Analysis of RecoilHe+andHe++Ions Produced by Fast Protons in Helium Gas

The cross sections for formation of ${\mathrm{He}}^{+}$ and ${\mathrm{He}}^{++}$ ions have been measured over the energy range from 0.15 to 1.00 MeV for the case of protons incident on helium gas. The individual ions were detected with an electron multiplier that was operated at near 100% efficiency. An observation of the angular distribution of the slow ions in a "field-free" environment yielded essentially an isotropic distribution in the laboratory reference frame. It was also observed that much less than 1% of the ions had energies greater than 1 eV. It was concluded that, in order for the recoil ions to have an appreciable angular distribution, there must be a substantial energy transfer in the collision, such as would be the case for heavy projectile-target combinations.