ENERGY CALIBRATION IN DPPMCA AND XRS-FP Energy calibratin is very imprtant fr quantitative X-ray analysis. Even a small errr in the calibratin f the energy scale can have significant cnsequences: if the phtpeaks are shifted even slightly frm their expected psitins, the sftware may misidentify peaks, cmpute errneus phtpeak areas, and make significant errrs when crrecting fr verlapping phtpeaks, sum peaks, etc. Amptek's Experimenter's Kit makes use f tw different sftware packages: the DPPMCA sftware and the XRS-FP sftware. Bth prgrams include the capability t calibrate the energy scale but they treat the calibratin differently. These differing appraches are the surce f many errrs fr users. This applicatin nte is intended t describe, very briefly, hw t minimize these issues. Backgrund Fr any spectrmeter, a key parameter is the "system cnversin gain", the ev/channel. It relates the system input, the energy depsited by a particle in units f energy, t the system utput, the crrespnding channel number in the utput histgram r spectrum. Anther key parameter is the "ffset", the channel crrespnding t zer energy. Ideally, the ffset wuld be zer and the gain exactly equal t a cnvenient value. In real hardware, the gain and ffset depend n many things, including parameters the user can change, n cmpnent tlerances, and n temperature cefficients f the hardware. DPPMCA is a "hardware-centric" sftware package. We recmmend using DPPMCA t ptimize a wide range f hardware parameters in the spectrmeter befre yu use XRS-FP t acquire and analyze the spectra. 1 The system cnversin gain and ffset depend n many different hardware parameters, sme which a user can adjust and sme which vary frm ne detectr t anther. The apprach f DPPMCA is t set these parameters, then t measure the channels crrespnding t the phtpeak energies, and then t perfrm a linear regressin, yielding the gain and ffset. XRS-FP is an "analysis-centric" sftware package; it cntrls the hardware t take a spectrum but then des extensive spectrum prcessing. It can read in a spectrum which has been previusly acquired with DPPMCA r XRS-FP. Fr efficient analysis, XRS-FP always assumes that the spectra have zer ffset and an exact integer system cnversin gain (5, 10, 20, 40, 80 and 160 ev/channel). Since the hardware is nt always set t exactly zer ffset r exactly this gain, XRS-FP adjusts spectra (i.e. re-bins the data) based n calibrated values and n check bxes which enable adjusting. Yu can use either DPPMCA r XRS-FP t change the DPP's gain and t calibrate the energy scale. The interfaces and dialgs are quite different in the tw sftware packages. Mst prblems ccur when the check bxes in XRS-FP are set incrrectly (i.e. incnsistent with each ther r with the settings when the data were acquired) r when the calibratin data stred in the file are nt cnsistent with the spectrum adjust ptins. 1 Energy calibratin and gain are discussed in the "Gain" sectin f the FAQ in DPPMCA's HELP. Nte that the "system cnversin gain" (ev/channel) differs frm the digital prcessr's gain (a dimensinless number and ne factr f many in the system cnversin gain). 1
Recmmendatins 1. Setting the gain 1.1. We recmmend first setting the DPP gain s that the system cnversin gain is clse t 5, 10, 20, 40, 80 r 160 ev/channel If it is clse, XRS-FP des nt have t adjust (rebin) the data very much, minimizing errrs. A typical value fr the ev/ch wuld be 20 assuming yu are using 2048 channels. I.e. a range f 40 kev typically cntains mst if nt all the peaks f interest fr XRF analysis using Si detectrs. 1.2. Yu can set the gain using either DPPMCA r XRS-FP. In bth cases, yu take an initial spectrum, determine the initial system cnversin gain and ffset, cmpute the new DPP gain and ffset, and send them t the DPP. 1.3. In DPPMCA, the calibratin dialg gives yu a gain and ffset. Yu must calculate the new values using a calculatr r spreadsheet (an example is belw). 1.4. In XRS-FP, the "Setup", "DPP/MCA" dialg has a utility t quickly cmpute the new gain (n calculatr needed) but nly changes the gain, nt the ffset. A detailed descriptin f its use is given n page 4. 