Angular correlation of positron annihilation radiation, positron lifetime spectra, and Doppler broadening of annihilation radiation data for silica aerogels were obtained at temperatures which ranged from 13 to 295K. Spin conversion of positronium at the surfaces of the amorphous grains was observed when the aerogel was sufficiently irradiated with positrons at low temperatures. This indicated that paramagnetic centers were introduced into the first layer of the surface of amorphous grains by the irradiation. The surface paramagnetic defects were unstable at temperatures greater than 150K, and were suggested to be non-bridging O hole centers.

H.Saito, Y.Nagashima, T.Hyodo, T.Chang: Physical Review B, 1995, 52[2], R689-92