THRESHOLD BEHAVIOR OF IONIZATION AND EXCITATION CROSS-SECTIONS OF HE AND AR BY POSITRON-IMPACT

Ionization cross section, Q(ion), measurements by positron impact on H e and Ar atoms have been performed using a time-of-flight method, and analyzed with interest in their threshold behavior. The experimental d ata of this work and our previous ionization cross section data are co mpared with those of Knudsen et al. and other data. The exponent (n) o f the excess energy, E-E(ion), is 1.0-1.3, which differs from the theo retical value n = 2.651. The threshold behavior of excitation cross se ctions, Q(ex), by positron impact on the same atoms, is also discussed .