1.5. Ntes In XRS-FP, under "Setup", "DPP/MCA", yu must enter the number f channels (e.g. 2048) and the energy range (e.g. 40 kev) These give the expected gain (e.g. 20 ev/channel). XRS- FP requires the actual gain t be clse; if yu tell XRS-FP that yu have 2k channels, and 40 kev full-scale, but the gain is really 5 ev/channel, it will generate an errr message. The cnfiguratin f the prcessr is stred in EEPROM in the DPP; when the DPP is pwered n, it recalls the cnfiguratin in use when it was pwered ff. If yu use DPPMCA, the sftware simply cnnects and uses the internal cnfiguratin. If yu use XRS-FP, it reads in a cnfiguratin stred in the DP5.TXT r PX5.TXT file (in the same directry as XRS-FP.EXE) and verwrites that stred in the DPP's EEPROM. Therefre, every time yu change the DPP cnfiguratin, yu shuld save the new cnfig t DP5.TXT (r PX5.TXT. Yu can d this using DPPMCA r XRS-FP. The chice f DP5.TXT r PX5.TXT is made autmatically depending upn which type f DPP is present. (If n file is fund then the XRS-FP sftware will use the existing DPP cnfiguratin, just like DPPMCA.) 2. Calibrating the energy scale 2.1. Yu can calibrate the energy scale using either DPPMCA r XRS-FP. In DPPMCA, yu can calibrate using any number f phtpeaks (the sftware perfrms a linear regressin n the centrids) and using any number f spectra (yu can take spectra frm several pure sample and then cmbine the results). When yu calibrate in DPPMCA, yu must manually transfer the calibratin results (gain and ffset) int XRS-FP. In XRS-FP, yu calibrate using tw and nly tw phtpeaks and they must be present in the same spectrum. Yu need a sample with tw knwn, strng, widely separated phtpeaks. 2.2. Calibrating in DPPMCA A detailed descriptin f DPPMCA's calibratin dialg is described in DPPMCA's HELP. DPPMCA yields tw parameters, dented "A" and "B", which are the spectrum ffset (A) and system cnversin gain (B) derived frm a linear regressin. XRS-FP adjusts the spectrum based n tw parameters, the ffset (ev) and gain (ev/channel), fund under "Calibrate", "Spectrum". Yu take the values fund in DPPMCA and enter them here in XRS-FP. Nte that these must be in ev; if yu calibrated DPPMCA in kev, then yu must rescale t ev. Prir t entering these values ensure that yu have previusly set the crrect target ev/channel, and clicked n the Reset Gain and Offset buttn. 2.3. Calibrating in XRS-FP A detailed descriptin is given n page 6. 2
3. Acquiring data 3.1. We recmmend enabling the checkbx in "Setup", "MCA/DPP" labeled "Aut Adjust Spectrum during Acquire". When this is checked, and yu acquire the spectrum in XRS-FP, it re-bins the data as they are acquired s the spectrum has zer ffset and the target integral gain (ev/channel). 3.2. We recmmend using XRS-FP t acquire the spectra which will be used fr analysis. We find there t be fewer errrs when XRS-FP is used fr all the steps: taking the spectrum, autmatically adjusting it, and prcessing it. Use DPPMCA nly t cnfigure the hardware. 4. Opening files 4.1. We recmmend yu d nt enable the checkbx in "Setup", "Prcessing" labeled "Aut Adjust Spectrum Gain & Offset n Lad". If this is enabled and the bx is enabled t aut-adjust during acquisitin, then when yu lad a file, it is adjusted twice, causing errrs. The XRS-FP sftware will autmatically warn yu against ding this. 4.2. If yu have saved a.mca file acquired in XRS-FP, and it was adjusted during acquisitin and is nt adjusted n lad, the energy scale will be crrect. Nte that XRS-FP stres the energy calibratin values in the XRS-FP.INI file. If yu take spectra ne day, then adjust the DPP gain the next day, yur riginal calibratin values are ver-written. We strngly recmmend that, fr every spectrum yu take using XRS-FP, yu save nt nly the.tfr file and the.mca file but als a cpy f the XRS-FP.INI file yu used. If yu will lad a file fr analysis, make sure t verwrite the XRS-FP.INI file befre launching the sftware. 