X-Ray Fluorescence (XRF) spectrometry for materials
Contents
1. e All the standard samples S 316 IARM 158B and 31XWSB7 are used one by one to have enough elements for the verification of the Moseley s law e The sample is placed inside the shielding box with the flat surface facing the source and detector e The spectrum will already be energy calibrated By initially setting the preferences in ADMCA to load the calibsteel mca file e The new spectra are saved Make sure you save all your spectra on the network drive 6 2 1 Data analysis 1 The characteristic peaks of various elements are located in the sample The ROIs are used to look at the details The spectrum can also be deconvoluted in the XRS FP software to distinguish between peaks 2 A graph is plotted between the atomic number and the square root of peak energies obtained from the ADMCA peak information MATLAB can be used for data plotting 3 The slope and intercept of the plot are obtained and the uncertainties in the values of both slope and intercept are also calculated The results of one such analysis is shown in Table 5 When we performed the experiment the energy of characteristic lines for each element were plotted against their atomic numbers yielding Figure 17 Moseley s law 3 is expressed as Geran 2 Ex sn o 33 Element Atomic number Z Ka enrgies keV Ka enrgies y eV Cr 24 5 39 73 42 Mn 25 5 91 76 88 Fe 26 6 39 79 94 Co 27 7 03 82 40 Ni 28 7 46 86 60 Cu 29 8 02 89 552. The number placement in the array represents the channel number in ascending order as shown in Figure 8 A calibration is a process in which specific energy values are assigned to the correct channel in the MCA This is done by sending X rays of known energy values to the MCA Once the the channels where the X rays were sorted are identified known energy values can be assigned to those channels Since it is not practical to assign energy values to each channel manually the calibration process consists of identifying two channels with known energy values separated by a large energy and then applying a curve fitting technique to those data points Through linear approximation in this method all the channels in the MCA can be indirectly calibrated 6 Si detector Multi channel analyzer 0 9 8 1 1 2 5 7 0 a Channels gt 1 2 3 Ab E panin 1024 Energy calibrated channels gt ikeV 10keV eee 40keV Known element 2 Energy keV Known element 1 b Channel number Figure 8 a Multi channel analyzer MCA histogram memory representation The small boxes represent channels The numbers represent a count of X rays for a specific energy b linear calibration curve converting number of channels to energy scale using two known X ray peak enrgies 15 3 5 Radiation shield Radiation safety is very important The X ray tube represents a personnel hazard if i
3. 5 3 Acquiring a spectrum in ADMCA The stainless steel standard sample SS 316 is mounted in the sample holder as shown in Figure 14 The PX4 is switched on by pressing and holding the Power Sample holder Shielding base plate Figure 14 Stainless steel sample mounted on the sample holder button on the front panel of the device for 3 seconds until the beep sound is heard twice Next the ADMCA software is launched and from the DPP Dialog Box the option Connect to PX4 is selected This will start the acquisition mode Next the X ray tube is switched on using the following three steps 1 The green safety interlock is plugged in carefully at the back of the tube as shown in Figure 15 a Next the adapter is plugged into the AC mains 2 The Mini X controller is launched by clicking on the Mini X icon in the taskbar and the Switchonthetube option is selected 3 A voltage 30 kV and a current 30 yA is set and then the X ray tube is switched on by selecting the HV ON button on the software window as shown in Figure 15 b A beep beep sound is heard a blinking red LED can be seen at the rear end of the tube which indicates that the tube is emitting X rays The Start acquisition tab is pressed and the spectrum is observed as it appears on the ADMCA window As soon as 100 counts are exceeded the y axis of the spectrum is observed to keep track the X ray tube is switched off by using the HV OFF control on the software window and then
4. intensities for the elements of the sample material The Gaussian deconvo lution method corrects for peak overlaps by synthesizing peaks for all the expected lines specified in the element identification step in the region of the analyte peaks so that a complete peak fitting occurs for both the analyte and the overlapping lines Gaussian method uses nonlinear fitting to give greater flexibility to the peak deconvolution by allowing the peak ratios widths and positions to change in order to better fit the acquired spectrum Gaussian peak profiles can be adjusted in position width and shape which helps when there are changes in the spectra from matrix effects or changes in the X ray spectrometer performance gain offset and detector resolution 22 4 2 2 Concentration analysis in XRS FP software Once the spectrum processing functions extract the peak intensities of all the elements the final step for the concentration analysis is carried out The concen trations are calculated using the fundamental parameters FP function for the quantitative analysis 19 20 This module solves a set of non linear equations which relate the intensity of the X ray peaks to the concentration of the elements in the sample These equations include terms due to attenuation and enhance ment matrix effects in the sample attenuation in windows and air tube spectra scattering of X rays and thickness of filters used in the tube For the XRS FP concentrat
