Core-loss spectra of thicker specimens are strongly influenced by plural scattering. Plural inelastic events increase the background Ib underneath an ionization edge by an amount dependent on t/λ, t being the specimen thickness and λ the total-inelastic mean free path. However, plural scattering also contributes to the integral core-loss signal Ic. In fact, if the latter were integrated over a sufficiently large energy window Δ, the signal/background ratio (SBR=IC/Ib) might be expected to be independent of t.Figure 1 shows K-edge signal/background ratios for elemental carbon and silicon, presented as measured data points (for a collection semi-angle of l0mrad, 120keV incident energy and Δ=100eV) and as solid curves calculated by convolving a power-law edge profile with a delta-function approximation of the low-loss region. For silicon, SBR falls off more slowly with increasing t/λ, as expected from its higher edge energy. This trend is confirmed by measurements on amorphous silicon dioxide depicted in Fig.2, which shows the signal/background ratios of the oxygen and silicon K-edges as a function of thickness.
