Ionization of atomic hydrogen by positrons

Abstract

New data on positron collisions with atomic hydrogen leading to ionization were taken by using the high-intensity positron beam at Brookhaven National Laboratory (BNL). The measured cross section values for positron-impact ionization, which supersede earlier ones, are in good agreement with other experimental and most theoretical results, while the new data for positronium formation, formerly normalized to impact-ionization data, now disagree with recent theoretical predictions.

Recommended Positron Scattering Cross Sections for Atomic Systems

“…The earlier results [150,151] favour a cross section with a peak amplitude around or above 3 Å 2 , and the results of [151] are largely in good agreement with the later work from Wayne State [148]. However, the more recent result of the Bielefeld-Brookhaven collaboration [152], which they claim is an improved measurement to that of [151], indicates a cross section with a lower peak magnitude -around 2 Å 2 . We can seek some guidance in this case from theory where there are now many reasonably reliable calculations of Ps formation.…”

“…Atomic hydrogen (H) is a notoriously difficult target to prepare for accurate quantitative scattering measurements in the laboratory. To our knowledge there have only been a few experimental determinations of absolute cross sections for positron scattering by atomic hydrogen, and these include measurements of the total scattering cross section [147][148][149], the positronium formation cross section [148,[150][151][152] and the direct ionization cross section (which does not include Ps formation) [151][152][153][154].…”

“…A variety of experimental techniques have been used to determine the positronium formation cross section for atomic hydrogen. A number of experiments in the Brookhaven-Bielefeld collaboration were carried out during the 1990's [150][151][152] with final values for the Ps formation cross section being provided by [152]. They used a crossed beam configuration and ion detection scheme to derive both Ps formation and impact ionization cross sections with absolute normalisation being provided via concurrent electron ionization measurements which were normalised to earlier literature values [156].…”

“…For positron impact ionization, the results of [152] were intended to supercede those of [151,153] from the same group/collaboration. These later results from the Bielefeld/Brookhaven collaboration are in good agreement with the results of the UCL group [154], which were undertaken primarily to validate the earlier measurements of [153], which were considerably larger than most contemporary theoretical calculations of the ionization process.…”

The Physics of Positron Annihilation in the Solar Atmosphere

“…He þ Ps, which were treated in x 2.2.) The cross section for impact ionization of H was measured by Jones et al (1993) from threshold to 700 eV and confirmed by Hoffman et al (1997). These results are significantly lower than those of Spicher et al (1990), which were used by Wallyn et al (1994) in their study of annihilation in the ISM.…”