Critical currents in Nb3Sn fast-neutron irradiated at 6 K

The short-section critical current density jc of neutron-irradiated Nb3Sn composite wires has been measured at 4.2 K in transverse magnetic fields up to 10 T. Irradiation was conducted at ∼6 K, and jc measurements were made after annealing to 77 K. A fluence of 4.8×1017 n/cm2 (E≳0.1 MeV) increased jc(H) (for H≳1.5 T) above high unirradiated values (about 1×106 A/cm2 at 4 T). The enhancement in jc increased with field, to 45% at 10 T. Samples irradiated to 1.1×1018 n/cm2 showed enhancements in jc a few percent less than samples given the lower dose. After annealing to 295 and 415 K, roughly half of the jc enhancement remained for H≳4 T. Changes in the defect structure caused by irradiation and subsequent annealing, and which are responsible for increased flux pinning, are discussed.