Measurement of the Total Cross Sections of Positrons on Helium Atoms in the Energy Range 15 to 23 eV

Abstract

Positron cross sections on helium have been measured by transmission using a weak beam of energy-analyzed positrons in the range 15 eV to 23 eV. The positrons are transmitted in a magnetic field parallel to the axis of the scattering chamber. Comparisons with theory have been made using several sets of phase shifts, and good agreement is found if the latest p- and d-wave phase shifts of Drachman are used. The experiments fail to show any sharp reduction in the transmission at the positronium threshold, from which we deduce an upper limit to the positronium formation cross section of [Formula: see text]. The agreement between our results and those of Costello et al. in the neighborhood of 17 eV is very good.