Single ionization of H2, He, Ne and Ar by positron impact

The single-ionization cross sections sigma + for positron impact on H 2 , 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. The authors' data agree well with those published earlier for H 2 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 2 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.