Position-dependent flux pinning by proton-irradiation damage in thick niobium samples

Pure vacuum-annealed niobium was irradiated at room temperature with 5-MeV protons to a total fluence of 6.8×1016 protons/cm2. The sample thickness was much larger than the range of the protons. The local magnetic induction as a function of distance from the surface was determined in the superconducting mixed state using a previously described ac technique. The measurements show a dramatic increase in pinning to a depth of about 100 μm, which corresponds to the approximate range of the protons. The field profiles were obtained at several applied-field values between Hcl and Hc2 and the magnetic field dependence of the volume pinning force Fv shows a strong ’’peak effect’’ near Hc2. The results are discussed in terms of several disclocation-loop–flux-line-lattice–pinning models using calculations of irradiation damage based on Rutherford scattering and the simple Kinchin-Pease model of atomic displacements. The results are found to be consistent with previous measurements by Agrawal, Kramer, and Loomis on neutron-irradiated niobium and suggest that a quadratic summation rule first proposed by Labusch should be used to obtain the effective volume pinning force Fv.