A pure Cu and a Cu (100 ppm Au) alloy were irradiated at 4 K with thermal neutrons. The isochronal recovery spectra (30 K-400 K) were compared with those of identical samples irradiated to equivalent defect concentrations using fast (>0.1 MeV) neutrons. The differences observed in the resultant recovery spectra could be interpreted (in view of the damage configurations) due to the different types of neutron energy involved. Direct comparison of these spectra with earlier results involving defect kinetics in thermal- and fast-neutron-irradiated Pt and dilute Pt alloys was possible.