ORTHOPOSITRONIUM STUDY OF POSITRON-IRRADIATION-INDUCED SURFACE-DEFECTS IN ALUMINA POWDER

Three-quantum-yield measurements and orthopositronium (o-Ps)-lifetime spectrometry at low temperatures are used to study the interaction of positronium with the surface in fine powders of aluminum oxide. It is found that electron and/or positron irradiation of the specimen induces surface defects which influence the positronium in three ways: (1) A surface positroniumlike bound state is created, (2) the fraction of o-Ps escaping from the particles is slightly inhibited, and (3) the escaped o-Ps is quenched into two-quantum decay upon collisions with the surface defects. It is found that the surface Ps state is not populated at the expense of the interparticle Ps. The most likely surface defects are Al2+ or Al0 due to the migration of irradiation-induced interstitials. The techniques of long-lifetime spectrometry and of three-quantum-annihilation-rate measurement could be used to study both the diffusion of bulk defects to the surfaces, and the interactions of o-Ps to surface defects.