5. use characteristic X rays to identify elements 1 3 acquire a spectrum calibrate it and use it for qualitative element identifica tion as well as quantitative elemental concentration analysis and finally 4 verify Moseley s law and the validity of an atomic number References and Essential Reading 12 13 14 15 16 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 BREMSSTRAHLUNG 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 Ame
6. Zn 30 8 89 94 29 Ge 32 9 89 99 45 Mo 42 17 47 132 17 Ag 47 22 17 148 89 Table 5 The atomic number of elements and their corresponding characteristic line energies obtained from standard samples SS 316 IARM 158B and 31XWSB7 Taking square root of both sides the equation becomes VE 4 Z Ka 4 A G which resembles the equation of a straight line y mg c with slope m 3RE 4 and intercept c Re Substituting the values of slope and intercept for this graph and calculating un certainties we arrive at the values of Rydberg s energy and scattering factor Rego 14 33 0 28 eV 1 60 0 33 eV 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 Rgs 13 60 eV Fong Calculate the uncertainty in your calculated value of R and 34 Ky energies Jev 150 T T T T iaol M 3 28 0 02 Ag c 5 26 0 76 130 J 120 J 110 J 100 J 90 J 80 a 70 Fe I 1 I I 20 25 30 35 40 45 Atomic number Z Figure 17 Typical result for the verification of Moseley s law 35 50
7. box appears The Expert mode is selected and the main screen pops up as shown in Figure 9 b The commands and controls of the software have already been described in Section 4 2 2 A new file is opened from the File menu and saved as steelanalysis t fr 3 The calibrated spectrum for the SS 316 sample is loaded into the XRS FP software from the Acquire menu 4 Next from the XRS FP Set up menu we need to set all 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 the geometry in which the sample tube and detector are mounted This information can be taken from XRF maunals 15 17 29 5 The desired quantization mode standardless or FP method is selected from the Quant option available under the Setup menu 6 The sample s geometry is defined in the Thickness Information Table Bulk mode is selected and the Normalize option is checked to 100 in order to get weight percentages of elements on a scale of 100 7 The voltage and current values used for the X ray tube are entered into the Measuring amp Processing conditions table After loading the spectrum into the XRS software and substituting all parameters and selecting the standardless mode from the Quant menu there remain three major steps in a standardless quantitative analysis 1 Element identification Known samples Elements present in the standard sample provided in Tables 1 2 and
8. 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 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 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 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 5 a Incident Primary X ray beam Fluorescent L X ray 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 of elect
9. referred to as Moseleys law 9 2 5 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 12 the understanding of the multielectron atom the true nuclear charge Z could be replaced by an effective charge given by Zef 2G 4 where was called the screening constant 10 11 neighboring electrons screen or shield the nuclear attraction Thus 3 could be modified to state that the energy of an electron in a multi electron atom could be given approximately by 3Rro Z Eg teole lt 5 Explain what does the screening factor indicate Is there a way to deter mine this factor experimentally 3 Description of equipment 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 3 1 Amptek s Hardware Components The XRF kit includes e XR100 CR detector with Si PIN diode e Mini X USB controlled X ray tube PX4 digital pulse processor e XRF mounting plate e stainless steel SS 316 test sample 3 2 Mini X X ray tube The X ray source tube contai
10. scientists but it provided no explanation for the pe riodicity of properties of the elements During the early decades of the twentieth 10 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 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 5 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 discovere
11. standard containing multiple elements is convenient to use In this Section the SS 316 sample and its calibrated spectrum calibsteel mca already acquired and saved in the above sections will be used and referred to 30 5 6 1 FP analysis for stainless steel After loading the calibsteel mca spectrum into a new file of XRS FP saved as F Pcalib tfr and inputting all the parameters in the Setup menu Thickness Information and Measuring amp Processing conditions tables there are five steps two addi tions to the standard less mode that are followed for FP calibrated concentration analysis 1 Element identification The known elements of SS 316 are entered to the Specimen Component Table 2 Initial estimates of concentrations Against each element the known concentration values for each element from Table 1 are entered in the Specimen Component Table 3 Spectrum processing All spectrum processing steps are run to extract intensities 4 Calibration coefficients calculation The Calibrate FP command is run which iteratively proceed and finally calculates the best estimates of the calibration coefficients or the sensitivity factors for each element of the stainless steel sample The coefficients are displayed in the Element Table against each element Save this file again to save the calibration coefficients into the F Pcalib tfr file 5 Concentration Finally the Process and Analyze commands are run to calculate the m
