Positronium formation in sol-gel-prepared silica-based glasses: temperature and positron-irradiation effect

Silica, titania-silica, and zirconia-silica samples were studied at room temperature (RT) and low temperature (LT similar to 50 K) by means of positron annihilation lifetime spectroscopy. It was confirmed that the samples were composed of oxide particles with finer pores dispersed within them and larger pores between the particles. The effect of e(+)-irradiation during the measurements was checked. The o-Ps lifetime originating from the larger pores was quenched at LT for all of the samples. For the zirconia-silica sample, this effect was observed even at RT. Quenching is an indication of ortho-para positronium conversion on paramagnetic centers formed due to e(+)-irradiation. These centers appear to be the reason why the longest lifetime as a function of the temperature does not follow the Goworek-Gidley model at temperatures below RT. Above RT, the Goworek-Gidley model fits well with the experimental values for the longest lifetime. (C) 2004 Elsevier Ltd. All rights reserved.