Student`s Manual
Contents
1. Table 3 Elemental composition wt age of Si brass standard sample obtained from Brammer 16 4 Experiment 1 Elemental analysis In the first experiment we will learn how to analyze a material sample to find its constituent elements and determine their relative concentrations We will start off by obtaining the spectral data of a stainless steel sample SS 316 Data will be acquired and analyzed using two softwares ADMCA 2 0 and XRS FP The assembly of the apparatus and its various components is shown in Figure 9 One should make sure that the PX4 X ray tube and the detector are all connected with the computer interface The equipment manuals 12 13 15 should be consulted for proper procedures and precautions The spectrum processing and concentration analysis carried out with the aid of the two softwares ADMCA and XRS FP is illustrated as a flow chart in Figure 8 Never switch the X ray source ON without the shielding in place Do not expose yourself to direct or reflected X rays Do not touch the X ray tube when it is switched ON Do not touch the Be window of the detector Be very careful while plac ing and mounting the detector any sudden movement or the slightest mechanical shock can damage the detector 4 1 Acquiring spectrum in ADMCA The ADMCA program 17 is the main display and acquisition software It is a Windows software package that provides data acquisition display and control for2. x Sample mount 110 220 VAC ADMCA software Mini X software XRS FP software HASP plug Computer 9V AC DC 9VD adapter Mini X een X ray tube USB P2774 Lr rm XR100 Si PIN n pus detector Digital pulse processor 5VD 5V AC DC co adapter mm 110 220 VAC Mini X Brass aluminum shield Sample XR100 Figure 9 a Diagram of EDXRF spctrometer components connected to com puter for spectral data acquisition and concentration analysis b X ray tube and Si detector mounted on the base plate to irradiate the sample material The brass aluminum box positioned to shield the experimenter from incident as well as reflected X rays Figure 10 Stainless steel sample mounted on the sample holder Sample holder Shielding base plate in the lower right corner Also check if the green LED in PX4 s power button is blinking as it indicates acquisition If yes stop acquisition 4 Make sure that the safety interlock is plugged in carefully at the back of the Mini X tube as shown in Figure 11 a Plug the adapter into AC mains 18 5 Start the Mini X controller software and click the Switchonthetube button In a couple of seconds the software should indicate that Mini X control is ready 6 Set the voltage to 30 kV and current to 30 uA and turn on the source by clicking the HV ON
3. 3 the radioactive decay of some atomic nuclei Each mechanism leads to a typical spectrum An X ray tube is a commonly used device for the generation of X rays by bom barding highly accelerated electrons on a heavy metal target X ray production in this manner results from the first two of the mechanisms listed above A schematic for producing X rays is shown in Figure 1 Electrons are ejected thermally from a filament behind the cathode and accelerated towards the heavy metal anode by a high voltage in kilovolts range Upon hitting the target anode these fast electrons decelerate and lose energy in the form of high energy photons 4 Heavy metallic target Cathode Acclerated electrons hitting the target anode Glass housing Heated filament emits electrons Emiited X rays Bremsstrahlung characteristic X rays Figure 1 A simplified picture of an X ray tube illustrating process of generating X rays These photons are the X rays with precise value of energy depending on the kind of target used The intensity of the X rays produced is dependent on the number of electrons hitting the target or tube current which in turn depends on the temperature of the filament emitting the electrons However increasing the X ray tube current at a constant X ray tube voltage increases the X ray intensity without affecting the energy distribution 1 2 The production of X rays by two different atomic pro
4. Amptek s signal processor PX4 It also calibrates the hardware by assigning energy values to its channels so that an energy spectrum of the sample can be visualized 15 a BITES Amptek ADMCA D MDMCA 2 0 IARM 158 mca File View MCA Display Analyze DPP Help Go amp SE isg x BE eo uuum p M HW Na ep D b Lum poem 7 me ne t ve penay Fua ox Te pup mur mi 0 Global Threshold Setings Clear Casc Method c ee Lom F oaeen ET en eea cee Tere Ta Taen vere feere uere verer ia oen re Terea j recer Monsurament Processing Conditions Menserament C Processing Active Condition Code 1 gf ey Source a chambar Tene sacs EA a e A O Fa a lal sas eames O O 4085 19 40 o Comracnt Strii Figure 7 Screenshots of a ADMCA software for elemental identification and b XRS FP concentration analysis software 4 1 1 ADMCA Features e full control of the hardware features available in the connected hardware PX4 and detector e live display of the spectrum Capable of displaying and calibrating up to 8142 channels of the MCA e spectral calibration and qualitative analysis and e an active link to the XRF FP Quantitative Analysis Software Package 16 channel to energy Raw spectrum channel Figure 8 Spectrum processing and concentration analysis steps carried out by the ADMCA and XRS FP softwares The first two steps are p