12. whose elemental composition and concentration are provided in Table 1 To start off a few characteristic peaks need to be identified and given as reference peaks to ADMCA for calibration During calibration care should be taken that the reference peaks are such that a wide energy range is covered For accurate calibration at least two peaks from the spectrum should be identified Two elements Fe at 6 40 keV and Mo at 17 48 keV are chosen as reference 27 peaks Fe is easy to identify as this will have the most intense peak in the spectrum having the highest concentration in steel The peak of Mo is chosen as it lies in higher energy range and is easily distinguishable although of lower intensity from the rest of the spectrum Now the following steps are performed to complete the calibration e From the ADMCA window the Define ROI command is selected which opens another small window an ROI is a group of channels around a peak To define an ROI the cursor is first placed at the left and then at then at the right edge of the peak both values at the opposite ends will be displayed under the start and end columns of the small ROI window The Add button is clicked to store the first ROI e The above step is done twice and ROIs are defined and stored for both Fe and Mo peaks The selected peaks will turn green e Next the toolbar button Calibrate is clicked to open the Calibration dialog box e The dialog box is moved such tha
13. 3 are manually input to the Specimen Component Table using the Insert key from the key board Unknown samples Elements are identified from ADMCA by selecting ROIs for all peaks The ROI detail will provide the list of elements 2 Spectrum processing All the spectrum processing steps discussed in Sec tion 4 2 1 are performed one by one by running the Process All command in the XRS FP window The spectrum in ADMCA would now be a pro cessed spectrum with background and noise clearly removed from it In the Element Table of the XRS FP window the intensities against each elemental line an be seen 3 Concentration extraction Finally the Process Analyze command is run which converts the intensities of each element into concentrations using all of the parameters input by the user to calculate and correct for the attenuation and enhancement matrix effects in the sample from physical models and mathematical equations 22 5 6 Fundamental parameter quantitative analysis with XRS FP It is simple to use the standardless analysis but the attenuation and enhancement coefficients and hence the concentrations are only approximate due to approxima tions inherent in the physical models and in the data entered by the user With the FP analysis method the correction parameters can be calibrated using ei ther a single standard or multiple standards Calibration is strongly recommended and will lead to much more accurate analysis results A single
14. As C Co P 5 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 16 3 6 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 Table 3 Elemental composition wt age of Si brass standard sample obtained from Brammer 16 4 Softwares for data analysis In this experiment the data will be acquired and analyzed using two kinds of softwares ADMCA 2 0 and XRS FP 4 1 ADMCA 2 0 The ADMCA program 17 is the main display and acquisition software It is a Windows software package that provides data acquisition display and control for Amptek s signal processor PX4 4 1 1 Features e It provides full control of all 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 Its spectral analysis features include energy calibration setting up regions of interest ROI and computing ROI information centroid net peak area FWHM e It also possesses an active link t
15. X Ray Fluorescence XRF spectrometry for materials analysis and discovering the atomic number Asma Khalid Aleena Tasneem Khan and Muhammad Sabieh Anwar LUMS School of Science and Engineering June 28 2011 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 Ka X rays KEYWORDS X Ray Fluorescence XRF Characteristic X Rays Bremsstrahlung Radiations Moseley s Law Atomic number Screening coefficient Rydberg s energy APPROXIMATE PERFORMANCE TIME 3 Week 1 Objectives In this experiment we will 1 differentiate between characteristic X rays and Bremsstrahlung radiations 2
16. XRF 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 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 displayed as the spectrum of the X ray irradiated sample The spectrum is further processed to identify elements and quantitatively analyzed to find the 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 5 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 5 1 A little bit of history 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
