POSITRON LIFETIME MEASUREMENTS IN POLYETHYLENE UNDER DIFFERENT CONDITIONS

Positron lifetime measurements were performed on polyethylene probes o f different densities under various atmospheric conditions within a ga mma-dose range from 0 kGy up to 1460 kGy. We present here experiments non destroying the sample. Four components could be resolved in each l ifetime spectrum; the components consist each of a lifetime tau and an intensity I. In the dose range investigated the behaviour of the life time parameters of the longest-lived (tau4, I4) and the second long-li ved (tau3, I3) component, respectively, could be explained by the gamm a-induced recristallisation and the creation of hydroperoxides. The av erage free volume as a function of the dose may be calculated out of t he lifetime of the fourth component (tau4).