THRESHOLD EFFECTS IN POSITRON SCATTERING ON NOBLE-GASES

The energy dependence of the positronium (Ps) formation cross section for positrons colliding with all the stable inert atoms is analyzed ne ar threshold. Using R-matrix and threshold theories, the corresponding variations with energy of the elastic-scattering and total cross sect ions are predicted and compared to experiment where possible. As the t arget atomic number increases from He to Xe, Wigner cusps are predicte d to develop in the elastic cross section near the Ps formation thresh old, reflecting the progressive increase of the interaction strength b etween the positron and the atoms.