One‐Photon Annihilation of Thermal Positrons in the Interstellar Medium

In the interstellar medium, one-photon annihilation of thermal positrons with bound atomic electrons is investigated. The total annihilation rate of thermal positrons with an interstellar element is calculated considering the K- and higher shell (L, M, and N) contributions from abundant many-electron atoms. The results show that the main contributions to one-photon annihilation are H, He, and C atoms in hot medium (≡ kT/Ry)=1, H, Mg, and He atoms in warm medium =0.1, Mg, H, and Fe atoms in cold medium =0.01, respectively. In a hot medium, positrons are mostly annihilated with the K-shell electrons; for example, the K-shell electrons of H and He atoms. However, in a cold medium positrons are annihilated with higher shell electrons; for example, the M-shell electrons of Mg atom and the N-shell electrons for Fe atom give main contributions.