4.3. If yu have saved a.mca file acquired in DPPMCA, and yu did nt save the calibratin values in the.mca file, then XRS-FP will nt autmatically d any adjustments. After yu have laded the file, g t "Calibrate", "Spectrmeter", and press "Adjust Spectrum". 4.4. If yu have saved a.mca file acquired in DPPMCA, and yu calibrated it in DPPMCA, then XRS- FP will autmatically adjust (re-bin) the data using the calibratin values stred in the.mca file. Nte that XRS-FP assumes yu calibrated in kev. If yu calibrated in ev, XRS-FP generates an errr message (the gain is ff a thusand-fld). Nte that the.mca file frm DPPMCA des nt stre the gain and ffset fr the calibratin. Instead, it saves the list f data pints used in the calibratin. XRS-FP nly reads the first tw f these. S if yu calibrated in DPPMCA using three r mre pints, the calibratin will be ff when reading the.mca file in XRS-FP. 3
Detailed Instructins Crrecting the gain using DPPMCA The plt belw shws the result f an energy calibratin in DPPMCA. A spectrum was taken frm a stainless steel 316 sample, the Fe and M phtpeaks marked, and their centrids used in the calibratin dialg. As the screen shws, the DPP gain was 30, and the calibratin yields a system cnversin gain f 20.56 ev/channel. The calibratin als yields an ffset f -51 ev (this means that channel zer is at -51 ev, r equivalently zer energy is at (51 ev)/(20.56 ev/channel)=2.5 channels. The spreadsheet n the right shws calculatins t get the crrect gain and ffset. T get the crrect system cnversin gain, the DPP gain shuld be cmmanded t (20/20.56) * 30 = 29.18. The spectrum ffset shuld be cmmanded t -2.5 * (29.18/30) = -2.4 channels. Nte that the ffset can be cmmanded t a fractinal number f channels. Crrecting the gain using XRS-FP 1) Open Calibrate, Spectrum and press Reset gain and ffset. This sets the ffset t zer and the gain t its ideal (10, 20, etc. ev/channel) and ensures that the sftware is nt adjusting the spectrum. 2) Ensure the Aut Adjust Spectrum During Acquire check bx is unchecked in the Setup-DPP/MCA panel. Then acquire the calibratin spectrum. The plt belw shws ne measured frm a Cu-Sn calibratin sample (n a lg scale). Duble click n the phtpeak, and then drag the cursr t the centrid f the phtpeak. Yu can read the energy f the cursr in the yellw bx at the bttm. In this case, the Sn K peak is at 23.66 kev (it shuld be 25.193 kev). 4
3) G t Setup, DPP/MCA. Click the Calibrate buttn in the Gain sectin.. In the dialg that appears (shwn belw), enter in the tp bx the expected peak energy (25.193 kev fr the Sn K peak in this example). In the middle bx, enter the measured energy (23.66 kev in this example). Click Calculate and a new DPP gain is cmputed. 4) Click OK and the value is written t the main dialg and sent t the DPP. Click Save DPP Cnfig File s that the new value will be written t DP5.TXT (r PX5.TXT), and thus be available when yu next launch XRS-FP. 5
Calibrating using XRS-FP 1) Make sure the Aut Adjust ptin n acquire is ff (in Setup -> DPP ) then acquire the spectrum t be used fr energy calibratin. Use a sample with tw strng, well-defined peaks, well separated frm ther peaks, and well separated in energy. The example belw uses a Cu-Sn sample. 2) Open Calibrate, Spectrum. First 3) Press Reset gain & ffset, t clear these values. 4) Enter the crrect centrid energies int lw energy peak and high energy peak bxes 5).Use the cursr t find the centrid f each peak in Spectra-X. Enter these values as the Lw energy and High energy peak values in the Manual Calibratin sectin 6) Click Calculate Gain & Offset (yu will see sme change in ffset and the gain at this pint). 7) Nw use the autmatic calibrate methd by clicking n the Calibrate Spectrum buttn t get refined values f the gain and ffset. Press Adjust spectrum t adjust the spectrum t match the new values. These new values are autmatically written t XRS-FP.INI, ver-writing any previus values, when the OK buttn is clicked (the Cancel buttn will nt save any f these values). Yu can use this "Aut Calibratin" sequence n a rutine basis, t crrect fr spectrmeter drifts frm ne day t the next. 6