Double differential cross sections for electron ejection from helium by fast protons

Measurements are presented of the cross sections for the ejection of electrons from helium atoms by protons, differential in the electron ejection angle and energy. The proton energies range from 20 to 100 keV, while the electron energies range from 5 to 100 eV and the ejection angles from 0 to 1000. The measurements are normalised by comparing the present estimates of the total cross section, derived from double integration of the measured distributions, with published absolute values of the ionisation cross section. Although the energy distribution agrees with existing data, there is substantial divergence in the angular behaviour. As the measurements include the zero-degree region, the electron capture into the continuum peak is examined in detail and an estimate of the total cross section for this process is made over the present range of proton energies. (author)