17. and its various components is shown in Figure 13 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 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 a Base plate 9V AC DC H 110 220 VAC IYD adapter P Radiation hield Colimator and filter Mini X a X ray tube USB HASP plug X Ray SI Radiation Hazard ADMCA software XR100 Si PIN mo PXA USB Mini X software detector ini XRS FP software g Digital pulse Computer Sample mount Sample processor 5VD 5V AC DC n adapter T 110 220 VAC XR100 i mpl Brass aluminum Sample shield Figure 13 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 25
18. ass the energy of an electron in its orbit n is given by 2 Fe pet n 2 1f 1 n2 4 where h is Planck s constant c is the velocity of light R 1 097 x 107 m is the Rydberg constant for an infinitely heavy nucleus Reo hecR 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 FE to a final lower energy state E y and the frequency v of the emitted radiation is given by the condition Le E hyp 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 Rge Z z BRL ee 2 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 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 SRrs Z 17 The above equation is usually
19. d 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 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 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 5 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 Ka line varied smoothly with that element s atomic number Z From Bohrs theory of atomic structure something you have already studied in 11 your Modern Physics cl
20. e cathode and anode while the intensity of the beam depends on the number of electrons emitting off the filament This number depends on the temperature of the heating element which in turns depends on the current flowing in micro amperes uA through the filament Increasing the X ray tube voltage results in both a higher X ray intensity and a higher energy distribution 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 2 2 Atomic processes involved in X ray production The production of X rays by two different atomic processes the X ray fluorescence and the bremsstrahlung radiation is discussed below 2 2 1 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 4 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 ca
21. eeping the geometry of the setup exactly the same as before 2 The spectrum is acquired in ADMCA and ROIs are defined to get information of the elements present in it 3 The F Pcalib tfr file with a list of calibration coefficients is loaded in the XRS FP software and the new spectrum loaded in to that file 4 The spectrum processing steps are performed 5 The Analyze command is run to find the concentrations 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 steel in Sections 5 4 and 5 6 1 Find the calibration coefficients for all the elements of the alloy and their concentrations 6 Experiment 2 Verification of Moseley s law In this experiment Moseley s Law is verified The screening constant for Ka X rays is also calculated 6 1 Apparatus and Setup 1 X ray Generator Mini X 2 Si X Ray Detector XR100 3 Digital Pulse Processor DPA 4 X ray Shield 5 Samples e stainless steel SS 316 32 e chromium copper IARM 158B e silicon brass 31X WSB7 Always use gloves when handling these samples Never touch them with bare hands as this will contaminate the samples Make sure the samples are placed in the desiccator after use each time 6 2 Procedure e The apparatus is assembled as shown in Figures 13 a and b
22. g from the source 7 Counts a 5 61 11 32 17 03 22 75 keV 21 Counts 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 wA the spectrum shows a triangular bremsstrahlung continuous spectrum along with a distinguished characteristic X ray peak of silver near 22 keV 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 8 Characteristic 7 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 possess an amount of kinetic energy equal to the difference between energies of
23. gy levels and 3 the radioactive decay of some atomic nuclei Each mechanism leads to a typical spectra of X ray radiation 2 1 1 X ray tube An X ray tube is a device commonly used for the generation of X rays by bom barding highly accelerated thermal electrons on a heavy metal target It produces X rays incorporating the first two mechanisms A schematic for producing X rays is shown in Figure 1 1 A voltage is applied to the cathode which heats up the high resistance wire filament Electrons in the filament become excited and tend to boil off the surface 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 2 A high voltage in kilo volts is applied across the gap between the filament on the cathode and the heavy metal target acting as the anode of the system which causes the thermal electrons to be attracted to the positively charged anode Due to high voltage the electrons speed across the gap with tremen dous velocity and finally hit the tungsten target so hard that they explode into a shower of high energy photons These photons are the X rays The energy of the X rays is controlled by the high voltage measured in kilo volts kV applied across th
