The damage introduced in a semiconductor is made evident by a change in minority-carrier lifetime. Irradiation by electrons produces relatively low-energy silicon-atom primary recoils which, in turn, produce isolated defect centers. Neutron irradiation produces high-energy recoils which create clusters of damage, as first discussed by Gossick. In the case of proton irradiation, both high–and low-energy recoils are produced. This physical process is used to derive a quantitative means of extrapolating proton damage from electron and neutron data. This is done by using cross-section data or theory to calculate the number of recoil atoms produced with energyEp. IfEp< 20 keV, the defects are “electron-like,”NE; otherwise, they are “neutron-like,”NN. The change in reciprocal lifetime per unit fluence (cm−2) is equal tokENEpluskNNN, wherekE(= 5.4 × 10−9cm2/sec) andkE(= 10−5cm2/sec) are found from appropriate experiments reported in the literature. This rule is used to calculate the lifetime in silicon that has been irradiated by protons. The calculated lifetime compares well with the lifetime that Berman has deduced from solar cell degradation data.