A remarkable similarity in the s-wave positronium formation cross sections for hydrogen and helium

Improved results for the s-wave contribution to the positronium formation cross section in positron - helium scattering in the Ore gap are found to be remarkably similar to the corresponding results in hydrogen, both in overall magnitude and in the details of the energy dependence. Each cross section displays a very steep rise from the threshold, followed by an abrupt change of slope to a gently rising plateau as the positron energy is increased. Possible explanations of these features, and the similarities between the two sets of results, are considered. The s-wave contributions to the total cross section are quite different for the two atoms, but each one appears to continue smoothly across the positronium formation threshold, apart from deviations which are associated with the increasing significance of virtual positronium as the threshold is approached from below. (author). Letter-to-the-editor