Neutron-scattering measurements of phonon perturbations in irradiated copper

Neutron-scattering measurements have been carried out on the transverse phonons with wave vectors →q=(0,0,ζ)2πa for 0.04≤ζ≤0.20 in a copper single crystal which had been irradiated at 4.2 K to give a Frenkel-defect concentration of approximately 1.3 ×{} 10^-4. The measurements were obtained at 4.2 K after the irradiation and after annealing the sample at 72, 300, 450, and 800 K. The data for the as-irradiated sample show defect-phonon perturbation effects which are similar to those observed in a previous preliminary investigation, carried out on a sample with a defect concentration of 0.4 ×{} 10^-4. The phonon peak shapes possess unusual structure consisting of a narrow component which displays a resonant-like ζ-dependent frequency shift and an additional component for ζ≤0.07 which appears only on the high-frequency side. An anneal at 72 K removes the resonant frequency shift but only little of the additional high-frequency structure. This structure is still clearly visible for ζ≤0.05 after an anneal at 300 K, but it appears to be removed by the 800 K anneal. The results cannot be explained entirely by simple model calculations for the $〈100〉$ split interstitial.