Positron annihilation studies of the effect of gamma irradiation dose in polymers

Positron annihilation lifetime (PAL) technique has been applied to study the effect of gamma irradiation dose on the structure of low-density polyethylene and polystyrene. The positron annihilation lifetime measurements were performed with a conventional fast–fast coincidence system. Gamma irradiation was carried out at room temperature with a 60Co gamma source up to a dose of 1000 kGy. The delayed lifetime spectra of irradiated and unirradiated samples were decomposed into three lifetime components. The lifetime parameters give indication of the free-volume size and concentration. It was found that the crosslinking efficiency in the polymer increases significantly with increasing irradiation dose. This appears as a decrease in the intensity of the long-lived positron lifetime components attributed to ortho-positronium formation. Measurements of the Doppler broadening energy spectra of the low-density polyethylene (LDPE) samples have also been performed using a variable mono-energetic positron beam. The trend of the Doppler broadening lineshape S-parameter as a function of the radiation dose supports the lifetime results.