposure on a CR plate taken of a valve with con uae A necting pipe The screen shot includes one of the selectable frames of the report module The image itself shows marks white lines super imposed by the workstation s operator to establish the remaining im co een ne wall thickness at those places Fig 38 16 On stream image and report of a valve to be calculated by the soft ware J6 9 S38 O 450 oe gee ee date e a 173 Figure 39 16 shows a detail of the selected pipe wall area with the reported results Wall Thickness
127. ide crystals in the emulsion These crystals carry traces of metallic silver and in doing so form the latent image Several chemical substances can reduce the exposed silver halides to metallic silver these are called developing agents 7 3 Characteristics of the X ray film The use of X ray film and the definition of its characteristics call for an adequate knowled ge of sensitometry This is the science which studies the photographic properties of a film and the methods enabling these to be measured The density or blackness of the photographic layer after development under closely def ined conditions depends on exposure By exposure is meant a combination of radiation dose striking the emulsion that is to say intensity symbol I and the exposure time sym bol t In sensitometry the relationship between exposure and density 1 t is shown in the so called characteristic curve or density curve Density optical When a photographic film is placed on an illuminated screen for viewing it will be obser ved that the image is made up of areas of differing brightness dependent on the local opti cal densities amount of silver particles of the developed emulsion Density D is defined as the logarithm to base 10 of the ratio of the incident light I and the transmitted light through the film It therefore D log I Iy Density is measured by a densitometer see section 9 2 Industrial radiography on conventional film co
128. ilable for many decades is the one in which ima ges are formed directly with the aid of once computerless radiation detectors in combi nation with monitor screens visual display units VDU s see chapter 17 This is in fact is an early version of DR These through transmission scanning techniques known as fluoroscopy the storage of images and image enhancement are continually improved by the gradual implementation of computer technology Nowadays there is no longer a clear division between conven tional fluoroscopy with the aid of computers and the entirely computer aided DR In time DR will to some extent replace conventional fluoroscopy Summarising the image of radiation intensities transmitted through the component can be recorded on The conventional X ray film with chemical development the wet process or one of the following dry processes A film with memory phosphors and a work station for digital radiography called computer assisted radiography or CR e Flat panel and flat bed detectors and a computer work station for direct radiography called DR A phosphorescent or fluorescent screen or similar radiation sensitive medium and a closed circuit television CCTV camera as in conventional fluoroscopy an early version of direct radiography 15 e By means of radiation detectors e g crystals photodiodes or semiconductors in a linear array by which in a series of measurements an image is built u
129. in extent combine the advantages of both These screens are provided with a lead foil between the film base and the fluorescent layer This type of screen is intended to be used in combination with so called RCF film Rapid Cycle Film of the types F6 or F8 see section 8 1 The degree of intensification achieved largely depends on the spectral sensitivity of the X ray film for the light emitted by the screens Due to the considerable exposure time reduc tion the application is attractive for work on lay barges and in refineries 55 To achieve satisfactory radiographs with fluorometallic screens they should be used in combination with the appropriate F film type When used correctly and under favourable conditions exposure time can be reduced by a factor 5 to 10 compared with D7 film in combination with lead screens This is not a con stant factor because the energy level applied radiation hardness and ambient temperatu re also affects the extent of fluorescence For example at 200 kV a factor 10 can be achie ved but with Iridium192 nominal value 450 kV it will only be a factor 5 compared to D7 film Table 1 6 shows the relative exposure factors for the RCF technique Film system Relative exposure time 200 kV Ir192 450 kV F6 RCF screens 0 1 0 2 1 0 1 0 D7 lead screens Table 1 6 Relative exposure factors for RCF technique A total processing cycle of a few minutes is possible with the use of an automatic fil
130. initial setting for the work period amg Vet 7 a Mie n e HY Eee DIE ei Fig 2 19 Pendosis meter Any radiation will cause the charge to leak away through its ionising effect and the fibre will move across the scale The amount of radiation received can be read off the calibrated scale This type of instrument is excellent for personal protection as it is small inexpensive and reasonably robust It can be easily read and records the total amount of radiation received for the work period with an accuracy of 10 Thermoluminescent dose meter TLD badge The TLD meter consists of an aluminium plate with circular apertures Two of these contain luminescent crystals Figure 3 19 shows an open TLD meter and the plate with crystals next to it The right side of the illustration shows the same meter now closed When the meter is read only one crystal is used to determine the monthly dose The other one is spare and if necessary can be read to determine the cumulative dose The TLD meter is sensitive to X and gamma radiation of 30 keV and higher The dose measuring range is large and runs from 0 04 mSv to 100 mSv with an accuracy of 5 The instrument measures 60 x 40 x 10 mm and is conve Fig 3 19 Open TLD meter Closed TLD meter nient to wear MESCAVE tte At 211 Film dose meter film badge The film badge consists of two pieces of X ray film con tained with filters in a special holder At the end of a
131. inium casting with coarse porosity Exposure on D7 film at 60 kV 5 mA 15 sec film focus distance 100 cm 140 141 Fig 15 11 Radiograph of 25mm thick aluminium copper alloy casting with gas porosity Exposure on D7 film at 140 kV 5 mA film focus distance 100 cm 142 Examination of assembled objects In addition to radiography for detection of defects in welds and castings it can also be applied to check for proper assembly of finished objects as figures 15 12 and 15 13 illustrate Fig 15 12 Radiograph of transistors Exposure on D2 film with 27 um lead screens at 100 kV 5 mA 2 min _film focus distance 70 cm Fig 15 13 Radiograph of electric detonators taken to check details of assembly 143 16 Digital Radiography DR 16 1 Introduction to DR As in other NDT methods the introduction of microprocessors and computers has brought about significant changes to radiographic examination Chapter 17 describes a number of systems such as Computed Tomography CT radioscopy and X ray microscopy that have been made possible by newly developed technology which involves rapid digital processing of vast quantities of data But as this chapter will show compu ter technology has also entered the field of conventional image forming radiography as applied in industry The driving force was the medical world where digital radiography already earned its cre dits and has become standard technology Along with a few other companie
132. ints are welded together by a circumferential weld a so called girth weld Onshore production rates can be far beyond 100 welds per day dependent on pipe diameter and terrain conditions On lay barges used for production of offshore pipelines more than 300 welds per day 24 hours are no exception According to applicable mandatory norms and specifications these welds have to be inspected either with radiography RT or automated ultrasonic testing systems AUT In the past the inspection was exclusively done by RT in more recent years AUT increasingly replaces RT One attractive feature of AUT is that contrary to traditional RT using film the results are instantly available Nevertheless for many pipelines RT using X rays or sometimes gamma rays is still in use to check weld quality To eliminate development time of the film several attempts in the past to replace traditional RT by RTR real time radiography providing instant results have only resulted in limited success mainly due to lack of image quality In the meantime some of such systems have entered the market and are in use Although no EN standards exist essential to conquer a market share some other standards accept digi tal real time radiography providing one can prove that the required image quality can be achieved see chapter 16 Attempts continue to develop better RTR systems than already exist Development of radiation sensitive sensor technology is still in f
133. ion dose is specified as the product of radiation intensity and exposure duration in mA min intensity x time The exposure chart shows the relationship between the thickness of the object in mm and the exposure value for X ray tubes in kV and mA min for sources in GBq h bis A The exposure chart is applied for eh ALE Be fi NALS ant Le a fae 1 a given density for example 2 or 2 5 2 agiven film screen combination for example D7 with lead screens X ray photograph of a Van Gogh painting presumably a self portrait on canvas 3 a given type of material for example steel X rays are made to prove a paintings authenticity check the condition of the canvas material or determine possible paint overs Cadmium and or lead in the paint absorb radiation thus forming an image The chart depends amongst others on Applied X ray process data 30 kV 10 mA min Agfa D4 without lead screens and a source to film l distance of 100 cm 4 type of X ray equipment or radioactive source 5 source to film distance usually 800 mm 6 development conditions for example automatic 8 minutes at 28 C 72 73 Type of X ray equipment Among the factors to be taken into account are the voltage in kV whether alternating or direct current the limits of voltage adjustment and the current through the tube in mA It follows that the exposure chart is unique for a particular X ray set The radioactive source Radiation intensity a
134. ion of circumferential welds so called girth welds during construction of pipe lines either cross country or on lay barges see section 16 11 For such systems CMOS type linear arrays are in use because of their effi ciency fast response and erase properties lt 0 2 msec and last but not least their robust ness an essential requirement for application under adverse field conditions Linear or curvilinear arrays are also commonly used in CT applications Direct linear arrays using CdTe Cadmium Telluride and other semiconductor materials are now available but most commonly linear arrays are of the indirect type with a scin tillator material to convert incident X rays into visible light and crystalline silicon photo diodes measuring the light These analogue signals are subsequently digitised and con verted into grey levels 154 2D detectors The simplest type of DR detector used in NDT is a two dimensional array of detection pixels to measure incident X ray intensity to directly create a radiographic image without the need for any motion of the component Small 2D detectors typically use a photo detector array made from a crystalline silicon integrated circuit optically mated to a powdered scintillator screen Both Charge Coupled Devices CCD s and Complimentary Metal Oxide Semiconductor CMOS devices are used Typically the screens use a powdered Gadolinium OxySulfide GOS material to convert the incident X
135. ith the requirements This would possibly require the use of a microfocus tube see section 5 1 182 Stationary real time installations Display monitor systems as illustrated in figure 9 17 are almost exclusively used in stationary set ups for production line testing of varying types of objects in particular in metal casting plants pipe mills and component assembly industries Often they provide some image magnification and software features to improve defect detectability Sometimes real time systems are utilised in the food industry to check for instance for the presence of glass fragments or other foreign objects Being part of a production line and due to the necessary radiation safety provisions such as cabins these systems can be very expensive The display monitors are located at a safe distance digital digital signal signal object flat panel detector image processing i Os ae system and image intensifier archiving X ray display monitor tube Fig 9 17 Radiography with image magnification The choice of a radiographic system to be used for a specific application depends on a number of factors e Hardness of the radiation required and appropriate detector e Resolution or detail discernibility required The type of defects to be detected in mass production is normally known Magnification factor required when it concerns small defects e Image dynamics density range with regard to object thickness range
136. le an objective method to indicate resolution is needed Resolution is dependent on contrast grey levels and separation distance Resolution is expressed as the number of line pairs black and white that can be distin guished in one mm see figure 20 16 160 Exposure parameters The quality of a digital image is affected by a number of factors The final image cannot be better than the quality of the X ray information arriving at the detector Just as with con ventional film radiography this inherent loss of information is determined by a variety of parameters These are the X ray spectrum kV filters and screens the part to be inspec ted thickness material and exposure conditions focal distance backscatter exposure time The overall effect is visible as loss of contrast and sharpnes of the final image on the screen of the workstation Some optimisation is possible for digital systems the majority of measures quite similar to those appropriate for good conventional film techniques MTF Modulation Transfer Function No imaging system is perfect All imaging systems record their inputs imperfectly One obvi ous shortcoming of paramount importance for radiography is a reduction in sharpness by imperfect contrast transmission throughout the total imaging chain The scientific method to quantify performance fidelity to transfer contrast information is characterised by the Modulation Transfer Function MTF for sho
137. le 3 2 shows the values for lead Element Isotope Symbol Average energy level Half value thickness in MeV in mm lead Ceasium137 Cobalt60 Iridium192 Selenium75 Ytterbium169 Thulium170 Table 3 2 Half value thickness for lead For a heterogeneous beam the HVT is not constant the second HVT is slightly larger than the first In general in industry where relatively hard radiation is used a fixed average HVT is applied Units and definitions 3 1 Units Until 1978 the International Commission of Radiation Units and Measurements ICRU used the conventional radiation units of roentgen R rad rd and curie Ci Since 1978 the ICRU has recommended the use of the international system units SI with special new units for radiation quantities the Becquerel Gray and Sievert Table 1 3 shows the relationships of these new units to the older units SI units Formerly used Conversion Designation of quantity nit Unit Old to SI _ spi tontin CO Cig R ntgen TR 2 58x104C kg lonisation dose rate cn C C ke s T rma energy a J kg Rad 1 Rad 0 01 Gy S ve dose H a J kg 1 Rem 0 01 Sv Table 1 3 Overview of new and old units disintegrations per second C Coulomb A s J Joule RBE Relative Biological Effect A Amp re In radiography and radiation safety units are preceded by prefixes Table 2 3 shows the ones mostly used Meaning Value
138. le so called application processes which describe how to apply a technique in a defined application Such procedures are often part of the formal contract between parties and are essential to achieve uniformity To speed up the instruction of a new NDT method or technique industry sometimes takes initiatives through JIP s Joint Industry Projects to develop procedures or recommended practices in order to obtain the quality and uniformity of results industry requires For example HOIS an international working group with members active in the offshore industry is working on an improved procedure for the application of CR Such documents indicate the require ments for NDT education level CR application training and image interpretation The results of such efforts are often partly or in their entirety implemented in documents issued by international standards issuing organisations Standards for weld inspection No EN standards exist 2008 for weld inspection However other standards are formulated in such a manner that digital radiography can be an alternative For example ISO 3183 2007 permits other means as formulated below E 4 2 1 The homogeneity of weld seams examined by radiographic methods shall be determined by means of X rays directed through the weld material in order to create a suitable image on a radiogra phic film or another X ray imaging system provided that the required sensitivity is demonstrated 166 Also API 1104
