Cartoon of the generation of a secondary-electron cascade 

Cartoon of the generation of thermalized secondary electrons. The cartoon is only semi-physical, insofar it conserves energy. However, it ignores binding energies, which is semi-valid for the higher electron energies but is certainly not so for the lower ones. It assumes that each inelastic collision divides the energy of the incoming electron equally between it and the electron it releases. Like I say, this is just a cartoon... 

The semitransparent cube has edges that are 180 Angstroms in length.