Slow Electrons Ejected from He by Fast Charged Particles

An accurate differential ionization cross section for the ejection of zero-kinetic-energy electrons from He by fast charged particles is deduced by extrapolating the Born cross sections for discrete excitations, in the spirit of the quantum-defect theory. Recent electron-impact data by Grissom et al. for secondary electrons of almost zero kinetic energy are in excellent agreement with the theory. The electron-impact data by Peterson et al. for slow secondary electrons show trends consistent with the theory also. The proton-impact data by Rudd et al. are in better agreement with the theory than those by Stolterfoht.