The (100) and the (111) faces of niobium single crystals were bombarded with noble-gas ions having kinetic energies between 100 and 1500 eV. Thermal release spectra were produced [E. V. Kornelsen, Can. J. Phys. 42, 364 (1964)] and characteristic activation energies were inferred as described previously [R. O. Rantanen, A. L. Moen, and E. E. Donaldson, J. Vac. Sci. Technol. 7, 18 (1970)]. These energies were compared with energies for thermally activated processes identified in niobium. Because the trapping probability for ions is greater in cold-worked metal than in an annealed metal, an investigation was made of samples which were annealed and mechanically stressed in vacuum.