Defect properties of neutron-irradiated YBa2Cu3O7 superconductors probed by positron annihilation

The results of positron lifetime measurements as a function of a dose of fast neutron irradiation and of annealing temperature in the YBa2Cu3O7 supeconductors are presented. The mean positron lifetime is found to increase as a function of a dose of irradiation up to 9.0×1017 cm-2. From the results of positron lifetime experiments, we found two types of defects generated due to fast neutron irradiation: cationic vacancies (220 ps) and large microvoids (465 ps). From the results of isochronal annealing experiments, we found that the intensity corresponding to the large microvoid decreases as a function of increasing temperature. The annealing also produces a multivacancy cluster. A direct correlation between the concentration of large microvoids (size, 25±5) and the critical current density due to fast neutron irradiation is found.