THE POSITRONIUM FRACTION BEFORE THERMALIZATION IN HELIUM

Positron interactions with the matter of various media have been studied for a long time in astrophysics, especially since the 511 keV line was detected in the direction of the Galactic center (Johnson, Harnden, & Haymes 1972) and confirmed by gamma-ray line experiments. The abundance of helium in the interstellar medium (the second most important atom after hydrogen) makes the study of the interaction of positrons in that medium a subject of great interest in astrophysics. The positronium fraction before thermalization in helium has already been measured in the laboratory (f1 = 80.7% +/- 0.5%; Brown & Leventhal 1986). The experimental value of the positronium fraction in molecular hydrogen (f1 = 89.7% +/- 0.3%; Brown & Leventhal 1986) has also been evaluated by a Monte Carlo simulation (Bussard, Ramaty, & Drachman 1979) (93%), and recently a reevaluation based on recent measurements of cross sections has been calculated by Wallyn et al. (1993) (90%). Theoretical calculations have already been done (Bussard, Ramaty, & Drachman 1979), (95%) and reevaluated (Wallyn et al. 1993) (98%) for the positronium fraction in atomic hydrogen. We pursue this investigation with helium atoms and obtain 78%. Our result is in excellent agreement with the experimental value of Brown & Leventhal (1986). Astrophysical implications will be emphasized.