FURTHER MEASUREMENTS OF POSITRON-ANNIHILATION LIFETIMES IN NEUTRON-IRRADIATED QUARTZ

Crystalline quartz samples, irradiated with fast neutrons, have been s tudied using positron annihilation techniques. An improved setup was u sed, allowing much better accuracy than in previous measurements. The positron annihilation lifetime characteristics were measured for the d oses: 1.67, 5.7, 9.9, 21, 47 X 10(18) n/CM2. A mean lifetime analysis was done as well as two three-component analyses: one for a lifetime f ixed at 120 ps and one without any fixed lifetime. The analysis shows the presence of free parapositronium decay as anticipated, but previou sly masked by the contributions of the sample holder. It confirms the existence of-a long lifetime of about 1000 ps for which the intensity increases with dose. Contrary to the previous results, a distinct incr ease of the length of the long lifetime is not found in the studied do se range, which is in agreement with X-ray studies.