Identification of ortho-positronium annihilation sites in Y zeolites by positron lifetime measurements in different environments

The annihilation sites of ortho-positronium formed by positrons incident on Y zeolites with different Si/Al ratios are investigated by positron annihilation lifetime spectroscopy in different environments. Comparison of the measured ortho-positronium lifetimes by exposure of the zeolites to argon, nitrogen, water or vacuum enables us to determine the annihilation process and the specific annihilation site within the zeolite samples. The third lifetime component is ascribed to ortho-positronium pick-off annihilation in or around the apertures of the sodalite cage and in the connecting channels, and is reduced in water because of the physisorbed molecules that hinder the ortho-positronium mobility. The longest fourth lifetime component is attributed to ortho-positronium self-annihilation in the interparticle gaps, but is significantly suppressed in the presence of polar molecules, which suggests a contribution from ortho-positronium pick-off annihilation in the supercage.