A Search for Wigner Cusps and Resonances in Positron Scattering by Atoms and Molecules

Detailed elastic cross-section measurements in the vicinity of the positronium (Ps) formation threshold for He, Ne, Ar, Kr and Xe show a cusp-like feature, made visible by a small and localised depression of between (2 to 15%) of the cross-section value. Other near-threshold structures such as scattering resonances have been demonstrated in close-coupling and convergent-close-coupling calculations for helium. This investigation is undertaken to explicitly search for the presence or otherwise of scattering structures using a high efficiency (∼ 1 % statistical uncertainty) and high resolution (< 70 meV) transmission based apparatus.