Low-temperature-irradiation study of flux-line pinning in type-II superconductors

Annealed wires of Nb and Nb--20 at.-% Ta were irradiated at liquid-helium temp. in two different modes. In one mode thermal neutrons produced 75% of the damage, while in the other mode fast neutrons produced 85% of the damage. The increase in critical current with fluence was measured to determine its dependence on pin type and density and material purity. Damage due to the fast-neutron component of the flux dominated the flux pinning in both modes. The observations were consistent with the collective statistical summation theory and inconsistent with the conventional single-particle theory. The elementary pinning interaction deduced from the data by using the collective statistical theory agreed well with a recent theoretical calculation of the interaction due to the electron scattering mechanism. 18 ref.--AA