An experimental facility was developed to measure in situ the degradation of crystalline silicon solar cells fabricated by the Solar Energy Group of the National Atomic Energy Commission (CNEA). The cells were irradiated with 10 MeV protons and fluences between 10⁸ and 10¹³ p/cm², using an external beam of the linear tandem accelerator TANDAR, at CAC-CNEA. This facility allowed observing the degradation of the electrical characteristics of the cells, by measuring the current-voltage characteristic curve. Furthermore, theoretical simulations were performed to establish the relation between the variation of the electrical parameters and the degradation of the lifetime of minority carriers in the base, which is directly affected by the irradiation. The influence of the illumination spectrum on the results is also analyzed. The damage constant for 10 MeV proton irradiated silicon solar cells of n⁺-p-p⁺ structure and 1 Ωcm base resistivity was determined. Measured and predicted lifetimes for intermediate fluences in the range studied are presented. Finally, a proposal of a new model of radiation damage for silicon solar cells is considered and discussed.