ONE-PHOTON ANNIHILATION OF NONTHERMAL POSITRONS IN THE INTERSTELLAR-MEDIUM

In the nonrelativistic limit, the direct one-photon annihilation rate of nonthermal positrons with bound atomic electrons is calculated. For various spectral power indices, the total annihilation rate of positr ons with an atom is calculated by considering the K- and L-shell contr ibutions. The total number of one-photon annihilation events for the a bundant interstellar elements (H, He, C, N, O, Ne, Mg, Si, S, and Fe) with respect to the hydrogen atom are evaluated. The result shows that in the interstellar medium the main contributors to one-photon positr on annihilations are Fe, O, and Si atoms for low power index (Gamma = 6), Fe, H, and He atoms for intermediate power index (Gamma = 7), and H, He, and O atoms for high power indices (Gamma = 8-10). The total nu mber of annihilation events is quickly decreasing with increasing powe r index. The L-shell contribution is increased as the power index incr eases; for example, the L-shell contribution of atomic oxygen is about 6% and 38% for Gamma = 6 and 10, respectively.