Annihilation of Positrons in Argon II. Theoretical

The results of measurements of the electric field dependence of the direct annihilation rate of positrons in argon are used to test the validity of simple models representing the interaction between a positron and an argon atom. Despite the fact that such models yield results in agreement with electron scattering data, they are inadequate in the case of positrons. By comparing this data with recent measurements of the temperature dependence of the lifetime of positrons in argon, a value of $(33\mathrm{}\pm 5)\pi a_0^{}{}_{}{}^2$ is obtained for the scattering cross section of thermal positrons in argon.