Threshold behavior of ionization and excitation cross sections of He and Ar by positron impact

Ionization cross section, Qion, measurements by positron impact on He and Ar atoms have been performed using a time-of-flight method, and analyzed with interest in their threshold behavior. The experimental data of this work and our previous ionization cross section data are compared with those of Knudsen et al. and other data. The exponent (n) of the excess energy, E−Eion, is 1.0–1.3, which differs from the theoretical value n = 2.651. The threshold behavior of excitation cross sections, Qex, by positron impact on the same atoms, is also discussed.