Further measurements of positron annihilation lifetimes in neutron irradiated quartz

Crystalline quartz samples, irradiated with fast neutrons, have been studied using positron annihilation techniques. An improved setup was used, allowing much better accuracy than in previous measurements. The positron annihilation lifetime characteristics were measured for the doses: 1.67, 5.7, 9.9, 21, 47x1018 n/cm2. A mean lifetime analysis was done as well as two three-component analyses: one for a lifetime fixed at 120 ps and one without any fixed lifetime. The analysis shows the presence of free parapositronium decay as anticipated, but previously masked by the contributions of the sample holder. It comfirms the existence of a long lifetime of about 1000 ps for which the intensity increases with dose. Contrary to the previous results, a distinct increase of the length of the long lifetime is not found in the studied dose range, which is in agreement with X-ray studies. ((orig.))