139. ls CMOS has a lower energy consumption and the effect of tempera ture is less than for amorphous silicon This in industry is an important feature because it requires less frequent recalibration for systems using CMOS detectors versus unregulated amorphous silicon devices For amorphous materials with every 5 C to 10 C of tempera ture variation a recalibration is recommended CMOS has a wider tolerance of up to 40 C With CMOS there is no risk of saturation causing blooming and edge burn out In addi tion they show no ghost memory effect thus no latent images Another fact is that the Fill Factor active portion of the detector of CMOS is better than amorphous sili con see figure 14 16 Similar to amorphous silicon CMOS is suitable for an energy range from 20 kV to several MeV CMOS even to 15 MeV Fig 16 16 Flat bed LDA scanner panel courtesy Envision So far no true large flat panels using CMOS detectors exist the maximum size known at present measures 100 x 100 mm with very small pixels sizes of approximately 50 microns and 200 x 300 mm with pixel sizes of 100 microns To nevertheless make use of the tech nical advantages a clever design provides the solution to mimic a large flat panel Figure 16 16 shows such a virtual panel detector in fact it is a flat bed scanner in which a fast moving linear detector array LDA is applied Such panels exist in many formats up to 600 x 1200 mm and can even be custom ma
140. lustrated in figure 25 16 Image A shows a 1 bit 2 2 level image and hardly contains any information Image B shows a 2 bits 2 image equal to density 4 grey levels with still lots of missing details Image C shows a 12 bit image providing more than sufficient informa tion and showing all details even far beyond what the human eye can distinguish 12 bits Ln as Te Fig 25 16 Depth resolution by number of bits 163 Lateral resolution Lateral resolution is determined by pixel size First of all the pixel size of the detector and secondly the pixel size of the display screen The effect of pixel size The graph also shows that the speed is much higher to achieve the same image quality of D type films Depending on the required image quality a time saving of at least a factor 20 D against E and roughly 200 F against E can be achieved however with poorer quality The range for true real time real instant images shows that exposures can be made with and number of pixels is illustrated in figure 26 16 The same text is displayed in four different resolu tions The text on top shows 6 pixels vertically step by step increasing to 50 pixels for the lowest text For this reason also the hardware of work stations is specified by standards to guarantee best possible image presentation without loss of information as contained in the original digital data Hardware performance should be equal to or Fig better than the re
141. m pro cessor which makes it a very attractive system to deploy offshore on lay barges where weld examination has to be done at a very fast rate and few concessions are made towards image quality Fig 5 6 shows that a time saving at 10 or 10 works out at approxi mately a factor 8 The actual time saving is often closer to factor 10 4 F6 RCF screen f D8 lead D7 lead D7 lead F8 NDT 1200 2 l I 2 2 5 3 log rel bel log rel bel Fig 5 6 Relative exposure time RCF and lead Fig 6 6 Speed comparison F8 film NDT1200 intensifying screen for 300kV and RCF versus D7 and D8 lead for 200kV These RCF screens are also used for on stream examination also known as profile radio graphy see section 18 6 whereby long exposure times and mostly hard gamma radia tion are applied because of the penetrating power required However the relatively long exposure time causing reciprocity and hard radiation Cobalt60 together considerably reduce the light emission effect as tables 1 6 and 2 6 show 56 On balance for on stream inspection the relative time saving is much smaller usually no more than a factor 2 for an F6 film at Ir192 and Co60 instead of 10 in the D7 lead screen technique See the bold figures 2 5 and 1 7 in table 2 6 Figure 6 6 gives an overview of graphs from which the relative exposur
142. mally be used For small component inspection where the image might be viewed under magnification to reveal small details a film of class C2 or possibly even a single emulsion film of class C1 would be desirable In megavoltage radiography because most equipment have a very high radiation output class C3 films can be used for objects of great wall thickness This has the advantage that a high film gradient can be achieved 8 3 Film sizes Film sizes in industrial radiography are to a large extent standardised according to ISO 5565 Non standard sizes are possible Standard film sizes and metal screens are supplied separately but can also be supplied vacuum packed so that the risk of film faults is considerably reduced For weld inspection there is so called Rollpac film strip on the market which is available on a roll together with the lead screen For very large projects the strip film can even be pre cut to suit a particular weld length or pipe Vvessel circumference 8 4 Handling and storage of unexposed films The conditions under which unexposed films are handled and stored play a very important role in the final quality of the exposed film Recommendations for handling and storage are contained in for example ASTM E1254 Pre exposure as a result of background radi ation must be avoided as it causes unacceptable fogging of the film If films are to be kept for a longer period the following storage conditions must be adhered t
143. minal value spectrum lines levels in MeV in MeV Cobalt60 2 1 17 and 1 34 MeV 1 25 MeV Caesium137 1 0 66 MeV 0 66 MeV Iridium192 gt 10 0 3 0 31 0 32 0 47 en 0 6 MeV 0 45 MeV Selenium75 gt 4 120 140 and 400 keV 320 keV Ytterbium169 0 06 and 0 2 MeV 200 keV Thulium170 52 and 84 keV 72 keV Table 2 5 Radiation spectra and nominal values a aa On the basis of these spectra data it is clear that Co60 Cs137 and Ir192 sources produce high energy radiation and are the refore well suited to irradiate thick materials Yb169 on the other hand is a source that produces relatively soft radiation and is of a very small size 0 5 mm which makes it particularly suitable for radiographic examination of circum ferential welds in pipes of a small diameter and thin wall thick i P Fig 8 5 Ytterbium169 source in ness with the source centrally positioned so that the weld can a position for exposure of be exposed uniformly in one exposure as shown in figure 8 5 urjerential weld 45 5 5 Source holders capsules All gamma ray sources for radiography are supplied in hermetically sealed corrosion resistant source holders capsules made out of monel vanadium or titanium The Atomic Energy Authority in the country of origin encapsulates the radioactive materi al The supplier will supply the source with a certificate which indicates the type of source its serial number the activity at a certain date
144. n energy used 13 5 IQI sensitivity values It is important to realise that any IQI acceptance value must be based on a particular type of IQI and the thickness of the object being examined When IQI sensitivity is expressed in a percentage of object thickness a lower recorded value indicates a higher radiographic sensitivity hence better image quality 117 118 Film exposure and handling errors Before a particular difference in density in a radiograph is attributed to a defect in the object examined it must be sure that it is not the result of incorrect handling or proces sing of the film It is therefore essential to be able to recognise such faults when exami ning the film in order to prevent their recurrence It is often possible to identify faults due to wrong processing by looking obliquely at the surface of the film while facing towards the light and comparing the two emulsion surfaces The X ray image usually is identical on both sides of the film while a fault in processing will frequently affect only one surfa ce and can be seen as a change in reflection on the surface The most common faults and their possible causes are listed below Insufficient contrast a with normal density 1 radiation too hard 2 over exposure compensated by reduced developing time 3 unsuitable or wrongly mixed developer 4 prolonged development in too cold a developing bath b with insufficient density 1 insufficient dev
145. n bodies for example metal particles between film and screen during exposure 9 small clear hollow spots usually with dark edges may occur when the emulsion has been subjected to local attack of bacteria This is generally the result of slow drying in a warm damp climate particularly if there are impurities in the wash water Clear lines or streaks 1 the film envelope has been scored with a pointed object before exposure 2 film insufficiently moved during development 3 uneven drying film has been carelessly wiped dry after washing 4 drops of fixer or stopbath have fallen on the emulsion before development Clear shapes 1 clear crescent shapes may appear when before exposure the film has been bent between two fingers 2 fingerprints may occur when the film has been touched with dirty fingers contaminated for example with grease fixer stopbath or acid Dark patches 1 drops of developer have fallen onto the film before development 2 drops of water have fallen onto the film before development 3 electrical discharge marks especially at low relative humidity of the air 4 marks from mechanical damage to the emulsion after exposure Dark lines or streaks 1 the emulsion has been scratched after exposure 2 the film envelope containing the film has been scored or written on with a pointed object after exposure 3 insufficient agitation of the film during development 4 uneven drying 5 water or developer has trickl
146. n with the CR film scanner of which the schematic principle is shown in figure 3 16 Contrary to the CR film scanner which mea sures reflected stimulated light the density measurement in the film digitiser takes place in transmission mode using a scanning light beam synchronised with a light detector Usually the density blackening blackness of a film is digitised in 12 bits equal to 4096 steps or grey levels For convenience these over 4000 levels are divided by 1000 resulting in relative digital density values from 0 to 4 This provides a mean comparison with traditional film density values Fig 1 16 Desk top film digitiser GE Inspection Technologies supplies film digitisers made by Agfa A desk top version is shown in figure 1 16 In these scanners films with a maximum width of 350 mm can be digitised in a single run Even for the smallest laser beam spot size of 50 um approximately 4 mm of film in length can be scanned per second so for the largest standard film size 350 x 430 mm this process would take approximately 2 minutes to complete Scanners exist without length limitation of film and adapters exist for digitisation of roll stripe films Apart from greatly reduced storage space and almost deterioration free archiving digitising also makes it possible to re analyse the film s images on a computer screen see work station in figure 33 16 with the possibility of electronic image adjustment enhan cemen
147. nd half life time of the source have to be taken into account Source to film distance The exposure chart for an X ray set is produced for a specified source to film distance If another distance is used corrections will be necessary using the inverse square law Intensifying screens When drawing up the exposure chart intensifying screens used must be recorded and the same type of screens used again when making radiographs Type of film The type of film must be indicated on the exposure chart since the various types of industrial X ray films are substantially different in sensitivity speed Density An exposure chart must be as accurate as possible Densities indicated are to be measured by a densitometer see section 9 2 The radiographs that form the basis for the chart must have been made under controlled and reproducible conditions whereby quality monito ring tools such as PMC strips as described in section 10 6 are used Developing process Developer formula processing temperature and developing time all influence the final result The exposure chart produced will be related to a particular well defined developing process 74 9 2 Densitometer A densitometer is used to accurately measure the photographic optical density at any spot on a radiographic film For most types of densitometers the size of the measured area is approx 1 mm The measuring range runs from density 0 to 4 Since it is a logarithmic scale this eq
148. nd screens are available in a wide variety of sizes and packings For example as sepa rate items to create a specific combination of film and screen or fully prepared for the job in daylight packing including lead screens and evacuated Vacupac to guarantee the best possible contact between film and screens Films with screens are available on a roll Rollpac to cut suitable lengths or even precut at a specified length for large jobs which require large numbers of identical film lengths e g for girth weld inspection of long distance pipelines Last but not least films can be made in sizes on customer demand 67 Figure 3 8 shows graphs of relative exposure time versus density for the entire Agfa D film range For density 2 the difference between a D8 and a D2 film is a factor 14 108 at 200 kV gt Fig 3 8 Density graphs of the Fa Agfa film range D2 through to D8 z with lead intensifying screens at 200 kV and automatic film development Re 3 0 Filmtype D gt 4 5 B f Ti F Aa ees 2 0 l l l l ks l l l l l 1 0 l l l l l 0 5 l l l l I 1 0 1 5 2 0 2 5 3 0 35 log rel exp Note Developing process for figure 3 8 above Automatic 8 min cycle 100 seconds immersion time in developer G135 at 28 C 68 Part of the Agfa film range with relative exposure factors and code classification has been listed in table 1 8 for various radiation intensities Relative exposure factors Film system Class 100 k
149. nd the expertise of preparation and use of radiation sources have been laid down in regu lations The purpose of practical protection against radiation is to prevent any individual receiving a harmful dose 208 As there is considered to be no totally safe lower limit below which no damage would be sustained the ALARA concept is being promoted ALARA short for As Low As Reasonably Achievable aims to achieve the lowest possible radiation dose whereby economic and soci al factors are considered The protection from unwanted external irradiation is based on three principles e Speed by working fast the exposure duration is reduced e Distance the greater the distance the lower the rate of exposure remember the inverse square law e Shielding and collimating materials with high radiation absorbing properties such as lead reduce the exposure rate to a level that can be calculated in advance Table 3 19 in section 19 8 shows the half value thickness of lead for various gamma sources 19 5 Permissible cumulative dose of radiation Although the subject of permissible cumulative dose of radiation is complex the values given below apply to external irradiation of the whole body The values have been established by the ICRP e Radiology workers category A 20 mSv year e Radiology workers category B 5 mSv year Public not being radiology workers 1mSv year The whole body level of 20 mSv per year is normally interpolat
150. ne Two step digital radiography CR is a two step process The image is not formed directly but through an intermediate phase as is the case with conventional X ray films The image information is elsewhere and later converted into light in the CR scanner by laser stimulation and only then transformed into a digital image Instead of storing the latent image in silver halide crystals and developing it chemically as happens with film the latent image with CR is stored the inter mediate semi stable phase in a radiation sensitive photo stimulable phosphor layer This phosphor layer consists of a mix of bonded fine grains of Fluor Barium and Bromium doped with Europium The CR imaging plate The phosphor layer has been applied to a flexible carrier and been provided with a protec tive coating An additional laminate layer mainly determines the mechanical properties such as flexibility CR imaging plates are not as flexible as X ray films Such plates can be used in a temperature range from 5 C to 30 C Figure 2 16 shows the layered struc ture of this type of plate which is generally called an imaging plate or sometimes wrongly imaging screen Note Screens in the world of NDT made of lead or another metal are used to intensify the effect of incident radiation or to reduce the effect of scattered radiation Protective coating Storage phosphor layer thickness approx 200 micron Flexible PET carrier base Laminate
151. nents when optical magnification is applied to allow very fine details to be observed D3 also exists as D3 s c single coating as alternative for D2 and is extremely suitable for optical enlarge ments in case of very small components which require large magnification factors of the image Moreover this type of film is suitable for neutrography see section 17 5 D8 is used for the examination of big castings and steel reinforced concrete D10 film is also produced for exposure monitoring purposes see section 19 6 Figures 1 8 and 2 8 show the rela tionship between film speed and image quality and film contrast respectively In addition to these graphs figure 27 16 gives a graphical representation of relative image quality as a function of relative dose and exposure time film speed for D films and computer assisted CR and DR techniques Agfa has developed special intensifying screens specifically for use in combination with F6 and F8 films see section 6 3 These so called rapid cycle film screens are usually referred to as RCF screens indicated as Agfa NDT1200 F8 has the highest film speed Depending on quality requirements F6 is mostly used for weld inspection on lay barges and on stream application profile radiography since it shortens examination time by a factor 10 see section 6 3 This combination can also be used for hand held flash radiography to enable lowest possible dose to make a quick but nevertheless suitable image Film a
