Fission neutrons from excited nuclei

The time required for a fission fragment to acquire a given velocity is calculated and compared with the computed lifetime of the excited nucleus with respect to neutron evaporation. On the basis of these calculations the hypothesis is advanced that nuclei excited in fission to energies ∼20 MeV or more may emit a certain fraction of the neutrons before the fragments have been fully accelerated (this fraction is assumed to increase with increasing excitation energy). If this factor is taken into account, certain contradictions in the treatment of the experimental data are removed. It is pointed out that for this reason the method which has been used for some time past of determining the dependence of the number of neutrons on the mass of the fragments by comparing the energies and velocities of the fragments may, in the case of large excitations, turn out to be inapplicable.