Double ionisation of noble gases by positron and electron impact

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.