Validity of the continuous-slowing-down approximation in electron degradation, with numerical results for argon

The electron degradation spectrum plays a key role in describing the electron slowing-down processes in matter. The validity of a method to obtain the electron degradation spectrum approximately, i.e., the continuous-slowing-down approximation with full accounting of electron production, is examined. Numerical results derived with this method are compared with the direct solution of the Spencer-Fano equation for argon using the same set of electron-collision cross sections. The method is effective at electron kinetic energies much higher than the first ionization threshold but fails at low electron kinetic energies even when the kinetic energy of an incident electron is high. Reasons for this failure are discussed in detail