Air quenching of positronium in mesoporous materials: positron porosimetry

The o-Ps lifetimes in mesoporous materials in vacuum and in the presence of air have been measured. The measured lifetimes are examined in terms of various available models for positron porosimetry. The o-Ps lifetimes measured in vacuum agree well with those predicted from the extended Tao-Eldrup model. In the presence of air, the o-Ps lifetimes are seen to reduce. The enhancement in the annihilation rate of positronium caused by air is seen to increase with decrease in the pore size. The positronium lifetimes measured in the presence of air in the present experiment and the literature data are fitted to a model based on the statistical probability of positronium interacting with the pore wall. This describes the increase in the quenching rate with decrease in pore size by taking into account the reactivity of air as well as the reduction in the average pore size due to adsorption of air on the pore surface. This simple model can be used in positron porosimetry for measurements carried out in the presence of air.