152. netrameter must be visible on the radiograph The thickness of the penetrameter in relation to the specimen thickness defines the contrast sensitivity The size of the smallest hole visible defines the detail sensitivity Equivalent Equivalent 4 ra 2 1T 1 4 4 2T 4 0 Table 3 13 ASTM Equivalent image quality indicator MIL STD IQI s military standards In the past for some applications specific MIL standard MIL STD IQI s should be used only They are very similar to ASTM IQI s Nowadays the MIL STD accepts that they are replaced by the almost identical ASTM type IQI s providing they meet the requirements specified in the MIL STD Nevertheless MIL STD IQI s are still available and in use 115 AFNOR IQI s The AFNOR type IQI s originate in France They consist of metal step wedges of the same material as the object to be examined The thick ness of the steps increases in arith metical progression Each step has one or more holes with a diameter equal to the thickness of that step There are various models of step wedges The most common types Fig 4 13 French AFNOR IQIs are rectangular with square steps measuring 15x15 mm and hexagonal with triangular steps measuring 14mm See figure 4 13 Steps thinner than 0 8 mm have two holes of the same diameter For a step to be regarded as visible all the holes in that particular step must be clearly seen on the film The French standard AFNOR
153. ng step wise image distortion 161 DQE Defective Quantum Efficiency Just as it is more difficult to discern fine detail when an object is dimly lit it is also more difficult to observe defect indications in a noisy X ray image with limited detected dose The effects of contrast and noise on defect recognition is illustrated in figure 23 16 Thus in addition to the two factors exposu re parameters and MTF already mentioned there is an additional loss of image quality if some of the X rays are not absorbed during the primary detection process determined by Decreasing noise the ability of the detector to accurately trans Fig 23 16 Defect recognition versus contrast and noise fer the information present in the incident X rays MTF does not take into account the inhe rent noise resulting from the X ray dose available for an image or noise added by the detec tion system The measure that combines MTF with detection efficiency and noise conside rations is known as Detective Quantum Efficiency or DQE for short Increasing contrast In mathematical terms DQE can best be thought of as the square of the signal to noise ratio SNRout of the X ray contrast measured by the detector divided by the square of the contrast SNR incident on the detector for each spatial frequency DQE f SNRout 2 SNRj 2 otherwise DQE Image quality Dose So DQE indicates a detector system s ability to accurately represent all of the co
154. not exist at all which hampers the strong potentials of this method to be used A first document ASTM E 2597 07 related to DR hardware has been issued It is intended for use by manufacturers of digital detector arrays to quantify the per formance of such devices This is also of importance for those involved in the selection pur chase of systems This ASTM document includes paragraphs describing the terminology for specification of the condition of pixels e g dead noisy over or under responding bad clusters etc as well as figures on noise contrast sensitivity etc Impact of standards Despite the lack of a full range of supporting documents CR and DR are in use for lots of appli cations that do not require international standards Example are e g in manufacturing plants castings corrosion detection profile or on stream radiography which is a fast growing market and a limited amount of weld inspections based on the ASME or DNV standards that allow CR and DR Certainly CR and DR would already have found more applications if standards would have been available Regardless of the availability of such documents if there is a choice economics will determine which method is selected for a particular application In addition to codes standards norms etc which permit a certain NDT technique plant owners often compile specifications which complement codes and standards with their own requirements NDT service providers often compi
155. ns of a computer and screen workstation without intermediate steps be presented as a coherent radiographic image A cable typically links the detector to the workstation from which the panel is controlled as well Assembled flat panel detector Physical data Dimensions 500 x 600 x 100 mm Weight 10 kg Fig 15 16 DR flat panel component with 400 x 400 mm active area The most common high resolution two step flat panels as illustrated in figure 15 16 use amorphous silicon technology First a scintillator made of structured Caesium Iodide CsI converts incident radiation directly and instantly into light The conversion is proportio nal to the radiation dose Secondly light is converted into a proportional electric signal by thin film transistors TFT s Each pixel contributes to the radiographic image formed on the screen of the workstation Each element is square in effective area with pixel pitch typically ranging from 50 to 400 microns The smaller the pixels the better the resolution but the poorer the imaging efficiency Figure 11 16 illustrates this two step process Reasearch and development is in progress to make sensor elements pixels smaller Depending on overall active area and detector pixel pitch a panel consists of up to several millions of such elements pixels 156 CMOS detectors and flat bed scanners For some applications CMOS detectors are an alternitive for temperature controlled amorphous materia
156. ntrast infor mation present in the incident X ray field as a function of spatial frequency A perfect detec tor will give a DQE of one 100 over all frequencies while a poor detector has a DQE that approaches zero For example assume there are two detectors with different DQE s If the same incident dose is applied the detector with the higher DQE will give a larger SNR and better image qua lity Alternatively the same image quality can be achieved with the other detector as well but requiring a higher dose which translates to increased exposure time or higher tube current In general DQE consolidates many individual parameters resolution efficiency exposure time noise etc into a single parameter describing the overall plate or panel performance Therefore specifying DQE for a detector will also help determine both the final image quali ty and inspection times required for a given application Like MTF DQE can range from 0 0 to 1 0 numbers in practice vary from 0 05 to 0 9 In summary MTF quantifies the maximum possible resolution of the total system but DQE quantifies the actual performance of the detector including its resolution noise and dose efficiency exposure efficiency The DQE function characterises the final image quality ver sus the inspection time required for a given application 162 Remark MTF and DQE are used to characterise detectors and systems Some users may find these scientific notions rather abst
157. ntry t thickness I intensity at material exit e logarithm 2 718 u total absorption coefficient Figure 7 2 shows the resulting radiation intensity logarithmic as a function of increased material thickness for soft and hard homogeneous radiation When radiation is heterogeneous the graphs are not straight see figure 7 2 but slightly curved as in figure 8 2 The slope of the curves becomes gradually shallower because of selective absorption of the softer radiation until it reaches the so called point of homogeneity After passing this point the coefficient of absorption remains virtually unchanged as if the radiation had become homogeneous The position of the point of homogeneity varies with the nature of the material irradi ated The graph shows that with increasing material thickness softer radiation is filte red out more than hard radiation This effect is called hardening 25 intensity Fig 7 2 Intensity of homogeneous radiation as function of increasing thickness _ hard radiation high tube voltage soft radiation low tube voltage penetrated material thickness Fig 8 2 Intensity of heterogeneous radiation as function of increasing thickness hard radiation lt __ _ hard radiation soft radiation points of homogeneity penetrated material thickness 2 8 Filtering hardening All materials for example a metal layer between the radiation s
158. o e background radiation levels below 90 nGray e temperatures below 24 C e relative humidity levels below 60 e away from X ray film chemicals e preferably stacked on edge In the long run minor fogging will occur to films stored Background fog to a maximum density of 0 3 is considered acceptable 70 9 Exposure chart 9 1 Exposure chart parameters Codes for the inspection of welds and castings specify the maximum allowed radiation intensity based on the type of material and the thickness of the object Exposure charts are necessary to etablish the correct exposure value A universal exposure graph or slide rule can be used for radioactive sources as these have a fixed natural radiation spectrum The radiation spectrum of X ray tubes varies with each tube even if they are of the same type This problem is easily solved by using a universal exposure chart for the specific type of tube and then individualise it for each tube the so called curve fitting The adaptation is normally limited to a zero point correction based on a few measured values obtained by trial Sometimes the gradient of the exposure graph needs to be adjusted as well An exposure chart is produced by making a series of radiographs of a step wedge as illustrated in figure 1 9 The radiation intensity level of most X ray equipment is expressed by the amperage of the current through the X ray tube measured in milliamp res mA The exposure radiat
159. of 2 0 above base and fog using a specific processing technique The various relative exposure values are shown in table 1 8 7 6 Graininess When a developed X ray film is viewed in detail on an illuminated screen minute density variations are visible in a grainy sort of structure This visual impression is called graininess and a measurement of this phenomenon establishes a degree of granularity 65 j g D2 D te gt oO D D3 E D4 D5 D7 D8 Fig 1 8 Image quality versus film speed faster The film speed is presented on a linear scale film speed f D2 9 g D3 D4 D5 S D7 D8 w 0 E faster Fig 2 8 Contrast versus film speed film speed 66 Film types and storage of films Industrial X ray films are produced by a limited number of manufacturers in an assortment for use with or without intensifying screens and filters The selection of a particular film type not only depends on economics but in particular on the required often prescribed image quality 8 1 The Agfa assortment of film types The films produced by Agfa are exclusively marketed world wide by GE Inspection Technologies The assortment of industrial standard radiographic film comprises the follo wing types in sequence of increased speed and granularity D2 D3 D4 D5 D7 and D8 complemented with the very fast films F6 and F8 The ultra fine grain D2 film is used in the radiography of very small compo
160. of in film based radiography Certain substances emit so much light when subjected to ionising radiation that they have considerably more effect on the light sensitive film than the direct ionising radiation itself e The term phosphorescence is used to describe the same luminescent phenomenon but once the electromagnetic radiation ceases light fades slowly so called after glow e NDT additionally uses the memory effect of some phosphorous compounds to store a latent radiographic image in order to develop it later into a visible image with the aid of laser stimulation see section 16 4 Fluorescent salt screens Fluorescent screens consist of a thin flexible base coated with a fluorescent layer made up from micro crystals of a suitable metallic salt rare earth usually calcium tungstate which fluoresce when subjected to radiation The radiation makes the screen light up The light intensity is in direct proportion to the radiation intensity With these screens a very high intensification factor of 50 can be achieved which means a significant reduction in expo sure time The image quality however is poor because of increased image unsharpness Fluorescent screens are only used in industrial radiography when a drastic reduction of exposure time in combination with the detection of large defects is required Fluorometallic screens Apart from fluorescent and lead intensifying screens there are fluorometallic screens which to a certa
161. oint determ 3 m J Adjustable ret ee si ines the brightness of the image In this a a graph 16 grey levels on the digital image result in one level on the monitor this can be gt o Black set at 1 to 1 These are a few examples of the white Digitized image nee versatility of the adjustment features provi Grey levels ded by the work station Fig 35 16 Flexibility of image grey values adjustment Figure 36 16 shows the effect on an image by contrast enhancement and sharpening Here the contrast improvement flat Z shape wide density window makes the interior of a valve clearly visible compared to the initial image It proves that the information is present in the ini tial image but has to be adjusted to make it visible for the human eye Original image Contrast improvement Sharpness improvement Fig 36 16 Image enhancement in two steps 172 In addition algorithms have been developed for e g the comparison of parts of an image with conformance criteria carrying out dimensional checks sizing for instance to measure remaining wall thickness see figure 37 16 For this latter function algorithms exist that takes the source to object distance and the nomi nal pipe diameter as a reference to calculate remaining wall thickness or metal loss due to cor rosion Also defect area measurement image statistics and a reporting module are part of the tools of the work station proie number m goup M
162. onfirm this behaviour Furthermore those sensitivi ty speed of CR is five to ten times higher as well compare point A and B at a density of 2 see also figure 27 16 Fig 9 16 Dynamic range of digitised film versus CR plate courtesy BAM Berlin 151 Exposure time and noise In addition to the wide dynamic range the dose sensitivity speed of CR plates is five to ten times higher compare point A and B in figure 8 16 at a density of 2 see also figure 27 16 This allows for shorter exposure times or weaker sources reducing the unsafe radiation area Unfortunately if a source with lower energy is chosen this will result in reduction of the image quality Iridium192 with a lower energy than Cobalt60 requires a longer exposure time and this in turn reduces image quality due to the larger quantity of scattered radiation For profile radiography applications sometimes also called on stream radiography Iridium can replace Cobalt for pipes with a diameter up to 6 150 mm with still an acceptable image quality or even 8 200 mm in case of thin wall pipe The general rule is the shorter the exposure time the less the scatter thus the better the image quality Note CR plates are more sensitive to low energy scatter more noise than conventional film Careful filtering and collimation of the radiation and control of backscatter are vital to good CR Fading After exposure the intensity of the stored information cassette cl
163. onsiderably extended the chapters on digital radiography and special techniques such as microfocus and X ray microscopy In addition the impact and non existence of norms codes and standards on new NDT technologies and their applications are addressed We trust that this new issue of Industrial Radiography will fulfil a need once again GE Inspection Technologies 2008 The author expresses his appreciation to all employed by GE Inspection Technologies and Applus RTD NDT amp Inspection who cooperated and provided ample information to update this new edition 3 Contents Introduction to the overview of Industrial Radiography Image forming techniques Preface 13 1 Introduction to industrial radiography 15 2 Basic properties of ionising radiation 19 2 1 Wavelengths of electromagnetic radiation 19 2d X rays 20 22 Gamma rays y rays 21 2 4 Main properties of X rays and y rays 22 2 5 Radiation energy hardness 22 2 6 Absorption and scattering 23 Photoelectric effect Compton effect Pair production Total absorption attenuation 2 7 Penetrating power 25 2 8 Filtering hardening 26 2 9 Half value thickness 26 3 Units and definitions 29 3 1 Units 29 3 2 Definitions 30 Radioactivity Ionisation dose rate Ionisation dose Absorbed energy dose Equivalent dose man dose 4 Radiation sources 30 41 X ray tube 33 4 2 The anode 33 Cooling the anode The focal spot Effective focal spot
