Neutron energy dependence of the initial dislocation pinning rate was studied from 1.9 to 23.4 MeV on a copper sample. By comparing initial pinning rates from 0.5‐ and 1.0‐MeV electron irradiations and the neutron irradiations, and using the defect production rate for electron bombardments, an estimate of the free interstitial production cross sections (FIPCS) for neutrons was made. These FIPCS values are compared with several theories which predict the total number of Frenkel pairs produced as a function of recoil energy. Comparison with the simplest of these models (Norgett‐Robinson‐Torrens) showed good agreement with the trend of the energy dependence of the FIPCS. Additional comparison showed that only 1% or less of the predicted number of interstitials survive as free interstitials. A gold sample was also studied and results of 2‐MeV electron and 14.3‐MeV neutron irradiations are also presented.