Positronium formation at low temperatures: the role of trapped electrons

Measurements have been carried out of electron spin densities (by electron spin resonance technique) and positronium (Ps) formation probability as fun ctions of Co-60 gamma-irradiation dose in poly(methyl methacrylate) and lin ear poly(ethylene) at 77 K. We observe a linear relationship between the en hancement of the Ps formation and the density of trapped electrons in both polymers. This clear correlation strongly supports the previous suggestion by the authors that the increase in Ps formation with time (that has been o bserved at low temperatures for a number of polymers) can be explained as a reaction of free positrons with trapped electrons produced by the previous ly injected positrons. (C) 2000 Elsevier Science Ltd. All rights reserved.