5. R and scattering factor C can be calculated 24 Plot the graph of your values and discuss your results Does your result verify Moseley s Law Use your graph to calculate the value of the Rydberg s constant and scattering factor How close is your value to the theoretical value of R 13 60 eV BONS Calculate the uncertainty in your calculated values of R and 25
6. a The tube voltage also influences the production of characteristic radiation No characteristic radiation will be produced if the voltage is insufficient to overcome the binding energy of the K shell electrons corresponding to a threshold voltage as shown in Figure 4 a and b 2 4 Detection of fluorescent X rays The detection of X rays is based on various methods The most commonly known methods in the past were photographic plates Geiger counters and scintillators but from 1970 onwards semiconductor detectors have been developed and used using silicon or germanium as the detection elements These detectors detect individual X ray photons that are reacting with the detector material Each individual photon is detected and then over time accumulated measurements make an accurate picture of the radiation coming from the source A PIN diode detector is today the most commonly used solid state X ray detector It consists of an intrinsic semiconductor region sandwiched between a p type and n type material as shown in Figure 5 The X ray photon enters the intrinsic region and causes an avalanche multiplication of charges and the reverse bias field sweeps the charges out of the region resulting in a detectable and measurable current The mechanism of current production is illustrated in Figure 5 Each X ray photon absorbed in the detector creates an electron hole pair The ejected electron will possess an amount of kinetic energy equal to
7. electrons hitting the outer zones experience a weaker force and produce lower energy photons The outer zones capture more electrons and create more photons For this extremely simplified model an X ray energy spectrum is predicted to be like the one shown in Figure 3 a a Electrostatic field regions around the nucleus Counts number of photons Energy keV b Ideal Bremsstrahlung curve Counts Corresponds to the maximum voltage set for the tube Experimentally obtained curve Energy keV Figure 3 a A model for bremsstrahlung production and the associated X ray photon energy spectrum b an ideal bremsstrahlung curve shown as dashed line compared to the experimentally obtained solid curve Discuss the bremsstrahlung curve and its shape From Figure 3 b discuss the ideal and the experimentally obtained bremsstrahlung curves and com ment on the reason for deviation from the ideal behavior The high energy end of the bremsstrahlung spectrum is determined by the tube voltage kV which establishes the energy of the electrons as they reach the anode Higher the tube voltage greater would be the number and energies of electrons 7 striking the inner zones of nuclear force resulting in higher energy X ray photons In the spectra of Figure 4 distinguish between characteristic X rays and bremsstrahlung radiation Why is the Figure b more spread out along the energy scale as compared to
8. American Journal of Physics 64 pp 335 338 1996 Mini X Users Manual Amptek Inc http compassweb ts infn it richi Stefano Amptek SW Mini X Mini X X ray detector http www amptek com pdf xriOO0cr pdf Digital Pulse Processor Amptek Inc http www amptek com pdf dp4 pdf Amptek Experimenter s XRF Kit Quick Start Guide Amptek Inc 3 16 http www dengeteknik com tr veri dosyalar metal chip nonferrous pdf 17 Amptek ADMCA Display and Acquisition Software http www amptek com admca htm1 18 Quantitative Analysis Software for X ray Fluorescence http www amptek com pdf fp pdf 19 XRS FP Quick Start Guide for Experienced Users version 3 3 0 Amptek Inc 20 XRS FP Software Guide version 4 0 4 Amptek Inc http crossroadsscientific com Documents XRS FPMA20SoftwareMV 20GuideN 420v404 pdf 21 Amptek K and L Emission Line Lookup Chart http www amptek com pdf xraychrt pdf 22 R M Rousseau The Quest for a Fundamental Algorithm in X ray Fluo rescence Analysis and Calibration The Open Spectroscopy Journal 3 pp 31 42 2009 2 Theoretical introduction X rays are part of the electromagnetic spectrum with energies ranging from 0 1 to 100 keV 2 1 Production of X rays X rays are produced by one of the three following mechanisms 1 deceleration of high velocity electrons in the vicinity of a target nucleus 2 atomic transitions between discrete energy levels and
