Positron lifetime measurements were performed on polyethylene probes of different densities under various atmspheric conditions within a Y-dose range from 0 kGy up to 1460 kGy. Ve present here experiments non destroying the sample. Four components could be resolved in each lifetime spectrum; the components consist each of a lifetime T and an intensity I. In the dose range investigated the behaviour of the lifetime parameters of the longestlived (74, 14) and the second long-lived (73, 13)

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... nt, respectively, could be explained by the y-induced recristallisation and the creation of hydroperoxides. The average free volume as a function of the dose m y be calculated out of the lifetime of the fourth component (7~).