POSITRONS FROM SUPERNOVAE

Positrons are produced in the ejecta of supernovae by the decay of nucleosynthetic Co-56, Ti-44, and Al-26. We calculate the probability that these positrons can survive without annihilating in the supemova ejecta, and we show that enough of these positrons should escape into the interstellar medium to account for the observed diffuse Galactic annihilation radiation. We assume current supernova models with possible mixing and consider the limiting conditions of positron propagation, where they are either trapped locally in the ejecta or allowed to stream through the ejecta. The surviving positrons are carried by the expanding ejecta into the interstellar medium where their annihilation lifetime of approximately 10(5)-10(6) yr is much longer than the average supernovae occurrence time of approximately 10(2) yr. Thus, annihilating positrons from thousands of supernovae throughout the Galaxy produce a steady diffuse flux of annihilation radiation. We further show that combining the calculated positron survival fractions and nucleosynthetic yields for current supernova models with the estimated supernova rates and the observed flux of diffuse Galactic annihilation radiation suggests that the present Galactic rate of Fe-56 nucleosynthesis is approximately (0.8 +/- 0.6) M. per 100 yr.