Enhancement of the superconducting critical temperature Tsub(c) of niobium by radiation damage

5 μm niobium foils were irradiated with 25 MeV oxygen ions at 20 K, 33 K and 145 K up to fluences of 1.3 x 1016 cm-2 corresponding to a damage rate decrease of 99%. Tsub(c) versus resistivity shows a minimum depending on the irradiation temperature. At high fluences and subsequent annealing Tsub(c) is enhanced compared to the unirradiated value and the transition is broadened by a factor of about 30. (Auth.)