Effects of low-temperature fast-neutron irradiation on ac losses in NbTi

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Abstract
The complex magnetic permeability of Nb-44 at.% Ti has been measured using a modified Hartshorn ac bridge and a lock-in amplifier. The out-of-phase signal can be directly related to the energy losses, and critical current can be calculated using the Bean model. Samples were irradiated in a fission neutron spectrum to a dose of 3.2×1018 neutron/cm2. All measurements were taken in situ at approximately 4.5 °K in an external magnetic field continuously swept to 6.7 kOe while applying ac magnetic fields at 88 Hz with amplitudes between 8 and 500 Oe. Irradiations and measurements were performed on an annealed and a cold-worked sample of the alloy. For the annealed sample, no changes were observed in VL and thus in Jc after 8.2×1017 neutron/cm2 and after an anneal to 320 °K. A decrease of approximately 8% was observed in Jc for the cold-worked sample after 3.2×1018 neutron/cm2. One-half of this decrease was recovered by 270 °K.