Threshold effects in positron scattering on noble gases

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Abstract
The energy dependence of the positronium (Ps) formation cross section for positrons colliding with all the stable inert atoms is analyzed near threshold. Using R-matrix and threshold theories, the corresponding variations with energy of the elastic-scattering and total cross sections are predicted and compared to experiment where possible. As the target atomic number increases from He to Xe, Wigner cusps are predicted to develop in the elastic cross section near the Ps formation threshold, reflecting the progressive increase of the interaction strength between the positron and the atoms.