Electron Probe Microanalysis EPMAGeneric EMP/SEMKey pointsSpectrometersX-ray pathWavelength DispersionWhat is nl ?Lots of CrystalsCrystals and PC/LSMsCrystals and PCs on the UW SX51Pseudocrystals/LSMsCrystals and PCs: Which to use?Crystal comparisonFull Width Half MaxSlide 15WDS detectorDetector amplificationDetector windowsSlide 19WDS pulse processingAr-escape peakPowerPoint PresentationSlide 23Slide 24Slide 25Slide 26Integral vs Differential PHAPutting it all together: Spectrometer = Crystal + Detector moving in a highly choreographed dance Dance floor =Rowland CircleRowland CircleSlide 30Electron Probe MicroanalysisEPMA Wavelength Dispersive Spectrometry (WDS) IUW- Madison Geoscience 777Revised 2-25-2013Generic EMP/SEMElectron gunColumn/ Electron opticsOptical microscopeWDS spectrometersScanning coilsEDS detectorVacuum pumpsSE,BSE detectorsFaraday current measurementUW- Madison Geology 777Key points• X-rays are dispersed by crystal with only one wavelength (n ) reflected (=diffracted), with only one wavelength (n ) passed to the detector• Detector is a gas-filled (sealed or flow-through) tube where gas is ionized by X-rays, yielding a massive multiplication factor (‘proportional counter’) • X-ray focusing assumes geometry known as the Rowland Circle• Key features of WDS are high spectral resolution and low detection limitsUW- Madison Geology 777SpectrometersAn electron microprobe generally has 3-5 spectrometers, with 1-4 crystals in each. Here, SP4 (spectro #4) of our SX51 (#485) with its cover off.Crystals (2 pairs)Proportional Counting Tube (note tubing for gas)PreAmpUW- Madison Geology 777X-ray pathOnly a small % of X-rays reach the spectrometer. They first must pass thru small holes (~10-15 mm dia; red arrows) in the top of the chamber (above, looking straight up), then thru the column windows (below, SP4).Thus, in our EMP, there are different vacuum regimes in the chamber vs the spectrometer, separated by the windows (not in JEOLs).BSE detectors alternateCrystalUW- Madison Geology 777Wavelength DispersionOf the small % of X-rays that reach the crystal, only those that satisfy Braggs Law will be diffracted out of the crystal.Note that exact fractions of will satisfy the conditions for defraction. Thus, there is a possibility of “higher order” (n=2,3,...11,?) interference in WDS (but there also is the means electronically to discriminate against the interference).n = 2d sinBA’ = A’C = d sinfor constructive reinforcement of a wave, the distance BA’ must be one half the wavelength. Thus, 2d sin = and by similar geometric construction = nUW- Madison Geology 777What is n? This is sometimes difficult to comprehend. Assume you have your spectrometer set to one particular position, which means for that sin and that 2d (let’s say they = 12 Å), there are several possible signals that the spectrometer can tune in to at that position: (1) an x-ray with wavelength of 12 Å, (2) an x-ray of 6 Å, (3) an x-ray of 4 Å, (4) an x-ray of 3 Å, etc --of course, if and only if such x-rays are being generated in the sample.n = 2d sinBA’ = A’C = d sin for constructive reinforcement of a wave, the distance BA’ must be one half the wavelength. Thus, 2d sin = and by similar geometric construction = nUW- Madison Geology 777Lots of CrystalsOver the course of the first 30 years of EPMA, ~50 crystals and pseudocrystals have been used.UW- Madison Geology 777Crystals and PC/LSMsConsider the order of 2d in Braggs Law: sin varies from .2-.8, and varies over a wide range from hundreds to fractions of an Å. Thus, we need diffractors that cover a similiar range of 2d, from around 1 Å to hundreds of Å. In our SX51, we utilize TAP, PET and LIF crystals for the shorter wavelengths. For longer wavelengths, there are 2 options:• pseudocrystals (PCs), produced by repeated dipping of a substrate in water upon which a monolayer (~soap film) floats,progressively adding layer upon layer, or • layered synthetic microstructures (LSMs; also LDEs, layered diffracting elements), produced by sputtering of alternating light and heavy elements, such as W and Si, or Ni and C. • Both these are periodic structures that diffract X-rays. The LSMs yield much higher count rates; however, peaks are much broader, which have good/bad consequences, discussed later.• In reality, people interchange the words PC, LSM, LDE, etc. Cameca uses PC and JEOL uses LDE, for same things.UW- Madison Geology 777Crystals and PCs on the UW SX51There is a more precise form of Braggs Law, that takes into account refraction, which is more pronounced in the layered synthetic diffractors, n = 2d sin(1-k/n2)k is refraction factor, n is order of diffractionUW- Madison Geology 777Pseudocrystals/LSMsGoldstein et al, p. 280Crystals and PCs: Which to use?The EPMA user may have some control over which crystal to use; some element lines can be detected by either of 2 crystals (e.g. Si Ka by PET or TAP, V Ka by PET or LIF), whereas other elements can only be detected by one (e.g. S Ka by PET). Each probe has its own (unique?) set of crystals and the user has to work out the optimal configuration, taking into account concerns such as• time and money (spread the elements out)• interferences vs counting statistics (sharper peaks usually have lower count rates)• stability (thermal coefficient of expansion)UW- Madison Geology 777Å ÅCrystal comparisonTAPPC0-Low counts-Good resolution-High counts-Poor resolutionGopon et al. 2013UW- Madison Geology 777Full Width Half MaxSi K on TAP sin = 27714 P/B= 4862/40=122 FWHM=0.038ÅÅ4862 cts2431 ctsMaxHalf maxFull WidthPeak (spectral) resolution is described by FWHMcountsUW- Madison Geology 777Crystal comparisonThe class project in 2002 was to collect data to compare the efficiency of different crystals/ PCs for certain elements.Si K on TAP sin = 27714 P/B= 4862/40=12 FWHM=0.038ÅSi K on PET sin = 81504 P/B= 207/1.3=159 FWHM=0.006ÅÅ ÅAcquiring accurate Si Ka counts is critical for geological EPMA. Always use TAP! (with rare exceptions) as the the wider peak (6x wider, see FWHM) is less sensitive to chemical peak shifts in silicates (and higher count rate is a plus)UW- Madison Geology 777WDS detectorP10 gas (90%
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