Measurements of Energy and Angular Distributions of Secondary Electrons Produced in Electron-Impact Ionization of Helium

Abstract

The energy and angular distributions of electrons produced in the ionization of helium by electrons with energies between 100 eV and 2 keV have been measured in a crossed-beam apparatus consisting of a fixed hemispherical energy analyzer and a rotatable electron gun. Distributions of secondary electrons (those electrons departing with energies less than one-half that of the incident primary electrons) were obtained for a wide range of energies and for angles between 30\ifmmode^\circ\else\textdegree\fi{} and 150\ifmmode^\circ\else\textdegree\fi{} with respect to the direction of the incident primary electron beam. The observed angular distributions were significantly different from the results of two early electron-impact measurements; however, they agreed to the extent expected with more recent results and with similar proton-impact data. For secondary energies above about 50 eV and for primary energies greater than 300 eV, the energy distributions (the cross sections integrated over angle) were observed to be very nearly equal to the distribution given by the Mott formula for free-electron-free-electron scattering multiplied by the number of electrons in the target.