Positron trapping by defects in vitreous silica at low temperature

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Abstract
Annihilation characteristics of positrons in vitreous silica (v-SiO2) were studied in the temperature range between 20 K and room temperature (RT). In the measured temperature range, almost all positrons are trapped by open spaces and annihilate mainly from positronium (Ps) states. Lifetimes of ortho-Ps increase with increasing temperature, where the distribution of the lifetimes at 20 K is shallower than that at RT. This fact was attributed to the thermal expansion of the open spaces and the broadening of their size distribution. In the temperature range between 170 K and 200 K, a certain structural transition of the open spaces was found. The transition temperature is decreased by electron irradiation or by the presence of hydroxyl groups. These results were attributed to the fact that an activation energy of the transition was lowered by the introduction of defects.

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