Positron lifetime measurements were performed on polyethylene probes of different densities under various atmospheric conditions within a [MATH]-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 [MATH] and an intensity I. In the dose range investigated the behaviour of the lifetime parameters of the longest lived ([MATH]) and the second long-lived ([MATH]) component, respectively, could be explained by the [MATH] 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 ([MATH]).