The annihilation characteristics of positrons in vitreous samples were studied at temperatures of between 20K and ambient. In this temperature range, almost all of the positrons were trapped by open spaces, and annihilated mainly from positronium states. The lifetimes of ortho-positronium increased with increasing temperature, and the distribution of the lifetimes was shallower at 20K than that at room temperature. This was attributed to a thermal expansion of the open spaces, and to a broadening of their size distribution. At temperatures ranging from 170 to 200K, a structural transition of the open spaces was detected. The transition temperature was lowered by electron irradiation or by the presence of hydroxyl groups. The results were attributed to the fact that the activation energy of the transition was decreased by the introduction of defects.

A.Uedono, T.Kawano, S.Tanigawa, A.Urano, M.Kyoto, H.Itoh: Journal of Physics - Condensed Matter, 1995, 7[26], 5139-49