Ionization Cross Sections of Gaseous Atoms and Molecules for High-Energy Electrons and Positrons

Ionization cross sections of forty gases have been measured for electrons of kinetic energies 0.1-2.7 MeV. The measurements are absolute and extensive precautions have been taken to minimize systematic and accidental errors. The energy dependence of the measured cross sections is accurately described by the Bethe asymptotic formula involving two parameters that represent important atomic properties. Comparisons have been made between H₂ and D₂ and between CH₄ and CD₄; the observed differences are of the order of 1% and too small to be resolved with certainty. A close comparison has been made between positrons and electrons in Ar at 0.67 and 1.1 MeV; the cross sections are observed to be equal within a probable error of 0.5%.