Annealing of defects induced by Ge irradiation of silica probed with variable energy positrons

Fused silica (Suprasil 2) has been implanted with 2 MeV Ge ions. Defects produced by the implantation have been studied using a variable energy positron beam. The measurements allow depth resolved characterisation of the implantation induced defects, which are found to be produced by both the electronic and nuclear stopping of the ions. It is suggested that the previously unidentified positron-trapping defect associated with the nuclear stopping is an oxygen-decorated vacancy. Heating the sample to a temperature of 600°C demonstrates an annealing stage for this defect between 350–400°C. A region with a low concentration of positron-trapping defects is found to be present close to the sample surface, even in the as-irradiated sample prior to annealing. The depth of this region increases with increasing annealing temperature.