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 Al + or Al 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.

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Positron annihilation lifetime‐, Doppler broadening‐, magnetic susceptibility‐, and density‐data, taken on isochronically annealed fast‐neutron irradiated α‐quartz single crystals, are reported. Data given on unirradiated samples allow comparison with results in literature. The sample behaviour versus the fast‐neutron fluences is observed to be related to the amorphization process in quartz. The annealed neutron irradiated samples respond differently to the heat treatment according to whether the irradiation dose is lower or higher than a certain dose Dt, referred to as the “threshold” dose. For samples irradiated with fast‐neutron doses higher than Dt, the free annihilation lifetime of positrons and the positronium pick‐off lifetime are lengthened under the heat treatment; the probability of positronium formation is drastically enhanced and the paramagnetic centres, which are induced by the irradiation, nearly disappear. It is sugggested that this alteration of the measured properties, due to the heat treatment, is rather related to an electronic phenomenon than to a bulk atomic rearrangement. In the high dose case, the observed density decrease of the samples is ascribed to a local distension of the matrix at the annealing. The “threshold” dose is explained with respect to the spike models as the fast‐neutron dose from which the number of spikes in the irradiated matrix is such that many of their interaction‐regions overlap.

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