9. X Ray Fluorescence XRF spectrometry for materials analysis and discovering the atomic number Asma Khalid Aleena Tasneem Khan Junaid Alam and Muhammad Sabieh Anwar LUMS School of Science and Engineering August 25 2015 Version 2015 1 X Rays were discovered in 1895 by the German scientist Wilhelm Conrad Roent gen This discovery opened doors for the development of X Ray Fluorescence XRF spectroscopy which has now become a powerful and versatile technique for the analysis and characterization of materials It distinguishes different elements present in a sample according to the characteristic X ray energies emitted by them and helps in determining their respective concentrations In this experiment we will use XRF spectroscopy to analyze a sample s elemental composition From the characteristic X ray energies we will also verify Moseley s Law which is a proof of the existence of a fundamental quantity the atomic number The atomic number increases in regular steps with an increase in the characteristic X ray energy We will use this realtionship to find the Rydberg s energy constant and screening coefficient for K X rays KEYWORDS X Ray Fluorescence XRF Characteristic X Rays Bremsstrahlung Radiations Moseley s Law Atomic number Screening coefficient Rydberg s energy Contents 1 Objectives 2 2 Theoretical introduction 4 Jd Production of Kays o 25 Len 2s QU 5 ate P E a e a G 4 2 2 X ray fluorescence p
10. acquired and saved in ADMCA has to be opened in the software named XRS FP a quantitative analysis software 18 19 package for X ray fluorescence It processes the raw X ray spectral data from Amptek s detector signal processing electronics and ADMCA spectrum to obtain the elemental peak intensities and the elemental concentrations XRS FP does spectrum processing requiring as input the parameters which de scribe the spectrometer itself e g type area and thickness of the detector the distance between the tube and the sample etc and parameters which control the processing Prior to running the analysis appropriate the settings in the Setup menu should be entered Figure 7 b captures a screenshot of XRS FP window Before starting this section students are strongly encouraged to refer to the XRS FP guide 19 20 to have a detailed information of these parameters and their effects on the analysis What are sum peaks escape peaks and background peaks 20 Why is it important to remove these peaks 4 2 1 Spectrum processing in XRS FP An XRF spectrum consists of characteristic peaks superimposed on a background bremsstrahlung radiation and detector effects Spectrum is processed to effec tively extract the signal net peak intensity from the noise the background peaks XRS FP carries out the following processes to arrive at a table of elemental con 21 a ADMCA 2 0 U steel mca 02 01 2011 15 47 24 Regions of I
11. ared to the other peaks and the latter due to its horizontal separation from the main chunk of the spectrum Now perform the following steps to complete the calibration e From the ADMCA menu bar click the Define ROI button Click on the start and end points of the desired peaks on the horizontal axis one by one A list of selected regions will show in a dialog box showing the start and end values for each ROI Also the ROIs should turn turquoise e Click the Calibrate button on the menu bar Clicking on ROIs will show their start and end points in the Calibrate dialog box Click the Centroid button to select the centroid of the peak and enter the energy value corresponding to the selected ROI e g 6 4 for Fe Click the Add button and repeat for the other ROI This procedure is illustrated in Figure 12 19 Amptek Mini X Controller L Seven MEM MiniX Serial Number 00001708 X RAY ON Set Monitor High Voltage and Current 30 MIN MAX D Hv OFF Set High Voltage 30 10 40 kV and Current 30 1 200 u e N High Voltage Monitor 30 0kV Exit Current Monitor 30 004 Board Mars opem Cors MT Tem mead 44 C M 9 w om s a 4 MiniX Controller ready Figure 11 a Safety plug inserted to complete the circuit for high voltage produc tion in the tube b USB controlled Mini X s software window to send the final command to allow the X ray emission e In the Units box choose the appro
