A comparison of experimental and theoretical total cross sections for low-energy positron-helium scattering

The experimental values of the positron-helium total cross section at 2 eV recently measured by Kauppila et al. (1976, 1977) and Burciaga et al. (1977) are compared with a theoretical value which is believed to be a conservative lower bound on the exact result. Both experimental values are found to be significantly lower than the theoretical value, and it is suggested that the discrepancy is due to the neglect of small-angle scattering in the experiments. The angle below which scattering would need to be neglected to explain the discrepancy is calculated for each experiment, and its value is consistent with the known or probably resolution of the apparatus.