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The decrease in the superconducting transition temperature Tc after fast-neutron irradiation has been measured in V2Hf (C-15 Laves phase), Nb3Al (A-15 phase), PbMo6S7 (Chevrel phase), and SnMo5S8 (Chevrel phase). The samples were irradiated at 6 K and warmed to 300 K during transfer to the measuring cryostat. The Tc of the samples was measured as a function of dose up to 3.0×1019 n/cm2 (E≳0.1 MeV). The different crystal structures showed a marked difference in the sensitivity of the Tc depression with irradiation. The total decrease in Tc of V2Hf was 5%, while ΔTc(Nb3Al) =23%, ΔTc(PbMo6S7) =61%, and ΔTc(SnMo5S8) =51%. Log(ΔTc) was linear with dose for all samples as expected for radiation saturation, i.e., the damage rate was proportional to the undamaged fraction. The recovery of Tc after a series of 5-min isochronal anneals up to 1200 °C has been measured, and the annealing stages observed are discussed in terms of the radiation-induced defects.
Multifilament copper-clad wires of Nb3Sn have been irradiated at 6 K to a dose of 1.8×1018 n/cm2 (E≳0.1 MeV). The critical current (Jc) was measured as a function of dose and applied magnetic field to 33 kOe. After 1.2×1018 n/cm2, an increase of 33% in Jc occurred at 33 kOe from an initial value of 1.5×106 A/cm2 at 4.5 K. The increase was smaller at lower applied fields, and Jc actually decreased for H<8 kOe. For all fields, the increase in Jc eventually saturated and then decreased as the dose increased, with the saturation dose increasing with increasing applied field. The critical temperature (Tc) changed by <1 K at the highest dose. Approximately 25% of the change in Jc recovered after a 295 K anneal. The Jc changes are explained as an increase in flux pinning from radiation-induced defect cascades that leads to an increase in Jc greater than the decrease in Jc which results from a decrease in Tc of 1 K and from the increase in Hc2 produced by the increase in normal resistivity. A model is proposed that allows the prediction of the dose dependence of various terms in the pinning-force density and explains the field and dose dependence of the Jc changes. Measurements of the superconducting–to–normal-state thermal transition indicate that the decrease in thermal conductivity of the Cu cladding is important when considering the degradation of the thermal transition current that occurs at high doses.
Articles you may be interested inEffects of neutron irradiation on the critical current of bronze processed multifilamentary Nb3Sn superconducting composites Fast-neutron irradiations have been performed on Nb 3 Sn wires to determine the roles of irradiation temperature and the unirradiated value of the critical-current density (J eO) on the changes in Ie' Four experiments were performed: low-(6 K) and high-(350 K) temperature irradiation of high-(2X 10 10 A/cm 2 at 5 T and 4.2 K) and moderate-(0.8X 10 10 A/cm 2 ) leO material. The large increases in Ie for the moderate-leO material were independent of irradiation temperature; the increases were smaller for the high-JeO material, and a significant difference was found for the two irradiation temperatures. The difference is attributed to the greater pinning capability of defect cascades over defect clusters in the high-J eO material. The smaller amount of Ie recovery after a 300 K anneal, when compared with NbTi, can also be understood in terms of the defects responsible for the flux pinning.
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