Experimental determination of total scattering cross sections for positron-helium collisions

A magnetic spectrometer for the determination of total scattering cross sections for low-energy positron-atom collisions is described. Results obtained for the scattering of positrons in the energy range 13 to 1000 eV by helium are given and compared with previous experimental and theoretical data. The present results are shown to be in agreement with recent theory and experimentally demonstrate for the first time that the Born approximation is valid in helium above 500 eV. Examination of the present results shows them to be consistent with the sum rule based on a forward dispersion relation. (author)