Slow-positron annihilation analysis on optical degradation of ZnO white paint irradiated by protons

The optical degradation in ZnO white paint under low energy proton exposure was investigated in terms of slow-positron annihilation spectroscopy. Experimental results show that with increasing proton fluence, the S-parameter of the Doppler broadening spectrum gradually decreases, and the W-parameter increases. The slope plot of the fitting S-W changes under the proton exposure. The decrease of S-parameter can be attri-buted to a decrease of zinc vacancy content and the formation of quasi-positroniums. The quasi-positronium is viewed as a bounded state of a singly ionized oxygen vacancy (trapping an electron) with a positron, the formation of which could reduce the positron annihilation rate and thus the S-parameter. The decrease of S-parameter demonstrates the amount increase of singly ionized oxygen vacancy of ZnO white paint caused by proton irradiation. The change of the S-W plot slope is related to the transformation of doubly ionized oxygen vacancies into singly ionized oxygen vacancies under proton irradiation.