Positron-Electron Scattering in Helium

Positron-electron scattering in a cloud chamber filled with helium at a pressure of 105 cm of Hg has been studied. The positron source was ${\mathrm{Na}}^{22}$, and the primary positron energies ranged from about 20 to 600 kev. On 2420 meters of track length 1129 scattering events have been found with a fractional energy exchange between positron and electron greater than 10 percent. The frequency of scattering is in good agreement with the theory of Bhabha, but not enough events are available to discrimate between the theory with the effect of exchange taken into account and the theory without exchange (i.e., "ordinary" scattering).