12. button as shown in Figure 11 b A periodic beep sound indicates that the tube is emitting X rays 7 Now start acquisition in ADMCA and observe the spectrum as it gradually builds in the display window Stop acquisition when counts exceed 50 000 Number of counts can be seen in the right panel of ADMCA 8 Stop the X ray source by clicking the HV OFF button in Mini X control window and plug out its adapter Make sure the source tube is never turned ON when you are not acquiring a spectrum 9 Save the acquired spectrum with a suitable name for example steel mca Do not exit ADMCA yet as you will be calibrating your spectrum next On the spectrum steel mca identify the characteristic peaks Use the X ray Chart 21 to identify peaks of Fe Cr and Mo 4 1 3 Spectrum calibration The spectrum saved in the last section shows only counts corresponding to different channels of the MCA To assign energy values to those channels is termed as calibration We can perform calibration by using a sample of known composition in this case steel 88 316 whose elemental composition and respective concentrations are provided in Table 1 For accurate calibration at least two peaks from the spectrum should be identified We can choose two elements Fe at 6 40 keV and Mo at 17 48 keV to be our references as they are reasonably apart on the energy scale Furthermore both the Fe and Mo peaks are easy to identify former due to its tallness comp
13. cesses the X ray fluorescence and the bremsstrahlung radiation is discussed below 2 2 X ray fluorescence X ray fluorescence is the emission of characteristic or secondary X rays from a material that has been excited by bombarding with high energy electrons or other X ray or y ray photons If the incident particle has enough energy it can knock out an orbital electron out of the inner shell of the target atom To fill the vacancy one of the electrons from the higher shells then jumps to the inner shell emitting in the process a photon with energy equal to the difference in binding energy of the two shells The process is illustrated in Figure 2 a The X ray fluorescence produces an emission spectrum of X rays at discrete en ergies These emission spectral lines depend on the target element and hence are called characteristic or fluorescent X rays We can use these spectra to identify the elements by comparing the peak s energy with the element s binding energy 3 XRF can yield results only for elements with Z gt 16 in air Explain why the lighter elements cannot be analyzed a Incident Primary X ray beam i Fluorescent L X ray u Fluorescence K X ray b Scaterred electron Incident electron Bremstrahlung photon Figure 2 X ray emission through a fluorescence and b bremsstrahlung radi ation a illustrates the characteristic emission of K and L X rays as a result
14. d Bremsstrahlung radiations use characteristic X rays to identify elements acquire a spectrum calibrate it and use it for qualitative element identifica tion as well as quantitative elemental concentration analysis and finally verify Moseley s law and the validity of an atomic number References and Essential Reading 10 11 12 13 14 15 http www dentallearning org course AdvancedRadiography DoctorSpiller x ray characteristics htm http hyperphysics phy astr gsu edu hbase quantum xtube html http www niton com portable xrf technology how xrf works aspx sflang en http www sprawls org ppmi2 XRAYPRO SBREMSSTRAHLUNG http www microsemi com micnotes 701 pdf http www byui edu physics Thesis Francom Brian2008 pdf http users skynet be xray corner xtb chap011 html H Holbrow N Lloyd C Amato E Galvez M Elizabeth Parks Modern Introductory Physics Springer New York Dordrecht Heidelberg London pp 536 542 2010 S B Gudennavar N M Badiger S R Thontadarya and B Hanumanaiah Verification of Bohr s Frequency condition and Moseley s Law An Under graduate laboratory Experiment American Journal of Physics 71 pp 822 825 2003 P J Ouseph K H Hoskins Moseley s Law American Journal of Physics 50 pp 276 277 1992 C W S Conover and J Dudek An undergraduate experiment on X Ray spectra and Moseleys Law using a Scanning Electron Microscope
15. e sample e detector and data collection and analyzing system 9 Characteristic A e X ray photon Figure 5 A PIN diode detector The characteristic photon produces a single electron hole pair if the electron produced has got enough energy the charge keeps on multiplying by collisions e and h represent electrons and holes respectively Si detector and preamplifier Multi channel analyzer X ray tube i A Sample Analysis software Element identification and concentration information Figure 6 The schematic of an EDXREF Spectrometer The X rays from the source irradiate the sample characteristic X rays are detected by the Si detector the multi channel analyzer separates different elemental peaks and the analysis software gives the final list of elements and their concentrations The EDXRF spectrometer helps plotting the relative abundances in terms of intensities of characteristic X rays versus their energy The characteristic X rays generated strikes the detector element in this case Silicon creating an electron hole pair which produces a charge pulse proportional to the energy of the X ray This charged pulse is converted to a voltage pulse by a charge sensitive preamplifier A multi channel analyzer MCA is then used to analyze these pulses and sort them according to their voltages This data is then sent to the computer interface where it is disp
