powder, irradiated in a high density thermal neutron flux, was partly converted to radioactive Zn‐65, Zn‐69, and S‐35. These radioisotopes decay to Cu‐65, Ga‐69, and Cl‐35, respectively. One week after removal from the nuclear reactor the sample was fired at 900°C to anneal out radiation damage occurring in the reactor. The brightness and spectral distribution under 3650Aå excitation and the diffuse reflectance spectra of the annealed sample were monitored during a period of 600 days. The principal results are interpreted in terms of an increasing concentration of uncompensated copper centers produced by the decay of Zn‐65.