Fast Neutron Irradiation Effects in Superconducting Niobium

Abstract

The superconducting magnetization behavior and transition temperatures of single crystals of Nb were investigated prior to and after a series of fast neutron irradiations (E>1 MeV) in the Oak Ridge Research Reactor at a temperature of 40°C.1 In addition to increases in the upper critical field Hc2 and small changes in the transition temperature after irradiation, it has been found that defects have been created with the ability to pin magnetic flux as evidenced by an increase in the nonequilibrium behavior of the magnetization. A greater degree of flux pinning in single-crystal Nb can be attained by fast-neutron irradiation than has heretofore been reported for pinning introduced by mechanical deformation, and for fast-neutron doses of approximately 2×1019 neutrons/cm2 the magnetization approaches that of a completely irreversible type II superconductor. The initial magnetization cycle and subsequent hysteresis loop for an irradiated sample are shown in Fig. 1 as well as the magnetization cycle prior to irradiation. Successive reductions in cross-sectional area of the highly irradiated sample and subsequent magnetization measurements reveal only a small size dependence of the hysteresis, indicating that rather steep flux gradients can be established even at applied fields close to the upper critical field. From size-dependent studies of the remanent moment it is concluded that a flux gradient has been established in a macroscopic surface sheath approximately 0.1 mm thick with the corresponding shielding current density calculated to be approximately 2×105 A/cm2.