16. e that the energy of an electron in a multi electron atom could be given approximately by UNE Ex 5 Explain what does the screening factor indicate Is there a way to deter mine this factor experimentally 3 Experimental Apparatus Amptek s XRF kit available and setup in our laboratory is a package designed to help the user quickly begin doing elemental analysis via X ray fluorescence Once this kit is assembled and the software configured and calibrated one can begin doing simple analyses The XRF kit consists of the following parts e XR100 CR detector with Si PIN diode to collect the X rays reflecting off the sample e Mini X USB controlled X ray tube being used as an X ray source e PX4 digital pulse processor is a pulse processor as well as a multi channel analyzer MCA in terms of counts it distributes the detected X rays over its physical channels with respect to their energy also working as the interface between the detector and the computer 13 e XRF mounting plate on which the X ray source and detector are mounted according to the guiding sketch imprinted on it and Be careful Be very careful when handling the XR100 Si detector the window of the detector is brittle and can be damaged beyond repair by mishandling Also touching the detector may interfere with its thermoelectric cooling system 13 Safety Note Before turning the X ray source ON make sure that the brass aluminium radiation
17. eaks by assigning them corresponding areas This process is called deconvolution the reverse of convolution 4 2 2 Procedure 1 After connecting the HASP plug available with the XRF unit to the USB port of the computer launch XRS FP Choose Expert mode to open the XRS FP main window The table on top left should be showing a list of elements From the Load dropdown menu select Spectrum which should import the acquired spectrum from ADMCA Next from the XRS FP Set up menu specify the parameters defining the detector and X ray tube types the thickness of the detector s window the filters used in the X ray tube and the geometry of the arrangement of source sample and detector This information can be taken from XRF maunals 15 17 In the Thickness In formation table define sample to be in Bulk mode and check the Normalize option to 100 in order to get weight percentages of elements Enter the voltage and current values used for the X ray tube into the Mea suring Processing conditions table Process the spectrum by choosing Spectrum gt All from the Process menu Finally click Analyze in the Process menu This should return you the percentage concentrations of elements Obtain a spectrum of the Cu film provided and using the ADMCA and XRS FP softwares perform the complete concentration analysis for the metal Run a complete ADMCA and XRS FP standard analysis for the Chromium alloy IARM 158B as done for stee
18. emissions are called X ray lines because they are analogous to the spectral lines in the visible light spectra of atoms and are a unique fingerprint of the emitter atom 8 2 6 3 Mathematical formulation of Moseley s law Moseley studied X ray line spectra and discovered a simple relationship that al lowed him to predict the frequencies energies of X rays for any element and to see that the charge of the atomic nucleus is the property that gives an atom its identity Moseley after studying the X ray line spectra in detail found that the most intense short wavelength line in the characteristic X ray spectrum from a particular target element called the K line varied smoothly with that element s atomic number Z From Bohrs theory of atomic structure something you have already studied in your Modern Physics class the energy of an electron in its orbit n is given by 2 TE E n REP Em 1 n2 where h is Planck s constant c is the velocity of light R 1 097 x 10 m is the Rydberg constant for an infinitely heavy nucleus Rpg hcR 13 06 eV is Rydberg energy Z is the nuclear charge and n is the principal quantum number used to designate energy levels The emission of radiation from the atom according to Bohr is due to the transition of the atom from an initial higher energy state E to a final lower energy state Ej and the frequency v of the emitted radiation is given by the condition oem
