Positronium annihilation in porous silica

Abstract

Annihilation of positronium is studied in silica gel. Analysis of annihilation spectra is shown to carry information regarding pore size, pore size distributions, pore surface chemical reactivity, and pore surface defects. Thermally induced D$\sb1$ and D$\sb2$ strained surface rings are detected and their density is estimated. Pore size distributions are determined from annihilation spectra with comparable accuracy to standard BET gas adsorption analysis. Interactions between positronium and air, nitrogen, oxygen, and ammonia are studied at room temperature and at condensation temperatures in the pores for untreated and derivatized surface samples. Pore surface with grafted species of F, Cl, I, and CH$\sb3$ groups are shown to be slightly less effective as quenchers than untreated OH surface samples. Nitrogen is found to behave as an inert substance with respect to positronium. Quenching by oxygen in the pores is shown to proceed by the conversion process at room temperature, and through the pickoff mechanism when adsorbed on samples at 77 K. Ammonia is found to produce higher annihilation rates when adsorbed on OH versus F surface samples, due to greater perturbation of intramolecular potential caused by adsorbent/adsorbate interactions.