The effects of fast‐neutron‐induced defects on the dislocation internal friction and modulus defect of copper crystals were studied by two procedures: (i) irradiation at 20°K followed by measurements during heating at a constant rate to room temperature and (ii) measurements during and after irradiations at various constant temperatures between 90 and 410°K. The measurements were made at low enough strain amplitude so that breakaway of dislocations from pinning points did not occur. Samples of known dislocation density were used in many of the experiments so that the ``vibrating‐string'' model of dislocation motion could be used to calculate approximate absolute values for the densities of radiation‐defect pinning points obtained at various temperatures. These were characterized by the ratiogof pinning points collected to freely migrating radiation defects created in the samples, assuming that on the average 20 defects can migrate away from the cascade region of each primary recoil. For the lower‐temperature pinning processes which showed no time delay, values ofgincreased monotonically from 10−6–10−5at 60°K to 10−4–10−3at 204°K. The higher‐temperature process is known to have a broad delay‐time distribution; by collecting all pinning points available at 410°K agvalue of ∼0.1 was obtained. Because of uncertainty as to the validity of the vibrating‐string model, pinning‐point densities were obtained primarily from the modulus‐defect data. The damping data often implied the same densities but sometimes disagreed seriously with the modulus‐defect results.