19. erformed by ADMCA conversion ________ gt Energy calibrated spectrum cl Energy keV background and escape amp sum peaks Processed removal gt _ gt spectrum lu Al Wd Deconvolution of peaks Table of intensities Intensity cts sec 551 08 Element Line Ka Ka Ka Ka Ka Ka Cr Mn Fe Ni Cu Mo 68 52 1 842 34 174 5 752 464 78 Matrix effect correction by standardless or FP calibration Table of conc Concentration 96 18 50 0 25 1 67 0 07 0 52 0 27 0 03 0 03 Cr Fe Ni Cu Ma 55 00 12 27 0 17 2 39 I l l lt and the remaining ones by XRS FP software It is advised to explore the available control and configuration options in the software by going through its drop down menus and buttons on the menu bar ADMCA allows you to choose peaks as Regions of interest ROIs and specify the respective energies they correspond to 4 1 2 Procedure 1 Mount the stainless steel standard sample SS 316 in the sample holder inside the shielding enclosure as shown in Figure 10 2 Switch ON the PX4 by pressing its power button until you hear it beep twice 3 Launch ADMCA Click ConnecttoPX4 when prompted by the software If properly connected to PX4 ADMCA will show a green USB connection sign 17 Base plat e a Radiation shield Colimator and filter LA X Ray l Radiation I Hazard A
20. hv Now a Ka X ray emission is due to transfer of an L shell n 2 electron to the K shell n 1 where a vacancy has been created by irradiating the atom with incident X rays prior to the transition Hence the energy of the Ka photon is using 1 1 1 2 Ex RZ x z Be oe which shows that the energy of characteristic Ka X rays is proportional to square of the nuclear charge In the X ray notation the subscript a refers to the transitions 12 of electrons from L to K shell A Kg X ray is emitted when electron jumps from an M n 3 to the K shell Moseley who was studying Ka X ray spectra at the same time as Bohr used this expression but modified Z to Z 1 to fit to his experimental data Thus Moseley s relationship was 3Rgs Z 1 Ex 1 3 The above equation is usually referred to as Moseley s law 9 2 6 4 Effective nuclear charge and the screening effect Moseley used Z 1 instead of Z in 3 which is attributed to the fact that the electron is not only attracted to the nuclear charge Ze but is also repelled by other electrons Within a few years this very idea had become commonplace in the understanding of the multielectron atom the true nuclear charge Z could be replaced by an effective charge given by Eg ET 4 where was called the screening constant 10 11 neighboring electrons screen or shield the nuclear attraction Thus 3 could be modified to stat
21. l Find the calibration coefficients for all the el ements of the alloy and their concentrations 23 5 Experiment 2 Verification of Moseley s law In this experiment we verify Moseley s Law as well as calculate the screening constant C for Ka X rays in order to have enough elements for the verification of the Moseley s law We will be using the following three known samples for the purpose e stainless steel 5 316 e chromium copper IARM 158B e silicon brass 31X WSB7 Always use gloves when handling these samples and place them in the desiccator after use 5 1 Procedure 1 Place all the standard samples SS 316 IARM 158B and 31XWSB7 one by one on the mount inside the shielding enclosure 2 Acquire their respective spectra in ADMCA The spectrum should already be calibrated if you have specified the path and filename of the calibration file 3 Save the spectra with appropriate names 4 Using the ROIDetail option in ADMCA the respective energy values for various elements can be seen 5 Use MATLAB to plot a graph between the atomic number and peak energies obtained 6 Linearize the graph to obtain values for its slope and intercept Moseley s law 3 is expressed as pA 2 ipe es une or which can be linearized by taking square root of both sides 3 R goo 3 goo VEx 4l Z Ka 4 4 G which resembles the equation of a straight line From the slope and the intercept of this line Rydberg s energy
22. layed as the spectrum of the X ray irradiated sample The spectrum is further processed to identify elements and quantitatively analyzed to find the 10 respective concentrations in a sample 6 What is a wave dispersive X ray fluorescence spectrometer What is the difference between EDXRF and WDXRF and advantages of using one over the other 7 2 6 Moseley s Law The power of XRF analysis was first realized by Henry Moseley in 1912 seventeen years after Wilhelm Roentgen had discovered the X ray 2 6 1 How atoms got their atomic numbers Mendeleev s periodic table of the elements was a significant advance in chemistry reflecting the similarities in the chemical properties of the elements and their peri odic recurrence with an increase in the atomic mass For over 40 years the atomic mass was a useful guide for scientists but it provided no explanation for the pe riodicity of properties of the elements During the early decades of the twentieth century dramatic advances in physics revealed the structure of atoms and uncov ered the physical basis of the periodic table The atomic number was explained as the number that specifies the position of an atom in the periodic table and is the number of positive charges in the atomic nucleus The basis was laid in 1911 when Rutherford discovered the atom s nuclear core after which Bohr in 1913 showed that the nuclear charge Ze determines the scale of the energy states of an atom In the
