Positron annihilation and magnetic susceptibility measurements on annealed fast-neutron irradiated crystalline quartz

Abstract

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.