POSITRON LIFETIME MEASUREMENTS IN POLYETHYLENE UNDER DIFFERENT CONDITIONS

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