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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.