The single-ionization cross sections U + for positron impact on H z , He, Ne and Ar have been measured from the ionization threshold to the high-energy region where the positron-and electron-impact cross sections merge. At the cross section maximum, the results are 1.4-1.6 times larger than the electron cross sections. Our data agree well with those published earlier for H, and He. For all but the lowest impact energies, agreement is found with recently published quantum and classical theoretical calculations. At low energies, for H, and He, the positron cross section falls below that for electrons and approaches zero faster than the electron cross section. The behaviour of the ionization cross section near threshold is discussed.

The ratio of the double to single ionisation cross section, R'", has been measured over a wide range of energies for positron and electron impact on He. At high velocities (>1-2 MeV amu-') the electron results, in accord with previous studies, merge with those for antiproton impact. Using a beam of low-energy positrons, a similar effect is found with proton data, further supporting the hypothesis that the observed difference in the electron and proton double ionisation cross sections is a result of their opposite charge.

Double ionisation of the noble gases Ne and Ar has been studied using beams of low-energy electrons and positrons. The results show that, except near threshold, the ratio of the double- to single-ionisation cross sections, R(2), is greater for electrons than for positrons, with, at high velocities, the former merging with antiproton data and the latter with the proton values. These data confirm that the double-ionisation cross section is dependent upon the sign of the charge of the projectile.