164. opment Making up processing solutions Nowadays chemicals are supplied as a liquid concentrate suitable for the particular type of film used The processing solutions can be prepared either directly in the tanks or in plastic buc kets In the latter case each type of solution must be prepared in a separate bucket which is never used for other chemicals 86 Developer Development fog graininess and contrast are dependent on the type of developer which is preferably made up to suit the film used If a concentrated manual developer is used for example G128 made by Agfa and the developer tank has a capacity of say 25 litres then all to do is pour 5 litres of the con centrated developer into the tank and add 20 litres of water ratio 1 part of concentra te to 4 parts of water G128 developer is also used as a replenisher in which case 3 parts of water are added to 1 part of concentrate Fixer Fixer too is supplied as a concentrated liquid G328 The same instructions as for pre paring developer apply here Developing times and bath temperatures The film is clipped on or slipped into a frame depending on the type of frame and hung in the developer tank As soon as the film is submerged in the developer the darkroom timer is set for the required number of minutes The optimal developing time is the time at which the most favourable contrast to fog ratio is achieved Minor deviations from the correct exposure time ma
165. or image chain Fig 3 17 System set up for X ray microscopy The set up of figure 3 17 is used for flat components For certain applications dependent on the geometry of the component the so called ovhm technique oblique view at highest mag nification is in use In such cases an open transmission X ray tube generates an adjustable oblique angled beam the detector is angled accordingly as illustrated in figure 4 17 Instead of tilting the sample which would result in a certain distance D that would limit the magnification factor the beam is tilted as illustrated reducing D to almost zero X ray tube Detector _ Adjustable detector position Fig 4 17 Oblique ovhm technique for maximum magnification 180 Effect of focal dimensions Figure 1 17 illustrates in one way the effect of focal spot size unsharpness figure 5 17 shows the same in another way for a micro and nanofocus tube head Source Object Detector i e D Fig 5 17 Effect of focal spot size on sharpness Figure 6 17 shows the effect of focus size on images of a connection wire diameter 25 micron in an electronic package Image A is taken with a focal spot size of 10 micron image B shows the result for a 5 micron focus and image C was taken with a focus of less than 1 micron Details of approximately 250 nm are visible Focal spot 10 microns 5 microns lt 1 micron Fig 6 17 Effect of spot sizes on image sharpness for differen
166. osed naturally decays over time resulting in some signal loss Scanning within 1 hour of exposure provides the best results typically 50 of the information is lost after 24 hours dependent on the manu facturer of the plate Fading is dependent on ambient temperature To avoid image fading scanning of the CR plate should not be delayed longer than necessary In critical applica tions where signal loss is expected due to delayed scanning the plates can be exposed with a higher radiation dose to compensate for this information decay Optimisation To optimise the use of CR imaging plates in practice a small handheld terminal as shown in figure 10 16 has been developed to superimpose specific project and exposure information to the images To this end the cassette contains a microchip which can receive wireless information from the ter minal On site and prior to the exposure the relevant information is sent from this terminal to the microchip on the cassette The specific data is ultimately added to the image in the CR scanner Once the data from the microchip has been erased the cassette is ready for re use Fig 10 16 Terminal for CR imaging plates Improvements Due to ongoing efforts for improvement the image quality of the phosphor plate one has already achieved a level equal to the quality obtainable with a medium grain conventio nal X ray film see figure 27 16 In fine grain films graininess is only a few microns while in
167. othed Fig 3 11 Inherent film unsharpness for X and Gamma radiation 99 Table 1 11 and figure 4 11 show experimentally determined values of inherent uns harpness for film exposed at various radiation energy levels These values are based on the use of filters and thin lead intensifying screens thicker screens produce slightly higher values If no lead screens are used Uf is 1 5 to 2 times smaller Ug is influenced mainly by radiation intensity and the type of intensifying screens used the type of film is hardly of any consequence The distance between film and intensifying screen is of great importance for the value of U Good contact between film and intensifying screen is imperative and can be achie ved by vacuum packing of film and screens together Radiation energy Upin mm 50 kV 0 05 100 kV 0 10 200 kV 0 15 400 kV 0 20 2 MeV 032 8 MeV 0 60 31 MeV 1 00 Se75 320 keV 0 18 Ir192 450 keV 025 Co60 1 25 MeV 0 35 0 1 1 Table 1 11 Empirical values of film uns Fig 4 11 Graphical representation of table 1 11 harpness Uf at various radiation energies Values of Ur for X and Gamma radiation at increasing radiation energies using lead intensifying screen From the above information it can be deduced that Ur increases at higher radiation energies Total unsharpness Total film unsharpness U is determined by the combination of U and Up The two values cannot be just added up to arrive at a figure for U
168. ource and the film cause absorption and filtering The position of the metal layer plays an important role in the effect it has A metal layer in front of the object will harden the radiation because it fil ters out the soft radiation The degree of hardening depends on the type and the thickness of the material This phenomenon is used to reduce excessive contrast variation in den sity when examining objects of which the thickness varies greatly A metal layer between the object and the film filters the soft scattered radiation that occurs in the object thereby increasing the contrast and consequently the image quality This method of filtering is for example applied in the use of Cobalt60 in combination with exposure time reducing intensifying screens which are sensitive to scattered radiation Lead copper and steel are suitable filtering materials 2 9 Half value thickness A convenient practical notion number of the linear absorption coefficient is the intro duction of the half value thickness HVT It quantifies the penetrating power of radia tion for a particular type of material and is defined as the thickness of a particular mate rial necessary to reduce the intensity of a monochromatic beam of radiation by half as shown in figure 9 2 This HVT value depends on the hardness of radiation D S x thickness Fig 9 2 Illustration of half value thickness 26 Table 2 2 shows the average HVT values for steel tab
169. own before making up the chemical solutions To remove chalk deposit from films they may be soaked in a solution of 7 ml glacial acetic acid to a litre of water 10 4 Silver recovery The silver halides in the emulsion which were not reduced during development are dissolved in the fixer Silver can be recovered from the fixer in order to keep the silver content of the fixer solution as low as possible so that the fixer lasts two to four times longer and sell the silver Silver recovery can for example be done by electrolysis In addition to electrolysis equipment there are other silver recovery systems commercially available It is worthwhile considering subcontracting this work to a specialised firm in view of secondary aspects such as organisation logistics storage and environmental requirements 90 10 5 Automatic film processing NDT U universal film processor Over the last few years there has been a vast increase in the use of automatic processors for handling industrial X ray films Not only is it a faster and more efficient process the uniform process also leads to improved image quality The total processing time may be between 1 5 and 12 minutes nominally 8 minutes significantly shorter than in manual processing Of these 8 minutes the film will be in the developer solution for only 100 seconds the so called immersion time These shorter processing times have been made possible by the use of special chemicals
170. p of a moving object This method is applied in systems for luggage checks on airports The source of radiation should be physically small a few millimetres in diameter and as X rays travel in straight lines from the source through the specimen to the film a sharp image is formed of the specimen and discontinuities This geometric image formation is identical to the shadow image with a visible light source The sharpness of the image depends in the same way on the radiation source diameter and its distance away from the surface on which the image is formed The classic film in its light tight cassette plastic or paper is usually placed close behind the specimen and the X rays are switched on for an appropriate time the exposure time after which the film is taken away and processed photographically i e developed fixed washed and dried In direct radiography DR a coherent image is formed directly by means of an computerised developing station The two methods have a negative image in common Areas where less material less absorption allows more X rays to be transmitted to the film or detector will cause incre ased density Although there is a difference how the images are formed the interpreta tion of the images can be done in exactly the same way As a result the DR technique is readily accepted The classic film can be viewed after photochemical treatment wet process on a film viewing screen Defects or irr
171. pixels The denser the better Fig 14 16 Fill Factor for amorphous silicon and CMOS This packing density is known as the Fill Factor and is illustrated in figure 14 16 This fac tor is dependent on the minimum possible spacing between individual elements The Fill Factor can be a reason to select a CMOS type detector for a particular application For CMOS this factor active portion is up to 90 for amorphous materials up to 80 155 16 5 3 Flat panel and flat bed detector systems There are different types sizes and suppliers of true 2D flat panel detectors A variety of flat panel systems exists with a wide range of pixel sizes and resolutions More and smaller pixels and a high Fill Factor increase the resolution of a panel As an indirect sensor material amorphous silicon is in wide use As direct sensors CCD s Charge Coupled Devices and CMOS Complementary Metal Oxide Semiconductors are also applied So far they have limited dimensions To mimic a large flat panel detector fast moving CMOS linear arrays are also in use providing an almost similar solution Amorphous silicon flat panels For industrial DR flat panel detectors knowm as DDA s Digital Detector Arrays in a variety of sizes are used up to approximately 400 x 400 mm maximum in 2008 as shown in figure 15 16 Thes detectors convert incident radiation intensity into proportional and digi tised electronic signals These digital signals can by mea
172. projected on the film it is straight forward to establish the cor rection factor which is the true diameter D divided by the radiographic diameter Dr This method should be used as much as possible With the projection technique the source is placed at a certain distance from the pipe At a film to focus distance of 3 X Dinsulation and a source size of 3 mm image quality require ment A of EN 1435 is met Fig 5 18 Projection technique for on stream radiography 198 The actual pipe wall thickness t is equal to the image on film tf multiplied by the correction factor see fig 5 18 Most common is on stream radiography of insulated pipes for which half the insulated diameter determines the sharpness In on stre am radiography it is important to know the direction of the product flow so that a existen ce of localised wall thickness reduction can be better deduced Films of 30 x 40 cm are gene rally used for pipe diameters up to 250 mm Larger diameters require more films D insulation Tangential technique In the pipe diameter range of 250 to 400 mm the tangential technique as shown in figure 6 18 is sometimes applied Only one wall is pro Correction factor jected The perpendicular projection produces 5 Pain 0 5 X Dsusie 2 5 X Disttin 0 8 a sharper image This allows a shorter focus to Fig 6 18 Tangential technique for on stream radiography film distance and consequently a shorter expo s
173. quired exposure detector quality 26 16 Effect of number of pixels on lateral resolution 16 8 Comparison of film CR and DR methods The choice of which technique to use lt g Faster extremely low dose but at cost of image quality 16 9 Impact and status of CR and DR standards Development of standards The application of established NDT methods is almost exclusively possible thanks to the existence of written stan dards codes norms guidelines procedures specifica tions qualification of personnel etc For the introduction and market acceptance of a new non established NDT method apartfrom economic considerations it is therefo re essential that standards support its use The standards for film radiography were written many years ago and did not envisage digital radiography so a whole set of new standards is required Terminology What s in a name Standard Standard practice Recommended practice Norm Code Procedure Specification Qualification depends first of all on the require ments with regard to the ultimate image quality In both the CR and DR methods the same IQI s also used in conventional radiography are applied PIE to check the radiographic processand image quality The major parameters D7 coarse grain to compare the three methods film CR and DR are speed dose needed aa for creating the image and image 40000 quality noise resolution contrast A Figure 27 1
174. r axis and the exposure value in mA min on the vertical logarithmic axis For a given kilovoltage for example 150 kV we can using the previously described intermediate kV thickness chart determine that for an exposure dose of 8 mA min a density of 2 can be obtained at a thickness of 4 5 mm and for an exposure dose of 200 mA min at a thickness of 15 2 mm These thicknesses and the corresponding exposures are then plotted on the graph paper to give points A and B see figure 4 9 Drawing a straight line linking points A and B the 150 kV line is obtained In a similar way the lines for other kV values can be drawn in the diagram eventually resulting in the complete exposure chart of figure 4 9 density 2 2 4 6 8 10 12 14 16 18 20 22 24 2 28 30 Fig 4 9 Exposure chart for a 200 kV X ray set 9 5 Use of the exposure chart While it may be possible to gradually build up a store of information which can be consul ted in day to day work it is better to make use of good exposure charts This system has many advantages to offer particularly when it comes to choosing the most suitable wor king method Apart from saving time it gives a guarantee of efficiency and moreover does away with or reduces to an acceptable extent the need for trial exposures on jobs which are a little outside the normal routine Different X ray tubes can in practice give quite different results even though they may be of the same type Even a different cable
175. r of exposures in accordance with EN 1435 A at maximum thickness increase of 20 See chapter 12 219 True real time Radioscopy DR Panels DR Panels Fay Sy A Y 5 D gt S w S D aa Fig 13 16 Relative image quality and speed of the various radiographic systems See chapter 16 220 CR Plates RCF Films D7 coarse grain 2 fine grain 10000 Relative dose wall thickness S A oS 25 50 75 100 150 200 250 300 Fig 7 18 Areas of application for selection of source screen and filter in on stream radiography See chapter 18 221 350 400 450 500 diameter in mm
176. r under or over exposed if there are great diffe rences in wall thickness This can be explained by the shape of the toe lower part of the characteristic curve of the X ray film used The film gradient contrast is lower and con sequently so is the defect discernibility In accordance with EN 1435 therefore there is a limit to the thickness range covered by one single exposure There are a number of practical ways to prevent over exposure of thinner and under expo sure of thicker sections These can be divided in two groups compensation by single film or by two film techniques For exposures on one film the following techniques can be applied Reduce contrast by utilizing a filter on the X ray tube to make the radiation harder e Reduce contrast by increasing the radiation energy using higher tube voltage or using Iridium192 or Cobalt60 sources Compensate the difference in wall thickness as the left sketch of figure 1 18 shows with material B of similar composition as object A Instead of insertion of B in the previous method use a special putty filling material mainly consisting of metal powder When two films are used the following techniques can be applied e Simultaneous use of two films of different sensitivity but similar screens for a single exposure For example an Agfa D7 and D4 type film could be used This is the least complicated and most practical method see figure 1 18 at right e Simultaneous exposu
