Interactions between positronium and paramagnetic centers on grain surfaces, induced by ultra-violet (254nm) irradiation or positron irradiation from 22Na, were studied by using positron annihilation and electron spin resonance methods. One oxide sample was a silica aerogel, one was silica powder and one was alumina powder. The positron irradiation from some 10mCi of 22Na, at low temperatures, produced marked changes in the angular correlation of the positron annihilation radiation spectra for all of the samples studied. This demonstrated the spin-exchange reaction of positronium atoms with irradiation-induced paramagnetic centers on the grain surface. Marked changes in the positron lifetime spectra were also produced by ultra-violet irradiation, of silica aerogel which was heat-treated at 800C, and of alumina. No change due to ultra-violet irradiation was observed in silica aerogel which was heat-treated at 800C. The results for silica powder indicated the formation of -OCH2 radicals on the surfaces of silica aerogel which was heat-treated at 200C. The spin-exchange cross-section for collision between positronium and -OCH2 radicals was estimated to be of the order of 10-21m2.

Quenching of Positronium by Surface Paramagnetic Centers in Ultraviolet- and Positron-Irradiated Fine Oxide Grains. H.Saito, T.Hyodo: Physical Review B, 1999, 60[15], 11070-7