Screening and distortion effects in positron impact ionisation of helium

Positron-impact ionization of highly charged ions in lowest-order QED theory

“…For helium the theoretical cross-section results shown in Fig. 2(a) are in even better agreement with the theory of Campeanu, McEachran, and Stauffer [7] and the experimental measurements of Fromme et al [8].…”

“…In Sec. III we test the validity of our algebraic and numerical methods by first calculating the positron-impact ionization of rare-gas atoms, where comparisons with nonrelativistic calculations [6,7] and recent experiments [8,9] may be made. We then move along the helium isoelectronic sequence to explore the comparison between electron and positron ionization cross sections and, in the case of U +, de lineate the size of transverse photon effects and the annihilation term.…”

Positron Impact Ionisation of H and He Atoms: The Continuum Model

“…2), the COM gives fairly good agreement with the measurements of Fromme et al (1986) at most energies except between 80-200 eV. In comparison, the DW calculations of Campeanu et al (1987) show much better agreement with experiment up to 250 eV. The DW calculations allow for screening and distortion effects which may explain the better agreement with experiment.…”

“…Ionisation cross sections for e+-He collisions. Experimental data (circles) are from Fromme et al (1986); the solid curve is the COM; and the dashed curve is the DW calculation of Campeanu et al (1987).…”

“…Generally, these methods predict lower cross sections than experiment, except at E::; 40 eV where the results of Ghosh et al and Mukherjee et al show some agreement with the experimental data. In the e+ -He case, there are the detailed DW calculations of Campeanu et al (1987) and Basu et al (1985). In particular, Campeanu et alhave studied the effects of screening and distortion in their calculation of ionisation cross sections.…”