177. ract and hard to understand While very useful in selecting a detector for a particular application in practice they do not replace the duplex IQI as final indicator of image quality for CR and DR applications Noise image averaging and DQE Fig 24 16 Effect of image averaging on noise and image quality Noise is a dominant factor in the DQE value System noise can be reduced by signal avera ging resulting in improved image quality as illustrated by the images of figure 24 16 Noise in turn depends on dose thus the time needed for an exposure to create an image that might include signal averaging to achieve the required image quality Reduction of noise by averaging the signals from a number of exposures increases the image quality but reduces the DQE value due to the longer exposure time 16 7 Resolution number of bits Resolution is a key word connected with digital radiography Apart from all digital proces sing inside the system it is ultimately the image resolution that determines its quality Two resolutions are of importance 1 depth resolution the number of grey levels in which a signal is presented 2 lateral resolution the pixel size Bit depth For depth resolution to present the densities in a map like image usually 12 bit 2 is applied This corresponds to 4096 different grey levels which corresponds when for con venience divided by 1000 to 4 in film technology The effect of the number of bits is il
178. rays to visible light These devices can have very good spatial resolution but are often used with thin scintillator screens that can limit X ray absorption efficiency detection over the full range of Xray energies used in common NDT applications Because they are made from singlecrystal silicon wafers they are also limited in size Thus detector designs that cover a larger area either require tiling of multiple devices or an x y motion of a single small device to simulate over time the effect of a larger device Larger 2D detectors up to the size of common X ray films are usually made from photo diode arrays of amorphous semiconductors Some early direct detector products were made from relatively thick film of amorphous selenium but these direct radiography detectors are no longer widely available for NDT applications More common are the indirect devices with photo detector arrays made from very thin film of amorphous silicon its schematic is shown in figure 11 16 These detector panels are available both with GOS screen scintillators and with thick layers of needle crystal Caesium Iodide CsI grown directly on the photodiode arrays The thicker scintillator layer in the CsI devices typically provides better absorption of the incident X rays and thus better imaging efficiency 16 5 2 Fill Factor Amorphous silicon Lateral resolution of a 2D digital detector array is determined by the packing density of the individual sensor elements
179. re on Agfa D7 and D4 films with different screens see figure 1 18 e Make two exposures on film of the same sensitivity and screen type one with the exposure time based on the thinner and one on the thicker section e Make two exposures on film of the same sensitivity but different screen types D7 film D7 D4 film Single film technique Double film technique Material A test object Section C evaluate on D4 Material B material of comparable absorption Section D evaluate on D7 Fig 1 18 Compensating for differences in wall thickness 194 18 3 Radiography of welds in small diameter pipes For pipe welds the single wall single image technique SW SI or if this is not feasible the double wall single image technique DW SI is to be applied For small diameter pipes this alternative is not really practical as a disproportionate number of double wall single image exposures needs to be made due to the limited effective film length see section 12 2 In such a case the double wall double image technique should be used DW DI Normally the DW DI technique is only applied on diameters lt 75 mm and wall thickness of lt 8 mm Both the weld on the source side and film side of the pipe are simultaneously interpreted Two more DW DI techniques are suitable for small diameter pipes e the elliptical technique and e the perpendicular technique Elliptical technique The elliptical technique as illustrated in figure 2 18 is the pre
180. record of the radiographic image which has recorded the intensity of X rays incident on the film as varying degrees of opacity shades of grey between black and white In digital imaging the intensity of X rays is first measured point by point and then individually digitised and con verted into many e g 12 bit 4096 levels discrete grey values including their corresponding coordinates This recording process is known as mapping a map consists of many millions discrete measuring points with their individual grey levels Finally these grey levels and their coordinates are displayed to form a coherent image on a video screen or printed as a collection of picture elements pixels for examination by the human eye Because of the 1 to 1 correspondence between each final image pixel and the discrete mea surement area sensor size the areas on a digital detector are also commonly referred to as pixels For digital radiography using panel flat bed or line array detectors this process of digitisation with assigned grey levels is done at once at the detector itself In case of imaging plates the digitisation and grey level assignment is done in the so called reader see section 16 4 The mapping process allows data to be measured and stored from a much wider dynamic range than the eye can normally perceive After an image has been stored different maps can later be applied to show different thickness ranges without affecting the origin
181. resolution display screen are recommended for digitised films as well as for displaying and processing the images obtained with CR and DR techniques The number of pixels of the display screen should at least match with the digitisation spot or pixel size of the applied CR plates or DR panels to achieve maximum resolution Radiographic images contain more information than the human eye can discern For this purpose workstations as shown in figure 33 16 are used as an image processing centre This workstation operates with powerful dedicated proprietary software e g Rhythm of GE Inspection Technologies to manage process and adjust images Fig 33 16 Work station 171 Versatility of the software Images can be adjusted and enhanced in many ways brightness contrast sharpness noise suppression averaging rotation filte ring inversion colouring magnification zoom pan scroll etc In this way hidden details can be made visi ble see figure 34 16 Figure 35 16 graphically shows the effect of two control mechanisms for selecting a part of the density range of the image for a closer look The Z shape can be shifted from left to right through the whole range of densities of 2 the image The angle of the vertical part can amp be changed to increase the width of the gt E window steeper or more flat to alter the 5 gt range of contrast densities z Selectable The position of the working p
182. rons for a wide variation of tasks have already been successfully applied To inter pret the results CT images can be freely rotated and virtually sliced in all directions for dif ferent views of a defect or other anomaly a unique and very useful feature Computing capacity and scanning time In NDT contrary to medical applications it is usually the object that rotates between the source and the detector as shown in figure 11 17 This can be done continuously or stepwise to obtain a great number of 2D images that ultimately are reconstructed into a 3D CT image The object is scanned section by section with increments of say 1 over 360 with a very narrow beam of radiation small focus X ray or collimated gamma ray The more incre ments the better the CT quality The receiver in this illustration is a flat panel detector Detector Rotating object a ao X ray source Focus object distance Focus detector istance a a i E E C ma FOD FDD Fig 11 17 Rotating component to create a 3D CT image 185 Each individual detector element measures during a short exposure period the total absorption across a certain angular position of the object This information including the coordinates is used to create a numerical reconstruction of the volumetric data This process produces a huge data stream to be stored and simultaneously processed in par ticular when an image of high resolution is r
183. rs rectifiers and smoothing capacitors The X ray tube and the circuitry of this equipment are usually placed in an insulated tank In most cases these tank type sets use oil for insulation and cooling and weigh approximately 60 kg Gas is used when weight is important the set than weighs approximately 30 kg Figure 3 5 shows an integrated all in one tank set for 300 kV with an asymmetric window At voltages over 300 kV housing eve rything in one tank becomes very difficult because the high voltage insulation would be inadequate Window Figure 4 5 shows a direct current X ray tube with a symmetric window Equipment up to 450 kV operating S on direct current is connected to a bun separate high tension HT supply Fig 3 5 All in one 300 kV tank set with an asymmetric window unit by means of HT leads As a result this equipment is bigger and heavier than all in one tank sets and mostly meant for stationary or semi ambulant use Window Fig 4 5 Direct current X ray tube for 450 kV with a symmetric window The 300 kV all in one tank set and the 450 kV direct current X ray tube only are of roughly the same dimensions Most tank sets are connected to a mains power supply with a frequency of 50 or 60 Hz At this frequency the supply voltage can be transformed upward This is followed by rectifying which occurs in various forms With some sets the X ray tube itself functions as rectifier so called single ph
184. rs up to 25 were sufficient The maximum magni fication factor was determined by the smallest possible focal spot size As illustrated in figure 1 17 larger magnification factors create unsharp images without providing more informa tion Moreover the intensity of the output is limited by the heat dissipation of the target anode For some time this was a physical barrier With the introduction of microfocus and more recently nanofocus X ray tubes new techniques have been developed for inspection of low absorbing objects like electronics applying large magnification factors with still high resolution Because of the need to inspect parts with ever decreasing dimensions such as electronic components and their joints or other products with extreme quality requirements and pro duct process control ever growing image magnification factors were necessary ending up in so called X ray microscopy The need for geometric magnification factors of up to 2 000 are no exception This was the incentive to develop X ray tubes with extreme small spot sizes Because there is a physical limit to the minimum focal spot size limited heat dissipation and output other measures tricks can be taken a combination of software lenses and cameras to further zoom in even up to 25 000 It should be realised that magnification only make sense if the initial image quality is sufficient a poor image just creates bigger pixels or results in a vague image Microfocu
185. rt MTF describes the relation between contrast and spatial frequency LU UUM In practice MTF characterises the unsharp ness blurriness that a digital system adds to an image thus indicating the level of distortion of contrast sharpness in the resulting image as illustrated in figure 21 16 Ideal MTF input output The graph shows the distortion of contrast of a square wave black white input and output for an ideal MTF of 1 100 and a low MTF Each step in an imaging chain has an individual MTF Low MTF with distorted output Fig 21 16 MTF and resulting contrast distortion The MTF of a complete system is the product of the MTF s of the individual steps In the end of an imaging process the effect is visible in the amount of loss of image quality MTF for a total system typically ranges from 0 to 1 0 to 100 Sharp features and small flaw indi cations will be more easily visible in images produced with a system that has a high MTF Figure 22 16 graphically shows the image distortion of for example an ideal pin shaped detail that through Initial information Final image successive distortions by the steps in x on monitor the system is presented as a blurred 5 ees spot on the screen of the work station Every step in the process widens the detail that was ideal in the beginning MIE X MIE A MIF A MIF MIF with a simultaneous reduction of Process steps contrast and sharpness Fig 22 16 MTF causi
186. s 188 In such cases 3D Computer Tomography lt Movement 4 X ray p sitions 3D CT can provide a solution In secti T on 17 3 CT systems for low to moderate energies and with extreme small foci are described to inspect small components with low radiation absorption For sizing and sometimes detection of defects in welds or cast stainless steel high energy 3D CT systems have been developed which are able to accurately size randomly oriented cracks or other planar defects with a minimum width T Detector positions as small as 20 microns Such systems use common high energy Fig 17 17 Schematic of 3D CT defect detection and sizing X ray tubes with common focus dimen sions of a few mm in combination with line detector arrays Figure 17 17 schematically shows the set up for a series of exposures to create a 3D CT image Synchronised and simul taneous movement of the X ray source and the detector causes only a particular volume of the material slice to be in focus All information from the adjacent area is out of focus and does not contribute to the image of the defect Many of such focus areas volume pixels or voxels are stored for instance a few hundreds per slice If the detector has sufficient length it does not need to move virtual movement In performing a CT scan the X ray beam goes through a wide range of angles including the angle s of the defect Numerous slices
187. s e Film screen combination e Film screen contact 3 Graininess depends on e Type of film Type of screen e Developing procedure e Radiation energy hardness 105 Fig 1 12 circular image Distortion of the image of a gas cavity due to beam orientation 106 elongated image Defect orientation image distortion and useful film length 12 1 Defect detectability and image distortion On a radiograph a three dimensional object is presented in a two dimensional plane the film The appearance of both the object and its defects depends on the orientation of radiation relative to the object As shown in figure 1 12 the image of a gas cavity in a casting may be circular or elongated depending on beam orientation In general the beam of radiation should be at right angles to the film and a specimen should whenever possible be laid flat on the film cassette Special angle shots are howe ver sometimes useful to detect defects which are unfavourable oriented with regard to the X ray direction This influence of X ray beam angles relative to the orientation of a defect is also described and illustrated in section 17 4 Figure 2 12 A shows a situation whereby detection of lack of side wall fusion in a V weld is not performed optimally Angled radiation B is more likely to show up this type of weld defect f source source A blurred image of the defect sharp image Fig 2 12 Lack of sidewall fusion in a V
188. s GE Inspection Technologies and its affiliated suppliers developed a variety of digital systems with a wide range of computer aided NDT applications Digital radiography partly repla ces conventional film and also permits new applications The growing number of availa ble standards norms codes and specifications essential for industrial acceptance and application supports this tendency Although the process itself is different from film radiography DR resembles traditional radiography to a large extent The optical impression of the X ray images is similar so that RT trained personnel can quickly adopt this new technology and adapt to it without great efforts Moreover the images can be interpreted in analogy to film Digitisation of traditional radiographs although not real digital radiography uses the same digitisation technology presentation on a display of a work station and image adjustment and therefore is part of this section too Digitisation of film is done for the purpose of archiving and or image enhancement adjustment Two main methods of real filmless digital imaging can be distinguished 1 digital radiography by means of phosphor coated semi flexible imaging plates compared with flexible film in combination with computer processing Digitally enhanced radiograph of a mermaid man The radiograph negated a myth by demonstrating that it concerns so called Computed Radiography CR for short a man m
