The photovoltaic properties including I-ri characteristics. junction capacitance (C-V), short-circuit current (I-sc), open-circuit voltage (V-oc), fill factor (ff), efficiency (eta) and spectral response of Cu2S/CdS heterojunction cells have been examined before and after exposure to nuclear radiation. This included gamma -rays of Co-60, and electron beams (at 1.5 MeV energy). The short-circuit current (I-sc) decreased, while the open-circuit voltage (V-oc), the fill factor (ff) and the efficiency (eta) increased after heat treatment (at 260 degreesC in air for 20 min). The I-sc effect during exposure to gamma -rays was studied. It was found that I-sc increases as the dose rate increases. The sensitivity dependence of the I-sc density on dose rate was observed to be linear, and hence a universal constant for its sensitivity is found to be 45 (nA/cm(2)) (rad/s). No permanent damage was shown until about 300 Mrad for gamma rays and 380 Mrad for electron beams. After these doses, the I-sc and V-oc slightly decreased on increasing the absorbed dose. After heat treatment, the spectral response was modulated. It was found that the wavelength response against the photocurrent decreased from 1000 to 800 nm and the photocurrent also slightly decreased in the range of wavelengths from 800 to 450 nm and increased from 350 to 540 nm. Heat treatment before irradiation improved the photovoltaic cells. After irradiation by gamma -rays and electron beams, the photocurrent went back to its original value by annealing (for 2 h at 500 degreesC). The capacitance-voltage behavior decreased after irradiation and hence the doping decreased.