An electron passing through a solid may be scattered

• not at all
• once (single scattering)
• several times (plural scattering ), or
• very many times (multiple scattering )

Each scattering event might be elastic or inelastic. The scattered electron is most likely to be forward scattered but there is a small chance that it will be backscattered.
Check that you understand the significance of each these terms.

The probability of scattering is described in terms of either an "interaction cross-section" or a mean free path.

When the solid specimen is thicker than about twice the mean free path, plural scattering is likely. This can be modelled using the Monte Carlo technique. The important features are the fraction of electron scattering forward and backwards and the volume of the specimen in which most of the interactions (scattering events) take place.

 

Experiment with the effect of specimen density and atomic numbers, electron energy and direction of incidents on the interaction volume.

1.What fraction of 200kV electrons are backscattered from 25mm of Al?
2. What is the diameter of the region from which 90% of the backscattered electrons are emitted during bombardment of solid titanium with 50kV electrons? [NB This would be the effective resolution of an SEM operating in these conditions.]