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 approximately 40°C. The experimental results indicate that the point defects and defect aggregates which are created lead to increases in the upper critical field H_c2 and small changes in the superconducting transition temperature. In addition, the increase in nonequilibrium behavior of the magnetization with integrated fast‐neutron flux establishes that defects are present with the ability to pin magnetic flux. For fast‐neutron doses of approximately 2×10¹⁹ neutrons/cm², the magnetization approaches in behavior that of a completely irreversible type‐II superconductor. Successive reductions in cross‐sectional area of this sample by chemical etching and subsequent measurements of the magnetization indicate only a small size dependence of the hysteresis, indicating essentially complete flux pinning in a macroscopic sheath approximately 0.1 mm thick. The flux‐pinning defects are considered to be dislocation loops as evidenced by other work, and the present experimental results are discussed in terms of these loops.