Positron–Helium Scattering Cross Sections and Phase Shifts below 19 eV

Abstract

The total cross sections for elastic scattering of positrons in the energy range from 4 to 19 eV have been measured by the method of transmission. By varying a magnetic field applied along the axis of the scattering chamber the transmitted fraction of the beam is altered, from which individual phase shifts can be extracted. s-, p-, and d-wave phase shifts are given over the entire energy range. The s-wave phase shifts are in agreement with values published by Drachman in 1968, while the p- and d-wave phase shifts are intermediate between values calculated by the same author in 1966 and 1971. The experimental results agree with those of Costello et al., and marginally with our own 1972 results, but are significantly different from those of Canter et al. We compute that the Ramsauer minimum in the diffusion cross section must be 0.04πa02 at 1.6 eV while the minimum in the total cross section is 0.11πa02 at 2.1 eV. The shoulder breadth observed in annihilation experiments is in nice agreement with what one would predict from our phase shifts.

Brownian local time

“…The experimental results are in reasonable agreement, both among themselves (except those obtained by Jaduszliwer andPaul (1973, 1974a)) and with the calculations. At an energy near E = 2 eV there is a minimum in the cross section.…”

“…The positron scattering total cross sections on inert-gas atoms were measured by Canter et a1 (1972Canter et a1 ( , 1973Canter et a1 ( , 1974 (Jaduszliwer andPaul 1973, 1974a, b), as well as those for Ne and Ar (Jaduszliwer and Paul 1974b,c). More recently the minimal positron energy was reduced to a fraction of an eV.…”

Recent progress in the description of positron scattering from atoms using the convergent close-coupling theory

“…Amusia et al [89] applied the random-phase approximation (RPA), based on many-body theory [49], to positron-helium scattering at low energies. By using an approximate account of virtual Ps formation they ob-tained good agreement with the elastic-scattering experimental data of Jaduszliwer and Paul [90] and Canter et al [91] at the lowest energies. A similar RPA method was used by Varracchio [92] to calculate positron impact excitation of He into 2 1 S and 2 1 P states.…”

Low Energy Cross-Sections for Positron Interactions with Cyclic Hydrocarbons

“…Measurements of total cross-sections in the low energy range were performed for several linear hydrocarbons by Sueoka's group [1][2][3] and for methane by Kauppila and Stein's group [4]. Numerous measurements exists also for noble gases [4][5][6]. The total cross-section can be measured precisely by an absolute method from the beam attenuation.…”