Resistivity changes in superconducting cavity-grade Nb following high-energy proton irradiation

Niobium superconducting rf cavities are proposed for use in the proton LINAC accelerators for spallation-neutron applications. The potential for accidental beam loss and continual halo losses along the accelerator path cause concern for the degradation of the superconducting properties of the cavities with the accumulating damage. Residual-resistivity-ratio (RRR) specimens of Nb, with a range of initial RRRs, were irradiated at room temperature with protons at energies from 200 to 2,000 MeV. Four-probe resistance measurements were made at room temperature and at 4.2K, both prior to and after irradiation. Nonlinear increases in resistivity simulate expected behavior in cavity material after extended irradiation, followed by periodic anneals to room temperature. Irradiations to 2--3 x 1015p/cm2 produce degradations up to the 10% level in materials that had a RRR = 316. Such a change is deemed operationally acceptable for an rf cavity. However, without periodic warming to room temperature, the accumulated damage energy would be up to a factor of greater than ten, resulting in unacceptable degradations. Likewise, the same damage energy imparted to higher RRR material would cause larger percentage changes in the RRR