Effect of oxygen impurities on positronium formation in voids of vanadium

To clarify the effects of oxygen impurities on positronium (Ps) formed in irradiation-induced voids, measurements of the angular correlation of two photon annihilation radiation (ACAR) have been done on vanadium samples doped with oxygen impurities and subsequently irradiated with fast neutrons of doses up to 5.5x1020 cm-2 at about 400 degC in the Japan Materials Testing Reactor (JMTR). It has been shown that slight contamination of voids with oxygen impurity atoms, presumably submonolayer adsorption on the void surface, causes Ps formation. On the other hand, the considerable contamination leads suppression of Ps formation. Energy loss process of Ps in voids is found to be also influenced by the surface contamination. (author)