Elastic-backscattering probability

The energy of an electron in a solid will be decreased by inelastic interactions. There is still a finite probability, however, that a certain fraction of the primary electrons will not be inelastically scattered and will enter the analyzer. The survival probabihty for an electron, with a given initial energy and a trajectory of length X, is related to the IMFP. Thus, the elastic-backscattering probability, rj, can be expressed by... 

The elastic backscattering probability can also be determined experimentally from measurements ofthe elastic-peak intensity, I(ASi). If the function G(xj AS2) can be obtained from MC simulations, the IMFP can be determined from Eq. (3.2.3.11). 

In all practical circumstances the characteristic angle that controls the angular profile near 6 = 0 (table 1) is quite small, and in reflection geometry the small angle loss must be accompanied by an elastic reflection in order for the electron to be detected in the spectrometer (fig. 1). Elastic backscattering is much more probable than the inelastic process, and furthermore the elastic momentum transfer is taken up by the whole crystal. 

Fig. 30. Schematic representation of the backscattered secondary-electron spectrum associated with an (isolated) external source of monoenergetic electrons of energy E. "ntis logj(E) vs. log (E) display mode emphasizes the separation of the spectrum into three parts (1) the secondary cascade which is bounded at high energies (at E p) by (2) the region of rediffused primaries which is bounded by (3) the elastic peak. At low energies the cascade is attenuated by the escape probability P(E). Auger and characteristic loss processes, among other things are not included in this idealized spectrum. (From Ref. " )...

The probability of elastic scattering varies strongly with atomic number, as Z, but the probability of backscattering, rj, varies much less. For 20keV electrons it is 0.06 for carbon and increases to 0.5 for silver [68] (shown in Fig. 3.21 of [38]). If the scattering is very efficient, in a short distance into the specimen, half the electrons will be going forward and half backward. Thus the backscattering coefficient saturates near 0.5 there is only a few percent difference between values for silver and uranium. 

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