Cross sections for singly differential and total ionization of helium by electron impact

Abstract

The binary-encounter dipole (BED) model for electron-impact ionization is applied to helium using the continuum dipole oscillator strength calculated from the relativistic random-phase approximation. The resulting total ionization cross section agrees with available experimental data well within the quoted experimental uncertainties. The singly differential $(=\mathrm{energy}$ distribution of ejected electrons) cross section agrees well in shape with available experiments. Because of the remarkable agreement with experiment at all incident and ejected electron energies and the compact analytic form of the cross sections, we propose that the BED cross sections for helium serve as a normalization standard with an accuracy of \ifmmode\pm\else\textpm\fi{}5% from the threshold to 1 keV in the primary electron energy.