Positron Annihilation in a Simulated Low-Density Galactic Environment

A low-energy positron beam is used to study the behavior of positrons in low-density (≈ ${10}^{-3\mathrm{}}$ Torr) ${\mathrm{H}}_2$ and He. Positronium is formed in flight and the collision-free Doppler-broadened singlet annihilation linewidth in ${\mathrm{H}}_2$ is found to be 6.4±0.1 keV in agreement with the calculations of Bussard, Ramaty, and Drachman. The measured fraction of positrons surviving below the positronium threshold is larger than predicted. We conclude that the neutral interstellar environment cannot be ruled out as a galactic annihilation site.