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Following an analysis of the basic radiation damage processes in semiconductors, and an analysis of existing radiation effects data, a proton-neutron damage equivalence was experimentally determined for silicon solar cells. The basis for comparison is the degradation of minority carrier lifetime during irradiation, as expressed by the lifetime damage constant, KÂ¿, or the diffusion length damage constant, KL . Both p on n cells and n on p cells were irradiated with 96, 69, 48 Mev protons, and with fission and moderated neutrons. Measurements were made on the degradation of the solar cell dark voltage-current characteristics, the forward bias characteristics under one sun illumination, and the short circuit current response to monochromatic illumination of one micron wavelength. The degradation of minority carrier diffusion length was calculated from the short circuit current response at one micron. From the analysis of radiation damage processes in silicon, the derivation of a theoretical proton-neutron damage equivalence was hindered by inadequately developed theory relating the number and type of active defects associated with the vacancies created by the incident particles. The analysis of available data indicate that a reliable proton-neuton damage equivalence in Si or Ge is not possible, due to the spread in the existing damage constants and the absence of damage constants for proton and neutron irradiations of equivalent samples. The experimentally determined proton to neutron ratio of minority carrier diffusion length damage constants in silicon solar cells ranges from 0.96 to 4.