Abstract
The results of positron lifetime measurements as a function of a dose of fast neutron irradiation and of annealing temperature in the ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ supeconductors are presented. The mean positron lifetime is found to increase as a function of a dose of irradiation up to 9.0\ifmmode\times\else\texttimes\fi{}${10}^{17}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. From the results of positron lifetime experiments, we found two types of defects generated due to fast neutron irradiation: cationic vacancies (\ensuremath{\tau}\ensuremath{\approxeq}220 ps) and large microvoids (\ensuremath{\tau}\ensuremath{\approxeq}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\ifmmode\pm\else\textpm\fi{}5 \AA{}) and the critical current density due to fast neutron irradiation is found.
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