The fraction of positrons which survive below the positronium (Ps) formation energy in a low-density galactic environment can directly annihilate with bound electrons in the gas. The details of the measurement of the survival (direct-annihilation) fraction using a low-energy laboratory positron beam are presented for H₂ and He. The fraction is found to be higher in H₂ than had been predicted by Bussard, Ramaty, and Drachman. The galactic positron line has been shown to be consistent with the line shape expected for annihilation in a neutral medium based on this measurement and on a theoretical prediction of the direct-annihilation linewidth.
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Brown et al. (1986) studied this question.
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