189. s also used to check complex assemblies for pro per construction and for many other technical applications The following selection of radiographs illustrates the wide variety of possibilities for detection possibilities of defects or errors Weld inspection The following examples are from the booklet published by GE Inspection Technologies called Radiographer s Weld Interpretation Reference Note All of these examples illustrating a variety of defects in welds are also issued on poster format 60 x 90 cm by GE Inspectio technologies 124 s Offset or mismatch Hi Lo An abrupt change in film density across the width of the weld image 125 Offset or mismatch with Lack of Penetration LOP An abrupt density change across the width of the weld image with a straight longitudinal darker density line at the centre of the width of the weld image along the edge of the density change External concavity or insufficient fill Excessive penetration External undercut The weld density is darker than the density of the pieces welded A lighter density in the centre of the width of the weld image and extending across the full width of the weld either extended along the weld or in isolated circular drops Internal root undercut An irregular darker density along the edge of the weld image An irregular darker density near the centre of the width of the The density will always be darker than the density of the pieces w
190. s and nanofocus X ray tubes Over the years industry developed X ray systems with ever decreasing focal spot sizes to meet the need for large magnification factors At present focal spot sizes expressed in a few hundreds of nanometres nm are on the market By conformation manufacturers of X ray tubes classify their tubes dependent on focus size in a few categories e macrofocus with spot sizes gt 100 microns 0 1 mm e microfocus with spot sizes ranging from 1 micron up to 10 microns e nanofocus with spot sizes far below 1 micron Sizes of down to 0 25 micron 250 nm do exist The Nanofocus see acknowledgements in chapter 20 X ray system is just one example of such a system The output of nanofocus X ray tubes is proportionally lower than for tubes with larger focal spot sizes Heat dissipation of the target anode generating the X ray beam puts a limit to the output The smaller the target the lower its output Over heating destroys the anode by burn in 178 Two types of X ray tubes exist e The closed X ray tube a sealed evacuated glass tube containing all components to generate X rays No part in it can be replaced or repaired The open X ray tube with a removable replaceable anode target and filament with its own high vacuum system for an almost unlimited life A closed system is cheaper and maintenance free but has a shorter life time than an open tube An open tube with an almost unlimited life time can operate at
191. s are joined or a pipe is joined to a 45 90 bend Three exposures equally divided over the circumference are sufficient for 100 coverage DW DI 60 of 120 ae ro 4 A Fig 3 18 Perpendicular double wall double image technique 196 18 4 Determination the depth position of a defect The depth position d of a defect can be determined by the parallax method as shown in figure 4 18 The radiograph is exposed from two opposite angles The required quantity of radiation is equally divided over positions A and B Only one film is used The shift in defect image on the film G in mm is a measure for the depth position the shift of the source A to B in mm and the source to film distance H in mm are important data The depth position is calculated with the formula d GxH AB G Another much more complex method of depth determination is stereo radiography by which two separate films are exposed which are viewed simultaneously via mirrors However this method is rarely used G defect shift Fig 4 18 Determining the depth position of a defect 197 18 5 Determination the depth position and diameter of reinforcement steel in concrete Similar to the method for determination of the depth position of a defect in metals is the determination of the depth position cover of reinforcement steel in concrete Subsequently the true diameter of the reinforcing bar D can be calculated
192. segment between density values 3 50 The specimen in figure 6 7 containing a and 1 50 and is a standard characteristic of a particular type of radiographic film small step is radiographed with an exposure time resulting in a density difference of 0 5 In all films for example D2 through to D8 the gradient a b increases with increasing B minus A If now using the same type of density within the for conventional viewing screens useful density range of D lt 5 film and the same tube voltage a longer exposure time is given the density difference AN E E ee The various types of films are not identical This becomes clear if plotting the values of is 0 9 D minus C The second radiograph at increasing density gradient Gp against the density resulting in the gradient density curves as shown in therefore shows more contrast figure 5 7 At higher film sensitivity the gradient is lower and hence the density curve less steep 62 63 Effect of developing conditions on the density curve The characteristic curve of an X ray film is not only determined by the emulsion characte ristics but also by the way the film is developed Parameters which can influence the cha racteristic curve are developing time and its temperature developer concentration and agitation The effect of for example the developing time on speed relative exposure factor contrast and fog has been made visible in figure 7 7 Initially up to approx 4 minutes speed and con
193. sensitive and the first signs of radiation are found in the blood which is why people working in radiology are subjected to periodic blood tests The most serious effects of radiation occur when a large dose is received in a short period of time Table 2 19 shows doses received over 24 hours and their effects Dose received Effects by the body 0 0 0 25 Sv No noticeable effects 0 25 0 5 Sv Limited temporary changes in the blood 05 10 Sv 2 0 2 5 Sv First lethal cases 5 0 Sv 50 lethal MLD medical lethal dose Table 2 19 Effects of radiation doses The consequences of excess radiation are not necessarily noticeable immediately after the irradiation Frequently they only show up after some time The time lapse between irradia tion and the moment the effects become apparent is called the latent period Genetic effects can only be assessed over generations 19 4 Protection against radiation The International Commission on Radiation Protection ICRP a division of the International Atomic Energy Agency IAEA is engaged in providing rules and regulations for the protection against radiation as the name suggests The ICRP has established the values for radiological and non radiological workers as indicated in section 19 5 Practically all countries have brought their national legislation laws on ionising radiation in line with the ICRP codes The conditions for registration transport storage protection a
194. shaped weld joint Shot A is unlikely to detect the defect shot B will 107 12 2 Useful film length When radiographing cur ved objects for example a circumferential weld in a pipe as figure 3 12 shows the resulting image will be distorted Variations in den sity will also occur As a result of the curvature of the pipe with a wall thick ness t the material thick ness to be penetrated incre ases to T so film density is lower at the ends of the film than in the middle source l l lt distorted Moreover if defects are pro NY image of crac jected nearer the ends of a film distortion of the defect image will become greater The film length suitable for defect interpretation is the refore limited This so cal led useful film length is depending on the nature of the work defined in codes e g in EN 1435 Fig 3 12 Image distortion caused by the curved shape of the object It is not always practicable to apply the single wall technique as shown in figure 3 12 In order to still achieve 100 examination the double wall single image technique DW SI is applied In NDT jargon the abbreviations DW SI and DW DI are frequently used for Double Wall Single Image and Double Wall Double Image respectively In that case several radiographs are made spaced equally around the circumference of the item under examination The number of radiographs to be made depends on the stan dard or code to be complied
195. size 4 3 Tube voltage and tube current 35 4 4 Radioactive sources isotopes 36 Natural radioactive sources 4 5 4 6 5 1 3 2 Sa 5 4 3 5 5 6 Oil 6 1 6 2 6 3 7 1 Ta 7 7 4 Artificial radioactive sources Advantages and disadvantages of artificial radioactive sources Properties of radioactive sources Activity source strength Specific activity Specific gamma ray emission k factor Half life of a radioactive source NDT equipment X ray equipment Types of X ray tubes Bipolar X ray tubes Unipolar X ray tubes Special types of X ray tubes High voltage generators Megavolt equipment The Betatron The linear accelerator linac Radioactive sources Average energy level nominal value Source holders capsules Transport and exposure containers Checking for container leakage Radiation images filters and intensifying screens Radiation images Radiation filters Intensifying screens Lead screens Steel and copper screens Fluorescent screens Fluorescent salt screens Fluorometallic screens The X ray film and its properties Structure of the X ray film Radiographic image Latent image Developing the latent image Characteristics of the X ray film Density optical Contrast Characteristic curve density curve 5 36 37 39 39 41 42 45 46 46 49 51 51 53 53 TH 39 39 ho 7 6 8 1 8 2 8 3 8 4 91 Die 9 3 a 9 6 9 7 03 10 10 1 10 2 Gradient of t
196. size thickness and inflexibi lity rigidness Contrary to DR CR plates are thin and can be bent to conform with to curva ture of the component which sometimes is a condition for certain exposures Moreover flat panel detectors require a considerably higher capital investment than the CR method 167 Although the electronics needed for both methods e g workstation cost approximately the same and partly can be shared a flat panel detector 150 000 is roughly 200 times more expensive than a phosphor plate 750 Hence selection of a DR solution requi res careful considerations with regard to return of investment pay back period Another aspect of paramount importance which influences selection between CR and DR is the availability or lack of industrial standards Insummary Numerous aspects with a great diversity such as image quality process speed productivity portability robustness fragility in flexibility of plate or panel available field space logistics environmental issues capital investment human investment non existence of industrial standards etc play a role in the ultimate choice between conventional film or CRor DR 16 11 Applications of CR and DR methods Corrosion detection For certain applications e g when the requirements for image quality are less stringent and normal or coarse grain film could be used the CR technique is an excellent alternative to film Examples include profile
197. specified period the films are developed and the densi ty measured The radiation dose received by the wearer can then be determined by consulting the density exposure curves and the type of radiation received can be established by i checking the densities behind the filters Film dose Fis 419 Filmdosismeter film badge meters as illustrated in figure 4 19 are a very cheap and reasonably accurate method of monitoring personnel in selected areas They measure 25 x 25 x 5 mm are robust and convenient to wear 19 7 Dose registration Due to legally required monitoring and registration of radiation doses received by radiolo gical workers over a specified period of time dose meters must be worn Generally these are TLD or film badges The TLD meter is preferred over the film badge as it is read out electro nically and can be linked to a data base Processing film badges is more complicated The films must first be developed before they can be viewed to quantify and register the radia tion dose Radiation dose monitoring is carried out by a government authorised organisation which is responsible for mailing processing and viewing of the badges This organisation generates reports which contain the individual irradiation doses over a specified period of time as well as the accumulated dose 19 8 Radiation shielding Protection from radiation best by distance can consist of ribbons or ropes and warning flags to demarcate the ar
198. t see section 16 12 Thus defect indication details not discernible on the original film using a viewing screen can be made visible For use in laboratory environments only high resolution film digitisation systems exist that use a scan spot size as small as 10 um This is an inherently time consuming process but enables detailed analysis of particular film areas e g to make tiny cracks visible at the work station 147 Because scanners vary widely in resolution dynamic range and ability to scan dense films evaluation is required to ensure that adequate scanning fidelity is achieved Depending on selected resolution many Megabytes are needed to store a single film see paragraph 16 12 Archiving of a digitised film identical to CR and DR images is usually done on an optical mass storage facility e g CD ROM DVD etc For uniform application of film digitisation norm EN 14096 has been issued 16 4 Computed Radiography CR Digital radiography using storage phosphor plates is known as Computed Radiography or CR for short This filmless technique is an alternative for the use of medium to coarse grain X ray films see the graph in figure 27 16 In addition to having an extremely wide dynamic range compared to conventional film CR technique is much more sensitive to radiation thus requiring a lower dose see figures 8 16 and 27 16 This results in shorter exposure times and a reduced controlled area radiation exclusion zo
199. t circuiting which might damage the tube or render it useless 5 3 Megavolt equipment The equipment described in sections 5 1 and 5 2 is used to generate X radiation up to approximately 450 kV However sometimes higher energy levels are needed Several types of equipment have been built to operate in the 1 MeV to 16 MeV range In industrial radiography almost exclusively Betatrons or linear accelerators linacs are used Operating high energy X ray installations requires costly safety precautions The Betatron The B tatron is an electron accelerator which can produce X radiation in the 2 30 MeV energy range The electrons are emitted into a round sectioned donut shaped glass vacuum tube as shown in figure 5 5 After several millions of revolutions the electrons reach maximum energy and are deflected towards the target On the target part of the electron energy is converted into a tangentially directed beam of X radiation To obtain a reasonably high radiation intensity most Betatrons have been designed to ope rate in the 10 30 MeV energy range as these voltages achieve maximum conversion rate of electron energy into radiation Even so the output of B etatrons is usually small compared to linacs Transportable low energy B tatrons 2 6 MeV have been built but these gene rally have a low radiation output which limits their application One advantage of Betatrons is that they can be built with very small micromillimeter focal spo
200. t focal spot sizes There are rules of thumb to determine resolution and potential defect detectability as a function of focal spot size For large magnifications exceeding 100x the following applies e resolution equals the focal spot size divided by two e detectability or feature recognition equals focal spot size divided by three 181 Imaging systems for high resolution radiography High resolution X ray inspection systems usually apply an image intensifier for presentation of results as shown in figure 7 17 This electro optical device amplifies and converts the invi sible X ray shadow to visible light by means of a scintillation crystal and photo cathode Electrons from the photo cathode are then accelerated and focused onto a phosphor screen where a bright and visible image is produced that is digitised by a CCD camera In order to avoid unnecessary losses of resolution it is crucial to at least use 2 mega pixel high resolu tion cameras To meet highest demands 4 mega pixel cameras are the best choice The advantage of the image intensifier based digital image chain is its relatively low cost and relatively sharp real time image Scintillator Diode array panni electrodes coils Object X rays Scintillator Photocathode Single component Figure 7 17 Schematic setup of an image intensifier Fig 8 17 Arrangement of a flat panel based digital image chain image system As an alternative a digital flat panel detector as s
201. the difference between the two values amounts to 2 45 2 15 0 3 This means that the exposure time must be doubled 10 2 resulting in a radiation dose of 30 mA min This answers the first question If the exposure time is doubled the log rel exposure of the lowest density value originally measured will increase by 0 3 i e 1 65 0 3 1 95 The corresponding density will be 1 0 fig 9 9 The average gradient between the upper and lower densities on the original radiograph was 1 5 0 5 2 15 1 65 2 0 The average gradient on the new radiograph is 3 0 1 0 2 45 1 95 4 0 so the average contrast has doubled 83 Preparation for an X ray exposure of a painting by Rembrandt 84 Processing and storage of X ray films Film developing is the process by which a latent image see section 7 2 is converted into a visible image The crystals in the emulsion carriers of the silver traces forming the latent image are transformed into metallic silver by selective reduction as a result of which the visible image is created The development procedure must be carried out carefully to achieve this and guarantee successful archiving over a longer period Manual developing is a laborious process that must be carried out meticulously in order to get the high quality results For increased efficiency and uniform quality X ray films are more commonly processed automatically The manual process is however still fr
