Irradiations of polycrystalline specimens of cobalt at various electron energies and at low temperature were performed. It was found impossible to explain the obtained data by using a simple step function for the probability of displacementP(T) as a function of the kinetic energyTimparted to a struck atom. More involved functions have to be considered. For irradiation temperatures lower than 6 K, the onset threshold is found to be:Td=22 ± 1 eV and the experimental results are best fitted by a theoretical cross-section curve calculated with:P(T)=O ifT<22.5 eV=0.50 if 22.5 <T< 31 eV=0.78 if 31 eV <T