23. nterest Details b Figure 13 a Output from ADMCA Region of interest defined for the six selected green peaks b ROI detail showing the initial and final positions for the each peak the centroids and the elements whose X ray line exists at that centroid value centrations e Spectrum smoothing Smoothing of the spectrum is the first step in spec tral processing This operation typically performs a Gaussian smooth of each channel in the spectrum for the specified number of times e Si escape peak removal Escape peaks result from fluorescence inside the detector material Si due to which a fraction of the parent characteristic 22 X ray gets lost as Si K escape photons with an energy of 1 75 keV This energy loss has to be accounted for before proceeding to final analysis Sum peak removal When two X ray photons arrive quicker than the PX4 hardware allows the corresponding counts bear energy that is the sum of the two photons Such coincidences are to be filtered out to get a precise result Background removal The background arises primarily from bremsstrahlung X ray continuum from an X ray tube whose shape depends on the anode atomic number and incident electron beam energy Only after subtracting the background from the acquired spectrum can a true spectral representa tion of the sample be obtained Deconvolution Finally to calculate the net peak intensities the spectrum is reconstructed as a sum of separate p
24. of electronic transition from L to K and M to L shells respectively b shows a decelerating electron emitting bremsstrahlung X rays 2 3 Bremsstrahlung radiation Bremsstrahlung is a German word for braking radiation Accelerating charges give off electromagnetic radiation In an X ray tube depicted in Figure 1 electrons travel from cathode with high speed towards the anode and penetrate the anode material When these electrons pass in close proximity to the strong electric field of the nucleus they get deflected and are decelerated by the attractive force from the nucleus hence radiating X rays which are called braking or bremsstrahlung radiation The production of these X rays is illustrated in Figure 2 b This gives off a continuous distribution of radiation which becomes more intense and shifts toward higher frequencies when the energy of the bombarding electrons or the tube voltage kV is increased 4 The bremsstrahlung spectrum can be described as follows An electrostatic field exists around the nucleus in which electrons experience the braking force The nuclear field can be imagined as a target with the actual nucleus located in the center as shown in Figure 3 a An electron striking anywhere within the target experiences a braking force and produces an X ray photon Now the electrons striking closest to the center are subjected to the greatest force and lose the most energy to produce the highest energy photons while the
25. priate units and click OK e Clicking the Enable calibration button on the menu bar will convert the horizontal axis to energy units finally showing the intensity versus energy spectrum for the sample e Save the calibrated spectrum file with an appropriate name Also opening Preferences from View menu specify the file path and file name and check the box for loading this calibrated spectrum every time the ADMCA is run 17 Our software and hardware has now been calibrated with the energy scale Once the spectrum has been calibrated a qualitative analysis can be carried out by importing libraries for Ka Kg La or Lg lines from the Analyze menu A typical result of the analysis is shown in Figure 13 20 File View MCA Display Analyze DPP Help Sust nAg BE EU wrtne ew m EE sd Mark ROI Button Calibrat Cursor Calibrate Button Channel Value Deviation 983587 64 D Add s 2705 91 17 443 0 Replace Remove Remove All Cursor Lj Centroid 6 Units Energy keV Plot Calibration Equation n A 0 0310732 B 0 00645774 Auto Calibration Ctrl F5 Units keV Value 5 89 Peak 2Value 5 43 Method 2 Peak Centioid az Energy kev 6 40 XT EX Count 33268 0 54832 CET oew zw Figure 12 The ADMCA display window showing the calibration dialog box 4 2 Quantitative analysis with XRS FP To run quantitative analysis the spectrum
