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)Torr) H2 and He. Positronium (Ps) is formed in flight and the collision-free Doppler-broadened singlet annihilation linewidth is found to be wider than the 511 keV galactic positron line. The measured fraction of positrons surviving below the Ps threshold is too small to account for this discrepancy. We conclude that the galactic line cannot be due to positrons promptly annihilating in a neutral diffuse interstellar environment.