Annihilation libre de l'ortho-positonium formé dans certaines poudres de grande surface spécifique

Abstract

The two long mean-lives observed for positron annihilation in some powders (Al2O3, MgO, SiO2) result from ortho-positronium formation. The longest mean-life corresponds to the triplet-state part that annihilates between the powders grains. Under vacuum, this annihilation is free and one observes the free triplet mean-life (14 x 10-8 s). In air, oxygen quenching reduces this mean-life by a factor two. The other positron mean-life (1.8 x 10 -9 s) corresponds to the ortho-positronium which annihilates by pick-off inside the grains. Contrary to the hypothesis that has been advanced, positronium presence is not due to adsorbed gases on area, but is a property of the amorphous state of these samples. The positronium abundance increases with the degree of disorder of the sample. The relative abundance of free ortho-positronium depends on the particle size of the powder ; it is a function of specific area