A measurement of the ionization cross-section of helium ions by electron impact

An apparatus, based on the crossed-beam principle, is described for measuring the ionization cross-section of ions by electron impact. The cross-section for the reaction He$^+$ + e $\rightarrow$ He$^{2+}$ + 2e is determined from the threshold at 54.4 to 1000 eV. The results are compared with the ionization cross-section of atomic hydrogen and with theory. The He$^+$ cross-section, when scaled by the appropriate factor, is very similar to that of atomic hydrogen but increases more rapidly in the neighbourhood of the threshold. At the highest electron energies the cross-section agrees well with that predicted by the Coulomb-Born approximation but is considerably lower at the cross-section maximum. For electron energies above 150 eV the error in the measurement is estimated to be less than $\pm$ 10%.