Scattering of 300-MeV positrons from cobalt and bismuth

Abstract

Positrons and electrons from the Stanford Mark III linear accelerator have been scattered from cobalt and bismuth at 300 MeV. The ratio R, equal to (σ--σ+)/(σ-+σ+), has been measured at a number of angles from 10° to 45° for cobalt and from 5° to 45° for bismuth. Two experiments are reported: a high-percision experiment with poor energy resolution, suitable for measuring the small values of R found at small angles, where inelastic scattering is not important; and an experiment with somewhat lower precision but better energy resolution, suitable for measuring the larger values of R found at angles where inelastic scattering must be taken into account. The elastic scattering data are in good agreement with phase-shift calculations of Herman, Clark, and Ravenhall, who used nuclear charge distributions which fit earlier electron scattering data. The inelastic data, for which no reliable predictions exist, indicate that Rinelastic is generally smaller than Relastic. This suggests that the inelastic scattering is better described by the first Born approximation, in which R=0, than is the elastic scattering.