Angular differential measurements of ionization in fast H+-He collisions

Experimental differential cross sections for single and double ionization of He by impact of fast protons and deuterons are presented. The cross sections are measured at projectile scattering angles in the interval 0.27-4.2 mrad and for energies between 0.1 and 2.0 MeV amu-1. Pure ionization events, in which the electrons that leave the He atom go into continuum states, are experimentally distinguished from ionization reactions associated with charge transfer. The results on pure single ionization by protons show how a structure in the differential cross section at 0.55 mrad first observed at 3 and 6 MeV, gradually appears as the proton energy is increased beyond about 500 keV. Results on double ionization of He at small impact parameters are presented in the form of ratios between cross sections for double and single ionization. Using reasonable models for the dependence on impact parameter of the probabilities for pure single ionization or for single-electron capture, we extract energy-dependent characteristic impact-parameter ranges for the reactions. For pure single ionization, the range is comparable with the K-shell radius of He, and it increases with increasing energy. For capture, the range is also of the order of the target K-shell radius, but it decreases with increasing energy. (author)