PowerPoint PresentationWhat’s the point?SummaryGeneric EMP/SEMEDS assemblageEDS WindowsHow it works: energy gapHow it works: inside the detectorArtifacts: Si-escape peak; Si internal fluorescence peakArtifacts: Si-escape peaks; Si internal fluorescence peak; extraneous peaksQuestion: Do all characteristic X-rays have Si-escape peaks in a Si(Li) detector? Why or Why Not?Signal processingThe first signals in the EDS detectorSlide 14Dead TimeDetector performance: peak resolution (FWHM)Why Mn Ka for EDS resolution?Spectral processing: background correctionSpectral processing: background modeling or filteringBackground ModelingBackground FilteringTop Hat FilteringMore Artifacts: Pulse Pile Up…Fools even the prosSum PeaksAnd More ArtifactsSlide 27…And beware of lazy peak IDs…and trusting softwareArtifical EDS spectrumArtificial spectrumEvolution of EDS spectrum: from the specimen to the monitor - 1Evolution of EDS spectrum: from the specimen to the monitor - 2Comments about LN2 and EDSEDS-WDS comparisonFurther EDS detailsEnergy Dispersive Spectrometry (EDS)Electron Probe MicroanalysisEPMA UW- Madison Geology 777Version Last Revised: 10/10/09What’s the point?Using X-rays to produce e-hole pairs (charges proportional to X-ray intensity), which are amplified and then “digitized”, put in a histogram of number of X-rays counts (y axis) versus energy (x axis). A solid state technique with unique artifacts.EDS spectrum for NIST glass K309(Goldstein et al, Fig. 6.12, p. 356)UW- Madison Geology 777Summary• X-rays cause small electric pulses in a solid state detector. Associated electronics produce ‘instantaneously’ a spectrum, i.e. a histogram of count (number, intensity) vs the energy of the X-ray• Relatively inexpensive; there are probably 50-100 EDS detectors in the world for every 1 WDS (electron microprobe)• Operator should be aware of the limitations of EDS, mainly the specific spectral artifacts, and the poor spectral resolution for some pairs of elementsUW- Madison Geology 777Generic EMP/SEMElectron gunColumn/ Electron opticsOptical microscopeWDS spectrometersScanning coilsEDS detectorVacuum pumpsSE,BSE detectorsFaraday current measurementUW- Madison Geology 777There are several types of solid state EDS detectors, the most common (cheapest) being the Si-Li detector. Components: thin window (Be, C, B); SiLi crystal, FET (field effect transistor: initial amp), cold finger, preamp, vacuum, amp and electronics (“single channel analyzer”).EDS assemblageGoldstein et al fig 5.21UW- Madison Geology 777EDS Windows Windows allow X-rays to pass and protect detector from light and oil/ice.Be: The most common EDS detector window has been made of Be foil ~7.6 m (0.3 mil) thick. It allows good transmission of X-rays above ~ 1 keV. It is strong enough to withstand venting to atmospheric pressure, and opaque to optical photons.Thin - Ultrathin: For transmission of light element X-rays (<1 keV), windows ~0.25 m thick of BN, SiN, diamond or polymer are used. They must use supporting grids to withstand pressure differentials; the grid (e.g., Si or Ni) takes up about 15% of the area, but the window material is thin enough that low energy X-rays pass through. “Windowless”: Here there is no film, and there is a turret that allows swapping with a Be window. Difficult to use as oil or ice can coat the detector surface. Not used much.Goldstein Fig. 5.41, p. 318This plots shows the transmittance of X-rays thru different types of window material. (Quantum [BN] 0.25 um, diamond 0.4 um). The higher the transmission number, the betterUW- Madison Geology 777How it works: energy gapX-ray hits the SiLi crystal, producing a specific number of electron-hole pairs proportional to X-ray energy; e.g. one pair for every 3.8* eV, so for incident Fe Ka, 6404 eV, 1685 e-hole pairs are produced. With a bias** applied across the crystal, the holes are swept to one side, the electrons to the other, producing a weak charge. Boron is important acceptor impurity in Si and degrades it (permits thermal excitation: bad); Li is drifted in (donor impurity) to counter its effects.Goldstein et al, Fig 5.19A semi-conductor like Si has a fully occupied valence band and largely unfilled conduction band, separated by an energy gap (1.1 eV). Incident energy can raise electrons from the valence to the conduction band.* 1.1 eV + energy wasted in lattice vibrations, etc**bias: a voltage is applied between 2 points; e.g. one +1500 v, other -1500 v.How it works: inside the detectorFig 9.5 Reed; Fig 5.22 GoldsteinX-rays are absorbed by Si, with photoelectrons ejected. This photoelectron then creates electron-hole pairs as it scatters inelastically. The Si atom is unstable and will either emit a characteristic Auger electron or Si ka X-ray. If Auger, it scatters inelastically and produces electron-hole pairs. If Si Ka X-ray,it can be reabsorbed, in a similar process, or it can be scattered inelastically. In either case, the energy will end up as electron-hole pairs. The result, in sum, is the conversion of all the X-ray’s energy into electron-hole pairs -- with 2 exceptions.UW- Madison Geology 777Artifacts: Si-escape peak; Si internal fluorescence peakFig 5.22 Goldstein et alThere are 2 exceptions to the previous neat explanation of how the Si(Li) detector works.Si-escape peaks are artifacts that occur in a small % of cases, where the Si ka X-ray generated in the capture of the original X-ray escapes out of the detector (red in figure). Since this X-ray removes 1.74 keV of energy, the signal generated (electron-hole pairs) by the incident X-ray will be 1.74 keV LOW. This will produce a small peak on the EDS spectrum 1.74 keV below the characteristic X-ray peak. Another artifact is the Si internal fluorescence peak, which occurs if an incident X-ray is absorbed in the Si “dead” layer (greenregion). This region is “dead” to production of electron-hole pairs, but Si ka X-rays can be produced here which then end up in the “live” part of the detector, and result in a small Si ka EDS peak. UW- Madison Geology 777Consider Ti …Artifacts: Si-escape peaks; Si internal fluorescence peak; extraneous peaksGoldstein et al Fig 5.39,p. 316The figure shows a real spectrum of a sample of pure Ti metal -- but there are 7 peaks besides the Ti K and K . At 1.74 keV below each, are the respective escape peaks (blue arrows). Also present is a Si internal fluorescence peak (green arrow). The Fe
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