We present an extensive irradiation study involving five state-of-the-art Nb3Sn wires which were subjected to sequential neutron irradiation up to a fast neutron fluence of 1.6 · 10 22 m −2 (E > 0.1 MeV). The volume pinning force of short wire samples was assessed in the temperature range from 4.2 to 15 K in applied fields of up to 7 T by means of SQUID magnetometry in the unirradiated state and after each irradiation step. Pinning force scaling computations revealed that the exponents in the pinning force function differ significantly from those expected for pure grain boundary pinning, and that fast neutron irradiation causes a substantial change in the functional dependence of the volume pinning force. A model is presented, which describes the pinning force function of irradiated wires using a two-component ansatz involving a point-pinning contribution stemming from radiation induced pinning centers. The dependence of this point-pinning contribution on fast neutron fluence appears to be a universal function for all examined wire types.

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The superconducting properties of Y 1 Ba 2 (Cu 1−x Al x) 3 O 7 /Y 2 BaCuO 5 bulk superconductors with different Al contents were studied (x = 0.0025-0.05). A peak effect was observed at intermediate magnetic fields over a wide temperature range below T c. The influence of standard oxygenation (SO) at 400 • C and high pressure oxygenation (HPO) at 750 • C on the critical current density J c was investigated. We show that the critical current density is higher by a factor of 3 in samples oxygenated at high pressure. The magnetization relaxation rates were investigated for an undoped HPO sample as well as for the lowest Al concentration for HPO and SO in the temperature range from 15 to 77 K and at magnetic fields of 0.5 and 1 T. The normalized relaxation rates, S, and the pinning potentials, U 0 , have been calculated from the time-dependent magnetization curves.

Fast neutron irradiation is a powerful technique for introducing additional pinning centers into high temperature superconductors. The spherical defects with sizes of a few nanometers are considered to be effective pinning centers, enhancing J c . Their morphology is well-known and has already been investigated by several authors in great detail. However, only very little is known about the nature and density of smaller and point defects, which are invisible in transmission electron microscopy. Positron annihilation lifetime spectroscopy was applied to investigate the nature and the concentration of small point-like defects. In this work, the influence of small point defects, such as vacancies and vacancy clusters, on the superconducting properties of YBa 2 Cu 3 O 7−x bulks was studied; these were introduced by irradiation in the TRIGA Mark II reactor in Vienna. J c and T c measurements were performed prior to and after each irradiation step. The samples were irradiated up to a fast neutron (>0.1 MeV) fluence of 6 × 10 21 m −2 . The two kinds of defects-the large collision cascades and the small point-like defects-contribute to the decrease of T c as well as to the J c enhancement in astonishingly similar ways.

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