Theory of the ionization yield in gases under electron irradiation

The total number N/sub i/(T) of ionizations that an incident electron of kinetic energy T causes in a pure gas obeys an integral equation known as the Fowler equation. Its solution is shown to closely approximate N/sub i/(T) = (T -- U)/W/sub a/ for T exceeding several multiples of the first ionization energy I, where U and W/sub a/ are constants having the energy dimension. Simple formulas express U and W/sub a/ in terms of various cross sections for electron inelastic collisions with a gas molecule. In particular, U - I represents the average kinetic energy of a subionization electron. (35 refs) (U.S.)