26. same year Moseley measured the wavelengths of X rays emitted by many different kinds of atoms and showed that each chemical element is uniquely identified by its nuclear charge In other words the nuclear charge number Z specifies the position of an element in the periodic table and is therefore the same as the atomic number which is the serial number of the element in the periodic table Hence the properties of X ray line spectra were the basis of Moseley s discovery and this is how elements got their atomic numbers 2 6 2 X ray line spectra In 1905 a decade after Roentgen discovered X rays the British physicist Charles Barkla found that a target struck by a beam of high energy X rays primary incident beam emitted secondary X rays distinctly different in behavior from those in the incident beam He discovered that the secondary X rays emitted by a target are unique to the chemical element the target is made of so he called them charac teristic X rays and pointed out that they could be used to identify the target material Barkla had infact discovered a new means of chemical analysis From his measure ments of the absorption of X rays Barkla found that an anode emits two distinctly 11 different types of characteristic X rays a more penetrating type shorter wave lengths higher energy that he called K radiation or K X rays and a more easily absorbed type longer wavelengths lower energy that he called L radiation These
27. shield is properly in place to avoid exposure to radiation Also be careful in placing the shield making sure that it does not bump into the outer windows of the X ray source tube or detector 15 Q6 How does the Si detector measures the energy of the X ray photon 3 1 Standard materials used in the experiment We will use three kinds of standard reference samples in our experiment 3 1 1 Stainless steel SS 316 The composition of the stainless steel alloy is given in the following table Cr Mn Fe Ni Cu Mo 18 45 1 63 65 19 12 18 0 17 2 38 Table 1 Elemental composition wt Yoage of Amptek s stainless steel standard sample 12 3 1 2 Chromium copper alloy IARM 158B The composition of the chromium copper alloy is given in the following table Cr Ag AI Fe Mn Ni Pb Si Sn 0 85 0 01 0 002 0 09 0 019 0 32 0 01 0 02 0 01 Zn Cu As C Co P S Sb O 0 014 98 5 0 001 0 002 0 002 0 005 0 003 0 002 0 005 Table 2 Elemental composition wt age of chromium copper standard sample obtained from Brammer 16 14 3 1 3 Silicon brass alloy 31X WSB7 The composition of the silicon brass alloy is given in the following table Si Zn Cu Al Pb Fe Mn Ni 4 25 7 581 72 74 3 87 0 025 1 95 03 39 3 03 P Sb Sn As Bi Cd Co Cr 0 188 0 636 1 93 0 103 0 190 0 007 0 012 0 014
28. the difference between energies of the incident photon and the band gap of the detecting material This electron will collide with other atoms and will cause further ejection of charge carriers in the detector producing an avalanche of charges The migration of the electron and holes takes place under the influence of a voltage maintained between the p and n type faces of the detector which constitutes a pulse of current The pulses created are then amplified recorded and analyzed to determine the energy number and identification of the elements The sensitivity of these detectors is increased by operating them at low tempera tures which suppresses the random formation of charge carriers by thermal vibra tion 5 2 5 Energy dispersive X Ray fluorescence EDXRF A schematic representation of an EDXRF spectrometer setup is shown in Figure 6 The setup of EDXRF instrumentation is quite simple consisting of four basic components Counts a 5 61 11 32 17 03 22 75 keV 21 Counts 10 b 5 61 11 32 17 03 22 75 28 46 keV Figure 4 Mini X X ray tube output Spectrum with Ag as target anode a at 15 kV and 2 uA Clearly the tube voltage is insufficient to overcome the binding energy K shell electrons and produce Ag Ka X rays b at 30 kV and 2 yA the spectrum shows a triangular bremsstrahlung continuous spectrum along with a distinguished characteristic X ray peak of silver near 22 keV excitation source
29. u dei aep uie Yee wee 2 3 Bremsstrahlung radiation y Jo ery amp Ace p Lene RE Sx IRA 2 4 Detection of fluorescent X rays 22s 2 5 Energy dispersive X Ray fluorescence EDXRF 2b Moseley s LAW a ake ce th ae aoe REP SE RE ghe dre tes 2 6 1 2 6 2 2 6 3 2 6 4 How atoms got their atomic numbers X ray line spectra usu wes eo eo ene ER a Mathematical formulation of Moseley s law Effective nuclear charge and the screening effect 3 Experimental Apparatus 3 1 Standard materials used in the experiment 3 1 1 3 1 2 3 1 3 Stainless steel SS 316 oa a a Chromium copper alloy IARM 158B Silicon brass alloy 91X WSBTYE 438 Suede eR 4 Experiment 1 Elemental analysis 4 1 Acquiring spectrum in ADMCA sss 4 1 1 4 1 2 4 1 3 ADMCA Features aa a a a a a a Proced re ds iesu t em a ace ae dae aa ae da a a i fee a Spectrum calibration o oo ce REI Ix 4 2 Quantitative analysis with XRS FP 4 2 1 4 2 2 Spectrum processing in XRS FP ProCcedub ee oss wa eo es Ea aed tee en AE a eas 5 Experiment 2 Verification of Moseley s law HL Procedure 2 2 nie S hee thie Bett blac held eo LEN 1 Objectives In this experiment we will 13 14 14 14 15 15 15 16 17 19 21 21 23 24 differentiate between characteristic X rays an
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