Energy/Wavelength Dispersive X-ray Analysis

Energy-DispersiveSpectrometry/Spectroscopy (EDS or EDX)Wavelength-DispersiveSpectrometry/Spectroscopy (WDS or WDX)Characteristic X-rays are always generated by interactions between the incidentelectron beam and the sample (They constitute a fingerprint of the local chemistry).Collect X-ray signal to determine local chemistry

A typical Energy Dispersive X-ray SpectrumA series of characteristic peaks on abackground of continuum X-rays, straypeaks and artifacts.

Continuum X-raysContinuum X-rays or (bremsstrahlung) aregenerated when electrons interact withmatter.Result of incident electrons decelerating infield associated with atom.Gives counts in the X-ray spectrumbetween zero and value of incidentelectron beam energy.http://www.youtube.com/watch?v=P0Xi-dJskY8http://www.youtube.com/watch?feature=endscreen&v=n9FkLBaktEY&NR=1Characteristic X-raysIf an incoming high energy electron ejects an electron from the K shell it will bereplaced by an electron from a higher shell.This will create a characteristic X-ray or Auger Electron.If replaced from the L shell it generates a Kα X-ray.If replaced from the M shell it generates a Kβ X-ray.

X-ray energies strong function of Z K Be (Z = 4) 110 eV Fe (Z = 26) 6.4 keV Au (Z = 79) 68.8 keVLFe 0.70 keVAu 9.71 keV Note that EDX detectors work well only in the range 1- 20 keV

EDX DetectorsPeak energy gives qualitative informationPeak intensity gives quantitative informationCrystal (Si-drift detector or X-ray microcalorimeter)detects X-raysLiquid nitrogen cools crystal to reduce noise.Window separates detector from columnvacuum.Collimator eliminates stray X-rays.

WDS• Yields best discrimination of emitted X-ray signal• Use a series of bent crystals to cover the range of wavelengths of interest• Scan wavelengths within each range by rotating the crystal and moving the detectorwhile keeping the position of the crystal fixed.• X-rays are collected from the sample at a fixed angle.The angle at the collecting crystal will vary with 2θ and the diameter of the focusingcircle will changeAll peaks are scanned sequentially. Youcannot record more than one characteristicline at once unless there are multiplespectrometers available.Data is collected sequentially. Takes moretime but the results are much more precisethan EDS.

EDS vs. WDS• Pulse height is recorded by the detector.It is related to the energy of the photonresponsible for the pulse.• Solid-state detectors are generally used.• EDS is faster than WDS• Problems with EDS:– Poor discrimination or energy resolution(3 eV or higher).WDS systems are much better, particularlywhen characteristic lines from differentelements overlap.– Need a windowless or thin windowdetector to pick up light elements.– WDS is more quantitative

Electron Back-Scattered Diffraction (EBSC)backscatter Kikuchi diffraction (BKD)Kikuchi patternCrystal orientation, structureand perfectionMicrostructure50000 points: color code gives theorientation of grainsInelastically scattered electrons diffract off of lattice planes.Diffracted electrons from two cones that strike a detection phosphor and appear as a band.Diffraction can be from a single grain for a fine enough e-beamApplicable to any crystalline material (with submicron resolution good for discriminingbetween phases)