202. the three absorption processes described above in which the primary X ray energy changes to a lower form of ener gy Secondary X ray energy arri ses of a different wavelength and a different direction of travel Some of this secondary scatte red radiation does not contribu te to radiographic image forming and may cause loss of image quality through blurring or fog The contribution of the various causes of X ray absorption to the total linear absorption coeffi cient u for steel plotted against radiation energy are shown in figure 6 2 X ray ejected electron ejected positron Fig 5 2 Pair production 107 107 u Uc tHe tp Fig 6 2 Absorption coefficient for steel plotted against radiation energy PE Photoelectric effect C Compton effect PP Pair production 24 2 Penetrating power The penetrating power of X radiation increases with the energy hardness The relationship of energy and penetrating power is complex as a result of the various mechanisms that cause radiation absorption When monochromatic homogeneous single wave length radiation with an intensity I passes through matter the relative intensity reduction AI I is proportional to the thickness At The total linear absorption coefficient u consisting of the three components described in section 2 6 is defined by the following formula Al sie O Expressed differently I 1 eu In which Io intensity at material e
203. ther filmless application of neutron radiography in NDT is moisture detection in insula tion This portable equipment that is on the market uses a very weak neutron source With the aid of this neutron backscatter method the presence of water actually that of hydrogen atoms is established The presence of moisture is generally an indication of exter nal corrosion in a pipe or the likelihood that corrosion will occur in the near future The portable real time equipment as described in section 17 3 or flash radiography descri bed in section 18 7 can in some cases confirm the presence or absence of corrosion without removing the insulation 17 6 Compton backscatter technique The Compton backscatter technique see section 2 6 benefited from the introduction of computer technology into NDT equipment just as most other methods discussed in this chapter This method is very attractive for objects with access from one side only It is now an accepted NDT technique for plastics and light metals 2 190 The scanner comprises an X ray tube and a detector consisting of a number of elements as illustrated in figure 13 17 A collimator reduces the beam of rays to 0 5 mm in diameter so that it cannot irradiate the detectors directly When a photon and an electron collide in the material the primary X radiation is scattered as somewhat softer radiation in all directions and thus partly also back from the material to the scanner This secondary
204. tive in cramped spaces e g offshore platforms see chapter 16 In industry this area is often called safety area which is wrong to the contrary it is an unsafe area with highest radiation close to the source 18 7 Flash radiography Flash radiography or pulse radiography can be carried out when information is required about the condition of the outer surface of an insulated pipe without having to remove the insulation Figure 10 18 shows flash radiography in progress Since only the aluminium cladding and insulation need to be penetrated relatively soft radiation is applied while exposure time is limited to only a fraction of a second In that time sufficient pulsed radiation is generated to create an image on the superfast F8 NDT 1200 film screen combination see section 6 3 It is safe to make radiographs manually without the need for additional safety arrange ments Systems up to 200 kV exist Golden Inspector suitable to penetrate 10 mm wall thickness of steel j i Ta z 5 pi a e TE Ra g k i i rete i Lin lt Pubeues ty ko Wes a ae Ca pien ee a TA TT oes 4 oe ang 7 faye gt SO ee a F x 5 e H _ i wale Gil aa h N Y s z TORIA c Fig 10 18 Flash radiography of an insulated pipe section 18 8 Radiography of welds in large diameter pipes To create an on or offshore pipeline individual pipes length usually 12 m or pipe sections double or multiple jo
205. to film distances longer collimators are used to restrict the beam of radiation This type of container is suitable for Iridium sources up to 1000 GBq and weighs only approx 20 kg A similar system Saferad with a weight of up to 15 kg exists using Selenium75 which almost eliminates the usual disruption to construction maintenance and process operations in the vicinity of the exposure 5 7 Checking for container leakage A sealed radioactive source capsule might start to leak and become an open source as a result of corrosion mechanical damage chemical reactions fire explosion etc Regular mandatory wipe tests by specialists serve to detect leakage at an early stage 49 objet Fig 1 6 The negative X ray image Fig 2 6 Image forming and non image forming radiation Only the radiation from source S that reaches the film in straight lines via beam section DE produces an image of cavity N at P The remainder not reaching P directly is scattered radiation no defect image forming thus reducing the image quality negative shadow image on the film radiation intensity after passing through the object source S 50 Radiation images filters and intensifying screens To influence the effects of radiation on an image filters and intensifying screens are used to e filter harden the radiation to influence contrast and or e to intensify the effect of radiation to improve contrast 6 1
206. trast are low but increase rapidly with developing time From 8 minutes on a further increase in developing time increases the background fog and eventually a decrease in contrast will occur developing time min 5 7 9 oO N o relative exposure factor O contrast effect average gradient for densities between 1 5 and 3 5 industrial X ray film developer G128 at 20 C development fog Fig 7 7 Film characteristics at various developing times Although it is possible to compensate to a certain extent for minor variations from the cor rect radiation exposure by adapting the developing time normally a fixed time is maintai ned In manual developing the standard time is 5 minutes Developer type film agitation in the tank and temperature also influence density That is why the overall developing pro cess should preferably be standardised or automated In most cases deviating from the optimum developing conditions leads to reduced image quality 64 7 5 Film speed sensitivity In radiography the relationship between exposure in C kg and resulting density is com monly referred to as film speed Other than in normal photography where film speed is indicated by a DIN or ASA number films for industrial radiography do not carry an inter nationally recognised speed number The generally accepted method of measuring the film speed of radiographic films is to mea sure the exposure required to achieve a density
207. ts A disadvantage is that with these very high energy levels the X ray beam is usu ally narrow and the coverage of larger film sizes is only possible by using increased sour ce to film distances The extended exposure times required can be a practical problem 42 rT tripe LET ETE EEE Ring shaped accelerator tube Anode Cathode filament emitting electrons magnetic fields Coils Auxiliary winding Fig 5 5 Betatron The linear accelerator linac The energy levels mostly used for linacs linear accelerators are 4 MeV and 8 MeV Linear accelerators can be constructed for one or two energy levels In the travelling wave linac the acceleration of electrons from a heated filament to very high energies results from the electrons riding a high frequency 3 10 MHz electromag netic wave travelling in a straight line down an acceleration tube the hollow guide The electrons are bunched into pulses at a frequency of a few hundred pulses per second The target which the electrons strike to generate X radiation is at the opposite end of the main wave guide of the filament assembly This is a transmission type target from which the radiation beam passes in a straight line The X ray output from a linear accelerator is many times higher than from a Betatron of the same energy An 8 MeV linac with a 2 mm diameter focal spot can deliver a radiation dose rate of 30 Sv minute at 1 metre distance from the focus Small light wei
208. ty spots that may be con nected in a straight line in the centre of the width of the weld image Transverse crack Feathery twisting lines of darker density running across the width of the weld image 133 Longitudinal crack Feathery twisting line of darker density running lengthwise along the weld at any location in the width of the weld image Longitudinal root crack Feathery twisting lines of darker density along the edge of the image of the root pass The twisting feature helps to distin guish the root crack from incomplete root penetration 134 Tungsten inclusions Irregularly shaped lower density spots randomly located in the weld image Casting radiography For the interpretation of X ray films of castings thorough knowledge of the specific manu facturing process is required The type of defects in castings varies for the different types of materials and casting processes Figures 15 1 and 15 2 show X rays of complex castings These radiographs were made to check the overall shape and possible presence of casting defects As it solidifies during the casting process metal contracts and unless precautions are taken shrinkage cavities can occur inside the casting These can take various forms such as piping worm holes figure 15 3 sponginess or filamentary cavities depending on the rate at which the metal has solidified When the contracting spreads slowly through the metal filamentary shrinkage fig
209. uals a factor 10 000 10 in density It is very important to regularly recalibrate these instruments particularly around values 2 and 2 3 since those are the minimum densities depending on class A or B which a film must have in accordance with standard EN 444 to allow it to be interpreted Densitometers are supplied with reference material density strips to re calibrate them Regular recalibration at least once a year according to code is essential The most commonly used density strips deteriorate quickly as a result of scratching and dis integration of the sealed transparent wrapping in which they are usually kept Their service life depending on use is usually not much longer than six months Agfa has developed the Denstep density step wedge film and has succeeded in considerably extending the service life of these strips by supplying them in special wear proof wrapping These density strips are certified and have a guaranteed minimum service life of four years The 15 steps of the Denstep comprise a density range from 0 3 to 4 9 3 Producing an exposure chart for X rays The step wedge The production of an exposu re chart calls for either a large step wedge or a series of plates of different thicknesses made from the same material to which the chart relates The increase in thickness between each consecutive step is constant but varies for different materials from 0 5mm to several millimetres ipgrawmemnsc
210. ude Exposure time and noise Fading Optimisation Improvements 16 5 Genuine Digital Radiography DR 153 One step digital radiography 16 5 1 Detector types 153 Direct versus indirect detection Linear detectors 2D detectors 16 5 2 Fill Factor 155 16 5 3 Flat panel and flat bed detector systems 156 Amorphous silicon flat panels CMOS detectors and flat bed scanners Limitations 16 6 Image quality and exposure energy 158 16 6 1 Exposure energy 158 16 6 2 Determination of image quality 159 16 6 3 Indicators of image quality MTF and DQE 160 Factors influencing image quality Image quality definitions Exposure parameters MTF Modulation Transfer Function DQE Defective Quantum Efficiency Noise image averaging and DQE 8 16 7 16 8 16 9 16 10 16 11 16 12 17 17 1 17 1 1 2 172 17 5 Resolution number of bits Bit depth Lateral resolution Comparison of film CR and DR methods Impact and status of CR and DR standards Development of standards Status of CR standards Status of DR standards Impact of standards Standards for weld inspection Data exchange and tamper proof standard Selection of CR and DR methods Applications for CR and DR methods Corrosion detection Weld inspection Dose reduction and controlled area Automated mechanised inspection Girth weld inspection Useful life of plate and panel Work station Hardware and software Versatility of the software Archiving and reliability of images E
211. ull progress and has the potential to ultimately meet the required image quality which at present only can be achieved by using traditional film To cope with the high weld production rates thus limited time available for inspection the RT process has been fully optimised On land with sufficient exposure time available it is common practice to develop and judge the films only once or twice the same day Repair can be done afterwards For offshore work this process is impossible Each weld has to be judged instantly and if necessary be immediately repaired before a new one can be made because a lay barge in general only moves forward For a full cycle exposure and development of the film and its interpretation approximately five minutes are available in case of an S lay situation For the more complex J lay situation applied in deep waters in general more time is available so inspection time although still at the critical path is less critical To optimise the inspection process first of all X ray crawlers with control units have been developed in combination with very accurate positioning devices as illustrated in figure 11 18 202 Positioning device Wrapped film Receiver Batteries Sanaa Crawler Fig 11 18 Concept of girth weld inspection This positioning device is used to stop the crawler at the correct X ray tube position within a few millimetres with regard to the weld plane in order to make a true panor
212. ungsten content 97 is used as shielding material This material is easily worked and finished and not prone to melting 46 Also greatly depleted uranium with the highest radiation absorption is used for shielding resulting in very compact exposure containers A disadvantage of this material however is that it has a certain minimal radioactivity which is reason that in some countries the use of depleted uranium is not allowed Regardless of the shielding material used all containers have a considerable weight in common There are several solutions to the problem of safely storing a source on the one hand and of putting it in a simple but absolutely safe manner in its radiation position on the other hand Two regularly used constructions for this purpose are source S is situated in a rota ting cylinder as shown in figure 11 5 or in an S channel container as shown in figure 12 5 open closed Fig 12 5 S channel container with Fig 11 5 Exposure container with source S in a rotating inner cylinder source S in storage position The S channel container is usually provided with a means to move the source out from a distance after all distance is the safest protection from radiation This may be done by means of a flexible cable in a hose Teleflex design as shown in figures 13 5 and 14 5 With this construction it is possible to extend the flexible hose in such a way that the source can safely be moved several metres out o
213. urce a E E m E m iz 4 E Ez E m E ROI Region of Region of interest interest isl Object l Focus object distance Focus detector FOD distance FDD Highest resolution magnification Fig 12 17 CT scan of object detail If too close to the X ray source the geometry of the object can hamper full rotation as illustrated in this figure The subject of CT is more elaborately described in the booklets German Die R ntgenpr fung and English The X ray Inspection see literature reference 3 17 4 CT for defect detection and sizing Effect of defect orientation Traditional radiography almost exclusively uses one single exposure from a fix position thus one direction of the X ray beam This can result in distortion of the defect image on the film see section 12 1 or even missing a defect This single shot practice also applies for weld inspection Welds and their adjacent heat affected zones might contain planar 2D defects possibly unfavourably oriented for detection The probability of detection POD of planar defects is strongly dependent on the angle between the centre line of the beam radiation angle and the orientation of the defect as shown in figure 13 17 Only transmission under an angle equal to or close to the orientation of the 2D defect will provide sufficient contrast Figure 14 17 illustrates this b Positions of X ray sourc
