The electrons are represented by a ``uniform attenuated beam'' model, hereafter
referred to as ``AB'';
electrons of a given initial energy and current density are incident
normal to the surface of the target, and are not
deflected in direction during passage through it ( i.e.,\
no backscattering).
The electrons are
uniformly attenuated in energy, with the energy loss given by the
Bethe-Bloch formula[6]. No ``energy straggling'' is included
in this calculation, so that there is one--to--one correspondence
of electron energy and electron depth in the target. The energy I in the
energy-loss formula that
characterizes the binding of the
electrons in the target is given by 
eV[5]. The
energy loss of a compound target is approximated by 
,
where subscript m designates the energy-loss for the element m in the target,
weighted by the appropriate fractional density[6]. The attenuation of
the electrons is stopped when the electron energy 
eV. Electron
production caused by x-ray photoionization is not included.