Free migration of vacancies in niobium at 250 K

Annealing of defects in high-purity niobium after low-temperature irradiation is investigated using perturbed angular correlation of $\gamma$ rays from the radioactive probes ${}_{}{}^{100}{}_{}{}^{}\mathrm{Pd}$ and ${}_{}{}^{111}{}_{}{}^{}\mathrm{In}$ which are deeply implanted by heavy-ion-induced nuclear reactions. The trapping behavior of the two different probes discriminates between interstitials and vacancies. When the data are combined with defect-orientation measurements in a single crystal we conclude that single vacancies are migrating freely at 250 K.