214. ure 15 4 or even inter crystalline shrinkage figure 15 5 may occur while if the solidification front shifts rapidly shrinkage cavities tend to occur figure 15 6 Gas cavities in the form of porosity or larger gas holes can occur either due to a damp mould or release of gas from the molten metal and can be particularly troublesome in cast light alloys figure 15 7 Cracks can also occur in castings If they are formed while the metal is still semi solid they are usually called hot tears figu re 15 8 if they occur when the metal has solidified they are called stress cracks or cold tears figure 15 9 A collection of radiographs of defects in iron steel castings is provided in ASTM E446 and for aluminium in ASTM E155 Fig 15 1 Radiograph of an aluminium casting 135 Fig 15 3 Shrinkage worm hole cavities in a high heat conductive copper casting Fig 15 2 Radiograph of an aluminium precision casting Exposure on D2 film at 75 kV 5 mA 3 5 min film focus distance 100 cm 136 137 Fig 15 4 Filamentary shrinkage in an aluminium alloy casting Fig 15 6 Shrinkage cavities in a bronze casting Fig 15 7 Gas holes and porosity in an aluminium alloy casting Fig 15 5 Micro shrinkage layer porosity in a magnesium alloy casting 138 139 y r AD Ls p s Fig 15 8 Hot cracks hot tears Fig 15 9 Stress cracks cold tears Fig 15 10 Radiograph of an alum
215. ure time Generally a focus to film distance of 2 5 X Dinsulation 1S chosen The correction factor would then be 2 5xD 0 5xD 2 5xD 0 8 insulation insulation insulation wall thickness Selection of source screens and filters The graph in figure 7 18 indicates which radio A 25 50 75 100 150 200 250 300 350 400 450 500 active source is the most suitable depending Ancien on pipe diameter and wall thickness The qua Fig 7 18 Areas of application for selection of source lity of the radiograph can be optimis ed by screen and filter in on stream radiography applying filters and screens SEE table 1 18 This graph appears enlarged in theappendix Source type Source size Screens front and back Iridium192 0 027 mm Pb Iridium192 0 027 mm Pb Cobalt60 0 5 mm Cu of RVS Cobalt60 3 mm 0 5 mm Cu of RVS 2 mm Pb Table 1 18 Selection of source screen and filter for the various areas in figure 7 18 199 Exposure time Obviously different exposure times are required for gas filled or liquid filled pipelines Below are a few examples For gas filled pipelines Depending on diameter and wall thickness Iridium192 or Cobalt60 see figure 7 18 Focus to film distance minimum 3 x Dinsulation Irradiated thickness 2 x nominal wall thickness Film type minimum C5 EN584 1 Film density minimum 2 5 in the centre of the pipe projection For liquid filled pipelines Depending on the diameter
216. used for so called 3D metrology CT metrology systems replace con ventional physical or optical measuring devices for components with complex geometries or measure dimensions at places with no access at all These systems include the software to transfer the part to be measured visible on the CT image into actual dimensions with accu racies of 1 micron High resolution and defect sizing In CT absorption values are determined with a very high degree of accuracy which means that the contrast of an image can be varied over an extraordinaraly wide range Absorption density variations of 0 02 can be displayed in a range of density 6 and over This offers great possibilities for image processing For the most challenging X ray inspections the best results are obtained by high resolution CT using microfocus or nanofocus X ray sources The achievable resolution or image sharpness is primarily influenced by the focal spot size of the X ray tube Defect detectability down to 250 nm 0 25 microns is possible Increasingly 3D CT is used on high quality castings often in combination with automatic object and defect identification Sometimes the magnification factor is not sufficient In that case the factor can be increased by scanning only the region of interest To achieve maximum magnification the region of interest should be within the X ray beam cone as illustrated in figure 12 17 186 Limited rotation Dedector lt 360 X ray so
217. utomatically completes the processing cycle After completion of this process including erasing the latent image the cassette is released from the CR scanner and ready for re use Figure 4 16 shows a typical tower type man size automated scanner In smaller and portable desktop scanner models intended for use at remote locations e g on offshore platforms the CR imaging plate is manually removed from the cassette and inser ted into the scanner which slightly increases the risk of the plates being damaged In addition to flat bed CR scanners drum type scanners exist Figure 5 16 shows an example This scanner can handle unlimited lengths of CR plates 149 For desktop scanners the cassette can be opened as shown in figure 6 16 CR plates can be exposed to subdued light lt 10 lux a candle creates 5 lux for one minute with acceptable consequences for the image quality The effect strongly depends on the type of the light source e g tube light causes most damage to the latent image The scanned image is ultimately or instantly made visible on a high reso ais lution monitor computer screen of the worksta tion see figure 33 16 Depending on the line distance selected typically 50 or 100 microns the plate speed lengthwise progress is 5 to 10 mm per second This is similar to the speed of digitisation of a traditional film In all scanners the latent image on the plate is not only read but also subsequently erased reset which t
218. vers a density range from 0 to 4 a differen ce corresponding with a factor 10 000 Contrast The contrast of an image is defined as the relative brightness between an image and the adjacent background The contrast between two densities D4 and Do on an X ray film is the density difference between them and is usually termed the radiographic contrast Film contrast or emulsion contrast are rather vague terms used to describe the overall contrast inherent in a particular type of film When an emulsion type shows most of the image contrasts present the film is said to be of high contrast or hard For the measurement of film contrast the term film gradient is used for which the sym bol is Gp Suffix p indicates the density at which G is measured 60 7 4 Characteristic curve density curve The characteristic or density curve indicates the relationship between increasing exposures and resulting density By exposure E is meant the radiation dose on the film emulsion It is the product of radiation intensity Io and exposure time t therefore E lgd The ratio between different exposures and related densities is not usually plotted on a linear scale but on a logarithmic scale i e density D versus log E The curve is obtained by applying increa sing exposures to a series of successive areas of a strip of film whereby each follo wing exposure is a certain factor for example 2 greater than the previous
219. vestment High performance DR detectors are most suitable on stationary locations for example as part of a production line where vast num bers of precision components are checked at high speed with the r lowest possible radiation dose or k ey in situations where mechanical automation robotising can be applied to achieve significant throughput improvements see figure 31 16 Fig 31 16 DR inspection of narrow beam weld and turbine blades Girth weld inspection Instant results as provided by DR systems would also very attracti ve to replace film to inspect the circumferential girth welds of long distance pipelines under construction either on land or at lay barges Until recently DR systems were more complex and vulnerable than equipment for film radiography thus not suitable Fig 32 16 CMOS we ee courtesy Envision USA for harsh field conditions But this has changed The resolution of DR although better than CR is still 2008 too low compared to film to meet the image quality requirements for the majority of such girth welds Nevertheless for some type of girth welds systems using CMOS line detectors with small pixels combined with a high contrast resolution which rapidly orbit around the pipe could provide a solution They can scan a weld in a few minutes with a reasonable image and contrast resolution Figure 32 16 shows the scanner of such a system The radiation source can be located insi de the pipe
220. with this type of equipment because of insulation problems 2 Upto 450 kV mainly for use on continuous stationary or semi ambulatory work because of their dimensions limited manageability and weight Tubes are of the bipolar direct current type 3 Upto 16 MeV so called Megavolt equipment Virtually exclusively applied to stationary work The first two categories are suitable for radiography on most common objects Objects of extreme thickness however require an energy even higher than 450 kV In this case Megavolt equipment is used if alternative sources such as Cobalt60 prove unsuitable It will normally involve stationary installations of large dimensions and high weight Lately portable versions have become available meant for ambulatory use Types of X ray tubes Depending on the shape of the anode X ray tubes produce a abeam of radiation in one direction directional tube b an annular beam panoramic tube X ray tubes are either unipolar or bipolar Bipolar tubes Figure 1a 5 shows a bipolar tube The bipolar tube has the advantage that the potential dif ference with respect to earth on both the anode and the cathode is equal to one half of the tube voltage which is a great help from the point of view of insulation The exit window is necessarily situated in the middle of the tube Bipolar tubes usually operate on direct cur rent and are generally air oil or water cooled They are designed to operate at voltages of
221. wn in example C The result is that image C not only becomes unsharp but also suffers a reduction in contrast compared to image A made with a point source and image B made with a relatively small source Inherent unsharpness Not only the silver halide crystals directly exposed to X radiation are formed into grains of silver but also albeit to a lesser degree the surrounding volume of emul sion This cross sectional area represents the inherent unsharpness or film uns harpness Ur test plate So even in the absence of geometric uns harpness if the radiation energy is high enough film unsharpness can occur the so called inherent unsharpness If a steel test plate with a sharp thickness transition is radiographed with high energy X rays there will be a gradual transition of film density across the image of the step from Ato B 7 S S Without inherent unsharpness the film would show an absolutely sharp transition between the two densities as shown in figu re 3a 11 In practice the density change across the image is as shown in figures 3b 3c and 3d 11 The width of this transitional area U expressed in mm is a measure of film unsharpness For clarity the density curves are magnified along the X axis a density distribution across image of sharp edge assuming Ur 0 b c and d density distribution due to film unsharpness b theoretical c with grain d smo
222. xchange of data Special radiographic techniques Image magnification techniques Common image magnification technique High resolution X ray microscopy Magnification factors Microfocus and nanofocus X ray tubes Tube heads System set up Effect of focal dimensions Imaging systems for high resolution radiography Fluoroscopy real time image intensifiers Stationary real time installations Portable real time equipment Computer Tomography CT Unique features Computing capacity and scanning time Reverse engineering CT metrology High resolution and defect sizing 163 164 165 167 168 171 177 177 177 178 182 185 17 4 Vo 17 6 18 18 1 18 2 18 3 18 4 18 5 18 6 13 7 18 8 19 19 1 19 2 19 3 19 4 19 5 19 6 7 19 8 CT for defect detection and sizing Effect of defect orientation 3D CT for sizing of defects in welded components Neutron radiography neutrography Compton backscatter technique Special radiographic applications Measuring the effective focal spot Radiographs of objects of varying wall thickness Radiography of welds in small diameter pipes Elliptical technique Perpendicular technique Determining the depth position of a defect Determining the depth position and diameter of reinforcement steel in concrete On stream inspection profiling technique Projection technique Tangential technique Selection of source screens and filters Exposure time Flash radiography
223. y factor is applied is the equivalent dose H The equivalent dose is the product of absorbed dose D and quality factor Q so the equi valent dose is calculated as H D Q Sv Sv Sievert The Q factors for various types of radiation are indicated in table 1 19 Type of radiation Quality factor Q X and gamma radiation y 1 Beta radiation f 1 Alpha radiation a 20 Fast neutrons 10 Table 1 19 Q factors for various types of radiation 19 2 Responsibilities The client It is the client s responsibility to consider possible alternatives before utilising ionising radi ation Considering its purpose the decision to use ionising radiation can only be justified when the radiation hazard remains at an acceptable level The radiographer It is primarily the radiographer s responsibility to protect himself and others from exposure to radiation 207 19 3 The effects of exposure to radiation The understanding of the effect that exposure to radiation has on human beings has grown over the past 50 years and has led to a substantial reduction of the maximum permissible dose There are two categories of biological effects that an overdose of radiation can cause soma tic and genetic effects Somatic effects are the physical effects A reduction in the number of white blood cells is an example of a somatic effect Much more is known about the somatic than about the genetic effects of radiation Blood cells are very
224. y be compensated by adjusting the developing time The recommended developing time for Agfa films in G128 manual developer is 5 minutes at 20 C In the automatic process using G135 developer the developing time is 100 seconds at 28 C Deviating from the recommended developing times and temperatures will almost always lead to reduced image quality e g increased coarse graininess Raising the tank temperature will speed up the development process as table 1 10 shows but the developer will oxidise more rapidly Should it not be possible to achieve a bath temperature of 20 C the following developing times can be used at the temperatures as indicated in table 1 10 This applies to all D type films Tempe 18 20 2 24 26 EEEN Taes 6 8 4 38 3 25 2 Table 1 10 Developing time versus developer temperature The temperature of the developer shall never be less than 10 C but is preferably higher than 18 C to obtain optimal image contrast It is best to always maintain the same developing conditions so that the exposure technique can be matched to these and uniform results obtained 87 Film agitation To prevent air bubbles from forming on the surface of the emulsion which will cause spots on the finished radiographs and to make sure that the developer penetrates all areas of the emulsion evenly the films should be kept moving during their first 30 seconds in the developer After that it is recommended to move
225. y take months before yellow fog becomes apparent Dichroic fog i e greenish yellow by reflected light pink by transmitted light 1 developer contaminated with fixer 2 film insufficiently rinsed after development and subsequently fixed in exhausted fixer 3 film stuck to another film when placed in fixer in which case the development continues in the fixing bath 4 prolonged development in exhausted developer 5 film partly fixed in an exhausted fixing bath exposed to white light and then fixed again Mottled fog A greyish mottled fog generally means the film is out of date or that it has been stored under unfavourable conditions e g in damp surroundings Whitish deposit 1 water used to make up developer or fixer too hard 2 wash water too hard 3 film insufficiently rinsed after development Clear patches 1 minute round spots with sharp edges the film was not kept moving in the first 30 seconds of development 2 drops of fixer or water fell onto the film before development 3 marks from mechanical damage to the emulsion before exposure 4 marks due to rapid and uneven drying of the film this occurs when there are still droplets of water on the film when placed in the drying cabinet 5 clear patches can occur from the film sticking to another film or to the tank wall during development 6 grease on the film slowing down or preventing the penetration of the developer 120 7 screen s in poor condition 8 foreig
226. yers The steel and magnet plate would otherwise absorb the low energy radiation 150 Dynamic range Exposure latitude CR plates have an extremely wide dynamic range exposure latitude In practice the phosphor crystals on a CR plate cannot be saturated and react almost linearly to incident radiation while in a conventional film the silver halide crystals react exponentially see the graph in figure 8 16 As a result the dynamic range of a CR plate is much Films CR plate E wider than for conventional DR p film which makes exposure times less critical reducing re shoots re takes and allows various material thicknesses to be examined AEN at the same time The wide l dynamic range of film dynamic range can also be useful in case of under expo sure this can be compensated 1000 10000 _ for by a more sensitive read ees out scan or image adjustment Fig 8 16 Density intensity versus dose for film and digital methods at the work station a Sg 5 Intensity digital methods Useful dynamic range of CR DR This wide range is illustrated imaging pae in figure 9 16 The images have been obtained from a stepwedge from 5 up to 25 mm thickness in steps of 1mm The digitised image of the film shows only a portion of the step wedge thicknesses the logarithmic CR image shows all steps proportionally The matching analogue records at right handside of this figure c

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