GaAs single-junction cells were irradiated with various fluences of 1 and 3 MeV electrons and 1 MeV protons. The light to V curves measured at room temperature exhibit a voltage-dependent photocurrent. The photocurrent is modeled taking into account the voltage-dependent width of the space charge region in combination with a strongly decreased minority carrier diffusion length. By extracting the width of the space charge region and the base layer diffusion length through measurements, the photocurrent is reproduced accurately.