24. helps to quickly scale the gain of the detector s preamplifier If the delta mode is enabled the spectra do not accumulate over time but a fresh spectrum is shown every second which allows one to see changes The Define ROI and Calibrate buttons are used to perform 18 the energy calibration Once a spectrum has been acquired the data can be saved and the file can be reopened Calibrate Define ROI Display Enable delta configuration mode Start stop acquisition EE Amptek ADMCA D ADMCA 2 0 IARM 158 mca File View MCA Display Analyze DPP Help SUSRH WFP Eoo s mM Sa mew ewww e Acquisition setup to configure PX4 Link to XRS FP Auto tune software Gain quick adjust Figure 10 ADMCA window s menu illustrating the key controls in Ampteks ADMCA software 4 1 2 Output of the ADMCA In the energy calibrated spectrum with regions of interest ROIs defined for every peak the software searches automatically via the library containing characteristic peak energies of Ka Kg La and Lg for all elements from H to Fm for the contents of each channel that lies within the specified ROIs which are then added together to form the Peak Integral The peak integral values are then divided by the acquisition time of the spectrum in seconds to produce the peak intensity in counts second A typical output of the ADMCA is shown in Figure 11 4 2 XRS FP The XRF FP is a quantitative analysis software 18 19 package for X ray fluo re
25. ion analysis one can choose either 1 standard less method or 2 FP calibration method In a standard less analysis all of the parameters describing the X ray tube spectra filtering attenuation in air attenuation in Be windows attenuation and enhance ment in the sample are computed from physical models based on the data the user has entered into the software It is simple to use standard less analysis but the parameters are only approximate This is due to approximations inherent in the physical models and in the data entered by the user The FP calibration method on the other hand provides correction parameters using either a single standard reference sample containing known concentrations of elements or multiple standards pure elemental standards FP calibration is strongly recommended and will lead to much more accurate analysis results A single type standard i e a single material containing known elemental concen trations can be used with convenience For example one can use a stainless steel SS 316 and then obtain very accurate analyses of other steel alloys or pure elements containing at least one of the components of steel 19 20 The spectrum processing and concentration analysis carried out with the aid of the two softwares ADMCA and XRS FP is conceptually illustrated in the Figure 12 5 Experiment 1 Elemental analysis In the first experiment we will learn how to analyze a material sample to find its consti
26. l Si and a fraction of a parent characteristic X ray gets lost as Si Ka escape photons with an energy of 1 75 keV Hence every feature in the spectrum will have an associated escape feature at an energy lower by 1 75 keV The software removes the escape peaks from the spectrum and adds the equivalent original X ray event at the parent peaks energy e Sum peak removal Sum peaks arise because two incoming X rays may arrive at the pulse proces sor within a time frame that is less than the minimum time interval required by the detector to discriminate two events This results in peaks that have energies with the sum of the two incoming X ray events This correction is not as accurate as the escape peak removal and may leave some residual sum peaks in the spectrum e Background removal The software uses the automatic background method to remove the back ground which distinguishes fast changing regions of the spectrum peaks from slowly changing regions background The background curvature arises primarily from bremsstrahlung X ray continuum from an X ray tube whose shape depends on the anode atomic number and incident electron beam en ergy e Deconvolution for Intensity extraction When the preliminary spectrum processing is complete the net peak inten sities must then be calculated Deconvolution is the process of assigning areas to the peaks of interest and fitting the spectrum to a sum of separate peaks to calculate the net peak
27. lled 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 2 2 2 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 that 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
28. n spectrum 28 File View MCA Display Analyze DPP Help Tea E o p TR pme a seHSaGh oe 284 EE Maat Qe Ie RW B x Mark ROI Button Calibrat Calibrate Button Cursor Channel Value Deviation 995 87 6 4 0 Add S 2705 91 17 443 0 Replace Remove Remove All Cursor 0 Centroid 6 Units Energy keV Plot Calibration Equation Lines yw A Bx A 0 0310732 B 0 00645774 Auto Calibration Ctrl F5 Units keV Value 5 89 Peak 2Value 6 49 a Method 2Peak Centtoid v Energy keV 6 40 moo pom Count 33268 0 54832 TE oen 20v Figure 16 The ADMCA display window showing the calibration dialog box 5 5 Standardless quantitative analysis with XRS FP 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 After successful calibration of the spectrum and qualitative analysis we now move onto quantitative analysis calculating the elemental concentration by extracting net intensities of the sample as discussed in Section 4 2 2 For this purpose the XRS FP software is needed The HASP plug available with the XRF unit is connected to the USB port of the computer and FP is launched from the taskbar or the ADMCA window 1 A yellow box appears with the version information The box is clicked any where and another grey
