Peak effect in superconducting niobium, induced by neutron irradiation

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Abstract
Xeutron irradiation at reactor ambient temperature gives rise to a peak in the magnetization curve of superconducting niobium near the upper critical field HC 1. It is correlated to a maximum in the critical current density Jc and it is referred to as the ‘peak effect’. The magnitude depends strongly on the irradiation dose. The peak effect can be explained qualitatively by a theory proposed by Pippard but quantitatively the agreement is not very good. A modification of Pippard's theory in which the pinning is assumed to be caused by point defects does not give any improvement, but a modification introduced by Campbell and Evetts, based on pinning by line defects, leads to results in very close agreement with our experimental data.