Low-energy quenching of positronium by helium

Very-low-energy scattering of orthopositronium by helium has been investiga ted for the simultaneous study of elastic cross section and pickoff quenchi ng rate using a model exchange potential. The present calculational scheme, while it agrees with the measured cross section of Skalsey et nl., reprodu ces successfully the parameter (1)Z(eff), the effective number of electrons per atom in a singlet state relative to the positron. Together with the fa ct that this model potential also leads to an agreement with measured mediu m energy cross sections of this system, this study seems to resolve the lon g-standing discrepancy at low energies among different theoretical calculat ions and experimental measurements.