Positron annihilation lifetime and coincidence Doppler broadening study of γ-irradiated polyethylene

We performed positron annihilation lifetime and coincidence Doppler broadening measurements on γ-irradiated high-density polyethylene (HDPE) at temperatures of 80, 293 and 373 K as a function of the measurement time. At 293 K, it is shown that more than one order of magnitude increase in free radical concentration (1018 spins/g) results in a reduction of the ortho-positronium (o-Ps) yield only by ∼2% for HDPE irradiated in vacuum. Stronger Ps formation inhibition was observed in an HDPE irradiated in air, which is attributed to a larger CO concentration. After temperature was raised from 293 to 373 K, free radicals were found ineffective to the Ps formation. Because of the stabilization of CO−e+ complex at 80 K, enhanced positron trapping by oxygen was observed.