29. ns an electrically heated filament that generates elec trons which are accelerated from the filament to the target Ag anode by appli cation of a high voltage When these high velocity electrons are decelerated or absorbed by the nuclei of the target atoms bremsstrahlung or characteristic Ag X ray photons of high energy are produced The Mini X X ray tube is fit with a beryllium window since beryllium is transparent to X rays 12 13 3 3 XR 100 CR Si detector This is a solid state silicon detector with a beryllium window The window is brittle and can be damaged beyond repair by mishandling Make sure that this window is not touched with anything at all neither it is dropped or mishan dled The detector requires a very low operating temperature and therefore has a thermoelectric cooling system Touching the detector with bare hands will disrupt this cooling system and damage it irreparably 13 Always wear gloves while handling the detector and be very careful during moving or positioning the detector 3 4 PX4 processor and analyzer The Amptek PX4 14 is an interface between Amptek s XR100 detector and a personal computer providing data acquisition and control through analysis soft wares The PX4 is acomponent in the complete signal processing chain of the XRF instrumentation system It consists of a digital pulse processor DP4 integrated multichannel analyzer MCA and power supply The input to the PX4 is the preamplifier
30. o the XRF FP Quantitative Analysis Software Package The ADMCA software performs the spectrometer calibration The spectrometer calibration refers primarily to the adjustment of the electronics analog amplifier ADC and MCA so that all peaks across a spectrum are located at their appropriate 17 a ZE Amptek ADMCA D ADMCA 2 0 IARM 158 mca Dok Fie View MCA Display Analyze DPP Help Sstre fg eoo t mMM fiile Qe ma NM Na a oe pS b a mo Te eror unes density Feea ok Toa gt o Global Threshold Setings pmo Clear Cosc Methad c Threshold intensity Pao Rome che Quere Cotwion af meren Eror Backar 1 cone mr i Mo Laem f vane Aone Meno memon f ine Hion f e f uenoa f rocceer Monsurmment Processing Conditions L Menserament C Processing Active Condition Code EJ E tector chamber Tene oace Ba ORO A RE OR PI OL E a Figure 9 Screenshots of a ADMCA software for elemental identification and b XRS FP concentration analysis software energies Unless this is the case the spectrum processing cannot correctly convert the X ray peaks to elemental intensities and therefore incorrect values will be generated resulting in incorrect assay analyses The important control buttons in the menu bar are shown in Figure 10 The Acquisition Setup button permits access to all PX4 configuration controls and options The Gain Quick Adjust button on the right
31. ost precise set of concentrations for each of the elements of the SS 316 sample using the calibration coefficients and net peak intensities The resulting table of concentrations and coefficients achieved at the end of the analysis is like the one shown in Table 4 Component Intensity Line Concentration Uncertainty Calib coef ficient c s wt wt Mn 98 64 Ka 1 63 0 16 7464 6 Mo 58 43 Ka 2 38 0 31 8165 6 Cu 31 75 Ka 0 17 0 03 38757 7 Ni 230 55 Ka 12 18 0 79 4586 4 Fe 2162 29 Ka 65 19 1 38 5304 3 Cr 1078 80 Ka 18 45 0 55 6856 5 Table 4 The concentration analysis of steel SS 316 completed through the XRS FP software The relative intesnities and calibration coefficients are also listed 5 6 2 FP analysis for unknown samples The advantage of running the FP calibrated analysis is that now we are able to find the precise concentration values for any unknown sample containing any one 31 or some or all elements of the SS 316 in any weight percentage Since we have successfully calibrated the responses calibration coefficients of the elements of steel i e Mn Cr Fe Ni Cu Mo we can use these response factors for any of these elements whether they are present in a pure sample material or with other elements in any sample and composition The analysis for an unknown sample with any elements of steel is outlined in following steps 1 The sample is mounted on the sample holder while k
32. output from the Si detector The PX4 digitizes the preamplifier output applies real time digital processing to the signal detects the peak amplitude digitally bins these values in its histogram memory and finally generates an energy spectrum The spectrum is then transmitted over the PX4 s serial interface to the user s computer The digital pulse processor DP4 and MCA inside the PX4 unit have six main function blocks to implement these functions as shown in Figure 7 A detailed description of MCA is described next Senate ae DTR eee MCA m Input from Anaic Analog to Digital Microcontroller detector amp gt digital i prefilter shaper amp interface preamplifer convertor gt selection Cc t supply logic omputer Figure 7 Block diagram of the PX4 14 3 4 1 Multi channel analyzer and channel calibration The characteristic X rays are the main factor in realizing an XRF measurement and it is essential to be able to accurately measure their energies This can only be achieved after a calibration of the detector and MCA has been performed Once an X ray impinges on the detector the detector sends an electrical pulse to the MCA The MCA will place each X ray in its appropriate channel depending on the X rays energy The data recorded is an array of numbers each number representing the total count of X rays for that energy intensity
