The pick-off annihilation process of ortho-positronium in zeolite ZSM-5 and Silicalite-1 under the influence of gas environment and adsorbed water

Abstract

This study evaluates the influence of gas environments and adsorbed water on the pick-off annihilation process of ortho-positronium (o-Ps) lifetime in zeolite Silicalite-1 and ZSM-5. The results indicate that short-lifetime components (Ï„₁, Ï„₂, Ï„₃) are minimally affected, whereas long-lifetime components (Ï„₄, Ï„₅) and annihilation intensity (I₄, I₅) vary significantly depending on the gas environment. Vacuum conditions are optimal for analyzing pore structures, while air significantly reduces o-Ps lifetime due to quenching by Oâ‚‚. Inert gases such as Ar and Nâ‚‚ have a lesser impact and can serve as alternatives when vacuum conditions are not available. The effect of adsorbed water is categorized into three drying temperature stages, primarily influencing the Ï„₅ and I₅ components. Below 200°C, retained water decreases o-Ps lifetime. Between 200°C and 250°C, rapid water desorption from the pores leads to a notable increase in Ï„₅ and I₅. Beyond 250°C, water is nearly eliminated, and these parameters reach a saturation state. Additionally, Silicalite-1 exhibits higher o-Ps lifetime and annihilation intensity than ZSM-5, reflecting differences in pore size and density between the two materials. These findings highlight the crucial role of environmental conditions in zeolite structure analysis. Vacuum remains the optimal choice for ensuring measurement accuracy, while the influence of adsorbed water must be controlled to prevent measurement deviations.