CHARGE-EXCHANGE REACTION-RATES OF THERMAL POSITRONS IN H, H-2, AND HE

The annihilation rates of thermal positrons by charge exchange in thre e different media (H, H-2, and He) are calculated using new experiment al cross sections. We show that the values previously used for atomic are more than one order of magnitude lower than those calculated here. We confirm the idea that the contribution of He atoms to the annihila tion rate in the interstellar medium can be neglected. Moreover, the c ontribution of interstellar grains to the decrease of the positronium fraction is found to be lower than the values published previously.