33. overlap the matrix effect The matrix effect is the absorption or enhancement of specific elemental X ray peaks by other elements present in the specimen This effect hinders the simple method of integration of regions around a peak to calculate the accurate elemental intensity To correct for the matrix effects the XRS FP software uses an empirical analysis methods called the FP calibration method which implicitly calibrate out the peak overlaps and other spectrum artifacts to arrive at the true composition of elements of a sample A brief introduction to the spectrum processing steps in place here e Element identification The element identification is required prior to extracting intensities It is implicit in this description that with every element there is a specified ele ment that will be used for the final analysis Using the Amptek ADMCA application the user can automatically mark peaks ROIs for analysis If the appropriate element library is loaded into ADMCA the software will as sociate the marked peaks with elements The corresponding elements can then be manually imported into the XRF FP element table 21 e Spectrum smoothing Smoothing of the spectrum is the first step in spectral processing This oper ation typically performs a Gaussian smooth of each channel in the spectrum for the specified number of times e Si escape peak removal Escape peaks arise when internal fluorescence occurs inside the detector materia
34. removing the power cable so that the high voltage inside the tube is completely turned off The spectrum must be saved now in the ADMCA folder with the name steel mca and the spectrum window must remain open for the entire steps and subsequent procedures described below On the spectrum steel mca identify the characteristic peaks Use the X ray Chart 21 to identify peaks of Fe Cr and Mo Now after obtaining spectrum with its peaks identified the spectrum is show ing intensity counts versus channels which really is of no use to us So this 26 a Amptek Mini X Controller Minix Serial Number 00001708 X RAY ON Set Monitor High Voltage and Current 30 MIN MAX D HV OFF 30 aw Set High Voltage 30 and Current 3 10 D E High Voltage Monitor 30 0kV h xit j Current Monitor 30 044 lee i i gt Mit a A Rar Toe wl i Me tf WH mw Mm 8 et Min s Controller ready Figure 15 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 spectrum should be calibrated to obtain graph between intensity counts and en ergy This vital step of transforming channels to energy called calibration can be subsequently used for qualitative and quantitative analysis 5 4 Spectrum calibration in ADMCA with a standard sam ple The ADMCA software is calibrated using SS 316
35. rican Journal of Physics 64 pp 335 338 1996 Mini X User s Manual Amptek Inc http compassweb ts infn it richi Stefano Amptek_SW Mini X Mini X X ray detector http www amptek com pdf xr100cr pdf Digital Pulse Processor Amptek Inc http www amptek com pdf dp4 pdf Amptek Experimenter s XRF Kit Quick Start Guide Amptek Inc http www dengeteknik com tr veri dosyalar metal chip nonferrous pdf 17 18 19 20 21 22 2 Amptek ADMCA Display and Acquisition Software http www amptek com admca htm1 Quantitative Analysis Software for X ray Fluorescence http www amptek com pdf fp pdf XRS FP Quick Start Guide for Experienced Users version 3 3 0 Amptek Inc XRS FP Software Guide version 4 0 4 Amptek Inc http crossroadsscientific com Documents XRS FP 20Software 20Guide 20v404 pdf Amptek K and L Emission Line Lookup Chart http www amptek com pdf xraychrt pdf 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 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 ener
36. ronic transition from L to K and M to L shells respectively b shows a decelerating electron emitting bremsstrahlung X rays a Electrostatic field regions around the nucleus Counts number of photons Energy keV b a 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 scale as compared to 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 threshhold voltage as shown in Figure 4 a and b 2 3 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 comin
37. scence It processes the raw X ray spectral data from Amptek s detector signal processing electronics and ADMCA spectrum to obtain e the elemental peak intensities and e the elemental concentrations There are three major steps to an XRF analysis after the system has been setup and the calibrated spectrum has been loaded from the ADMCA 1 Subtracts the detector s response to recover the incident X ray peaks This step corrects for escape peaks sum peaks background continuum and back ground peaks The output of this step is a processed spectrum ideally show ing only the incident characteristic X ray peaks 19 a AADMCA 2 0 U steel mca 02 01 2011 15 47 24 Regions of Interest Details b Figure 11 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 2 Deconvolve the peaks to determine the intensity of the X rays interacting in the detector The output of this step is a table of the intensities for each peak to be analyzed 3 Account for attenuation and matrix effects to determine the concentrations of the elements in the sample The output of this step is a table of concen trations which is the final result of the analysis 20 XRS FP does the spectrum processing requiring as input the parameters which describe the spec
