ACAD SINICA,INST HIGH ENERGY PHYS,BEIJING,PEOPLES R CHINA
Angular correlation of positron-annihilation radiation, positron-lifetime spectra, and Doppler-broadening spectra of the annihilation radiation from silica aerogel have been measured in the temperature range between 13 and 295 K. Spin conversion of positronium at the surfaces of the amorphous-SiO2 grains is observed when the aerogel is irradiated sufficiently by the positrons at low temperatures. This shows that paramagnetic centers are induced on the first layer of the surface of the amorphous-SiO2 grains by the irradiation. The surface paramagnetic centers are unstable above 150 K and are attributed to a nonbridging oxygen hole center (Si-O-.).