Features of the spectra of neutrons accompanying the neutron-induced emission fission of 238U

A satisfactory description of the shape of the experimentally observed spectrum of neutrons produced in the emission fission of 238U by 14.7-, 16.0-, and 17.7-MeV neutrons is obtained within a statistical theory and the exciton model of preequilibrium emission. An excess of soft neutrons (low-energy anomaly), which was previously observed in comparing experimental spectra with the results of theoretical calculations, is now reproduced fairly well. This becomes possible under the assumption that, at high excitation energies of a compound system, the emission of postfission neutrons is governed by a two-step mechanism. According to this mechanism, some part of postfission neutrons can be emitted by nonaccelerated fragments, whereas the remaining neutrons are emitted by fully accelerated fragments. The low-energy anomaly in the spectra is associated with neutron emission from nonaccelerated fragments