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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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