Quasifree-electron process in the single ionization of He by fast protons

Double-differential cross sections for the single ionization of He by protons are measured at collision energies of 2, 2.8, and 6 MeV. At energy losses larger than 0.75 keV, single ionization is shown to be dominated by binary encounters between the proton and a quasifree target electron. The measured scattering angle and energy loss of the protons show a simple two-body kinematic relation. Furthermore, the measured shapes of differential cross sections are in good agreement with calculations of Rutherford scattering from electrons.