Electrons, high-energy secondary

Physically the generation of X-rays is often a secondary process preceded hy the ionization of an atom. There are, therefore, several possihilities of X-ray generation depending on the type of the exciting medium - neutrals or charged particles such as electrons and ions and high-energy photons, i. e. X-rays themselves. 

In Eq. (4.18), it is implicitly assumed that the ionization is a direct, one-electron process that is, the contribution of superexcited states to ionization is not included. The latter process is indirect and essentially of a two-electron nature. When the energy loss is much larger than the ionization potential, however, ionization is almost a certainty. For high energies of the secondary electron, Eq. (4.18) approaches the Rutherford cross section, or the Mott cross section if the incident particle is an electron. 

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