Electron intensities obtained during backscattered-Mössbauer spectroscopy: II. Theoretical and experimental comparisons of emergent angular distributions

Abstract

When gamma-rays from a 57Co source impinge on an absorber containing 57Fe, they can be resonantly absorbed, resulting in the generation of conversion electrons and subsequent Auger electrons, or they can be nonresonantly scattered via photoelectric and Compton processes. The ratio of resonant to nonresonant electrons escaping from the absorber represents the absolute peak height/area obtained from a backscattered-conversion electron Mössbauer spectrum and provides important information helpful in the depth-profiling of multilayer specimens. A simple theoretical model which accounts for resonant and nonresonant electrons emerging from the surface at angles from zero to 2π radians with energies above zero eV has been developed previously. The signal-to-background ( S/ B) ratios from 5 nm iron films and 2.5 × 10 6 nm iron foils have been calculated using this model, and good agreement between calculated and experimental results has been obtained. The intensity of resonant and nonresonant electrons emerging from the absorber in specific angular intervals has also been calculated using this model for a 1.0 μm thick 92.8% enriched 57Fe foil mounted on a stainless steel substrate. The simulated angular distributions for backscattered-resonant electrons and backscattered-nonresonant electrons both provide cosine-type curves and are similar to experimental data, resulting in an isotropic distribution of the S/ B ratio. The predicted S/ B ratio, however, is about twice the S/ B ratio measured experimentally. This discrepancy may result from the neglect of: (i) electrons below 50 eV and (ii) multiple scattering events. Despite these shortcomings, the model proposed in this study is capable of comparing the relative magnitude of the resonant and nonresonant signals, thereby calibrating the resonant signal against the nonresonant background. This procedure appears to provide additional information helpful for nondestructive depth-profiling studies.