Positron and electron impact double ionisation of helium

The ratio of the double to single ionisation cross section, R (2) , has been measured over a wide range of energies for positron and electron impact on He. At high velocities ()1-2 MeV amu -1 ) 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.