Electron beam and gamma irradiation effects on high density polyethylene studied via positron annihilation lifetime spectroscopy

Abstract

Positron annihilation lifetime spectroscopy is applied to the study of electron beam and gamma irradiation effects on high density polyethylene (HDPE). The electron and γ-irradiations were carried out in air, with a flux of 80 kGy s−1 and 26 10−3 kGy s−1, respectively. In four-component analyses of the spectra, two long-lived states are found, with lifetimes τ3=1100 ps and τ4=2570 ps, ascribed to positronium (Ps) triplet state annihilation in the free volumes present in the crystalline and amorphous phases of HDPE, respectively. After irradiation no changes are observed in the lifetimes. In contrast, the intensity of the second longest-lived component, I3, decreases with the fluence, up to about 8 kGy, then levels off and remains constant up to 300 kGy. For the intensity of the longest-lived component, after electron irradiation, two regions may be identified as the fluence increases: a rapid drop, followed by a much smoother decrease. The initial rapid decrease seems to be absent in the case of γ-irradiation. These results are discussed on the basis of the spur and free volume models.