Measurement of displacement cross sections of aluminum and copper at 5 K by using 200 MeV protons

To validate the Monte Carlo codes for prediction of radiation damage in metals irradiated by > 100 MeV protons, we developed a proton irradiation device with a Gifford-McMahon (GM) cryocooler to cryogenically cool two 0.25-mm-diameter wire samples of aluminum and copper. By using this device, the defect-induced electrical resistivity changes related to the displacement cross section of copper and aluminum were measured under irradiation with 200-MeV protons at 5 K at the beamline of the cyclotron facility at RCNP, Osaka University. After irradiation to a 3.89 × 1018 proton/m2 flux, the damage rate of the aluminum sample was 1.30 × 10−31 Ωm3/proton at 185 MeV and that of copper was 3.60 × 10−31 Ωm3/proton at 196 MeV. Based on measurements of recovery of the accumulated defects in aluminum and copper through isochronal annealing, which is related to the defect concentration in the sample, about 50% of the damage remained at 40 K, with the same tendency observed in other experimental results for reactor neutron, fusion neutron, and 125-MeV proton irradiations. A comparison of the measured displacement cross sections with the calculated results of the NRT-dpa and the athermal-recombination-corrected displacement damage (arc-dpa) cross sections indicates that arc-dpa with the defect production efficiencies provided by Almazouzi for aluminum and Nordlund for copper provide better quantitative descriptions of the displacement cross section than NRT-dpa.