POSITRON-INERT-GAS-ATOM ELASTIC DCS MEASUREMENTS

In this paper we review prior measurements and report new measurements of elastic differential cross sections (DCSs) obtained in our laborat ory for positron scattering by the inert gas atoms He, Ne, Ar, Kr, and Xe. The gases (Ne, Ar, Kr, and Xe) for which low-energy measurements have been made below and above the positronium (Ps)-formation threshol ds show evidence that the Ps-formation channel is having an observable ''absorption'' effect on the elastic scattering channel when compared with calculations that only consider the elastic channel. At low ener gies where only the elastic channel is open, good agreement is found w ith available calculations. DCS measurements made at high energies (gr eater than or equal to 200 eV) for He, Ne, and Ar are also found to be consistent with theoretical calculations. At intermediate energies (a bove the Ps-formation thresholds and below a few hundred eV) theory ha s not yet been able to match the relative DCSs obtained experimentally owing to the difficulty of including the effects of the most importan t inelastic channels (i.e., Ps formation and direct ionization). Some initial attempts by our group to obtain absolute elastic DCSs at fixed scattering angles versus positron energy revealed unexpected structur e between 55-60 eV for Ar and at 25 and 200 eV for Kr. Our most recent measurements reported here do not confirm the prior observations of t hese structural features.