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 $\frac{({\sigma }_{-}-{\sigma }_{+})}{({\sigma }_{-}+{\sigma }_{+})}$, 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 $R_{\mathrm{inelastic}}$ is generally smaller than $R_{\mathrm{elastic}}$. This suggests that the inelastic scattering is better described by the first Born approximation, in which $R=0\mathrm{}$, than is the elastic scattering.