Correlation of Orthopositronium Annihilation and the Surface Properties of Silica Gels as a Function of Temperature

Abstract

The lifetime spectra of positrons annihilating in silica gels of various pore sizes as a function of temperature are investigated. There are two long lifetime components which are attributed to the annihilation of o-Ps under various processes. The intensities of the two long lifetime components change with temperature. These changes cannot be attributed solely to the diffusion process. Other processes, such as the formation, quenching, and thermalization of o-Ps in silica gels, must also be taken into consideration.The mean life of the longest lifetime component also changes with temperature. The pattern of this change varies with the pore size and grain size of the silica gel. It seems that positronium atoms are adsorbed on the pore surface of silica gels. A potential model including the effect of adsorption is proposed to explain this phenomenon. The change of annihilation rate of o-Ps is not found to be solely due to the vibration of surface atoms. The quenching due to the presence of the "strained" surface silica groups and its change with the temperature is offered as an explanation.