Observation of Threshold Effects in Positron Scattering from the Noble Gases

Abstract

Channel coupling is a phenomenon that has been investigated for many scattering processes, and is responsible for the formation of cusps or steps in the cross sections for open scattering channels at, or near, the onset of a new scattering channel. It has long been speculated that the opening of the positronium formation channel may lead to the formation of such cusp features in the elastic positron scattering cross section. In this work, elastic scattering of positrons has been measured in the region of the positronium formation threshold for the noble gases He-Xe. Cusplike behavior is observed and, while the features which are observed appear broad, they represent a magnitude of between 4 and 15% of the total elastic cross section. No evidence is found of any other features in this region, at least within the uncertainty of the present data, discounting the possibility of scattering resonances.

Low-energy positron interactions with krypton

“…The [43], and Stein et al [44] and (b) theoretical models from McAlinden and Walters [36], Gilmore et al [4] and Dunlop and Gribakin [45]. present ROP result shows good qualitative agreement with the previous calculations of Parcell et al [2] and Schrader [40] below about 15 eV. Above this energy qualitative and quantitative differences are observed between the result of [2] and the present ROP cross section, with the ROP result lying in excellent agreement with the present experimental result between 20 and 60 eV.…”

“…Such effects might be expected at or near the thresholds for Ps formation, electronic excitation, and ionization. This has been the subject of a recent detailed study by our laboratory [40]. In this work strong channel coupling, manifesting as Wigner cusps, was observed in all of the rare gases at the Ps threshold.…”

Resonance phenomena ine+-Li collisions

“…Opening a new scattering channel may influence the cross sections in the old channel. Threshold effect has been a focus of discussion for many decades [19,20], since Wigner first predicted the presence of cusp structures. Experimental observation and theoretical calculation have demonstrated the existence of threshold cusps for a low-energy electron collision with a lithium target (Eyb and Hofmann [21] and Burke and Taylor [22]).…”

Positron scattering from molecular hydrogen

“…As discussed previously [22], the positronium (Ps) formation cross section can be determined by measuring the number of positrons which are lost upon transmission through the gas cell, when the RPA is set to transmit all positrons which have lost any energy (as they are no longer magnetically confined, the positronium atoms are lost from the beam). By comparing the intensity of the positron beam below the positronium formation threshold to that at the energy of interest, it is possible to determine the proportion of positrons undergoing the positronium formation process.…”