Binary-encounter stopping cross sections. I. Basic theory and calculations for helium in hydrogen

A model to describe the slowing down of fast-moving heavy particles in matter owing to inelastic collisions is devised using the binary-encounter approximation, and stopping cross sections are calculated specifically for helium in hydrogen. In performing the calculations, each incident-beam charge-state component is treated separately, and the results are combined using the best available experimental information on the equilibrium charge-state fractions. The results, both for the partial stopping cross sections of the individual charge-state components and the total stopping cross section of the equilibrium mixture, are compared as a function of incident energy with the results of various experiments and with other theories. The calculated total stopping cross section is in reasonable agreement with experiment over a rather wide energy range and is superior at energies below 1 MeV to the results of the Bethe theory, but discrepancies exist between calculated and experimentally determined partial stopping cross sections. These discrepancies are discussed and possible explanations for them are suggested.