ENERGY-SPECTRA OF NEUTRONS ACCOMPANYING THE EMISSION FISSION OF U-238

The spectra of fission neutrons emitted from U-238 measured for the fi rst time by the time-of-flight method at incident-neutron energies of 16.0 and 17.7 MeV. Analysis of the neutron spectra shows that experime ntal results at incident-neutron energies of 14.7, 16.0, and 17.7 MeV (above the threshold of chance fission) differ significantly from thos e obtained at a neutron energy of 2.9 MeV (below the threshold of chan ce fission). Owing to the prefission emission of neutrons, the observe d spectra of neutrons from emission fission exhibit a characteristic g rowth of the neutron yield in both hard and soft sections of the spect rum of secondary neutrons. This growth manifests itself as a step in t he first case and as a rise in the second case, where it results in a noticeable excess of neutrons over the statistical-model predictions f or E < 2 MeV. The first feature in the spectra of neutrons from emissi on fission can be associated with the nonequilibrium decay of an excit ed fissile nucleus. On the contrary, the origin of the second feature has yet to be clarified. Additional measurements of angular distributi ons of secondary neutrons may prove helpful in this respect.