38. t both peaks are visible and then cursor is clicked on to the first peak The peak should be highlighted in blue color and the Peak Information section on the right hand information panel should be filled in e The Centroid button is then clicked on the dialog box This will enter the centroid value of the peak into the Channel memory Then the energy value of that peak is entered into the Value box e g 6 4 for Fe and then the Add button is clicked This procedure is illustrated in Figure 16 e Now the cursor is clicked on to the second peak and its Centroid and energy value e g 17 48 for Mo is also entered and added The information for the two peaks will then be shown in the box e In the Units box the energy units keV are selected e Click OK e On the ADMCA window the Enable calibration option is checked This command will convert the horizontal axis from channel to energy hence we finally see the intensity versus energy spectrum for the sample e Next the calibrated spectrum is saved with the name calibsteel mca so that we can use this calibration for element identification of unknown samples The View menu is opened andthe Preferences option is selected the file path and file name are specified and the box checked for loading this cali brated spectrum every time the ADMCA is run 17 Our software and hardware has now been calibrated with the energy scale We can now use the software for qualitative analysis of any unknow
39. t is not shielded properly The shielding must prevent accidental personnel exposure but must permit easy access to the samples In addition the material in the shield should not produce its own X rays since these would interfere with measurements in the detector The enclosure around the tube and detector must stop primary and scattered radiation A cubical shield was designed with 0 25 inches brass as the inner walls of the cube and 0 125 inches aluminum for the outer walls Any backscattered X rays absorbed in the brass can produces Cu and Zn lines which can be stopped easily by the aluminum walls The aluminum X rays have a very short distance in air and hence protect the experimenter and do not contaminate the measurement 15 Q6 How does the Si detector measures the energy of the X ray photon 3 6 Standard materials used in the experiment We will use three kinds of standard reference samples in our experiment 3 6 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 age of Amptek s stainless steel standard sample 12 3 6 2 Chromium copper alloy IARM 158B The composition of the chromium copper alloy is given in the following table Cr Ag Al 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
40. 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 4 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 e excitation source e sample e detector and e data collection and analyzing system 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 9 Si detector and preamplifier Multi channel analyzer X ray tube 1 Sample Analysis software Element identification and concentration information Figure 6 The schematic of an ED
41. trometer itself e g the type area and thickness of the detector the distance between the tube and the sample etc and parameters which control the processing Figure 9 b captures a screenshot of XRS FP window Most of the parame ters are found in the Setup menu item and are self explanatory The parame ters describing the detector are in Setup Detector menu Some of the param eters for the X ray source are in the Setup Source menu and others are in the Measurement Conditions Table which include the filter thickness tube voltage kV and tube current uA Some columns of the Element Table are inputs while some columns of this table are outputs e g intensity an output of spectrum processing The Specimen Component Table is used to select which elements will be analyzed The Element Table is used to se lect which X ray line is the basis of the analysis The Thickness Information Table is used to define the sample The functions and processing of XRS FP are explained in the next sections What are sum peaks escape peaks and background peaks 20 Why its 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 It is the job of spectrum pro cessing to effectively remove the signal net peak intensity from the noise the background peaks An additional complication is caused by peak
42. tuent elements and determine their relative concentrations We will start off by obtaining the spectral data of a stainless steel sample SS 316 We have already seen in Section 2 that each element has a set of characteristic X ray peaks unique in their energy Therefore the peak energies can be used as a fingerprint for element identification 23 channel background and to energy escape amp sum peaks Raw spectrum conversion Energy calibrated ramoval Processed spectrum spectrum channel MiM isi wn Nii Energy keV Deconvolution of peaks Table of intensities Intensity Element Line cts sec Cr Ka 551 09 Mn Ka 68 52 Fe Ka 1 842 34 Ni Ka 174 5 Cu Ka 7 52 Mo Ka 464 78 Matrix effect correction by standardless or FP calibration Table of conc Concentration Cr 18 50 025 Mn 1 67 007 Fe 65 00 0 52 Ni 12 27 0 27 Cu O17 0 03 Mo 2 39 0 03 Figure 12 Spectrum processing and concentration analysis steps carried out by the ADMCA and XRS FP softwares The first two steps are performed by ADMCA and the remaining ones by XRS FP software 5 1 Apparatus We will use the Amptek s XRF components listed below e X ray tube Mini X e detector XR 100 CR e digital pulse processor PX4 e ADMCA 2 0 software e XRS FP software e base plate e sample mount e radiation shield 24 5 2 Procedure The assembly of the apparatus
Download Pdf Manuals
Related Search