Abstract
The differential cross sections for double and single ionization and the corresponding charge-state fractions of He resulting from impact of fast protons have been measured as functions of both the scattering angle (0.25-4.1 mrad) and the impact energy (300-1000 keV) of the proton. The differential cross sections smoothly decrease with the scattering angle, but the fraction of doubly charged ions, F2, exhibits a distinct peak at ≅0.9 mrad. This peak is not easily explained in terms of the single- and double-ionization mechanisms thought to operate at these energies.