FEATURES OF THE SPECTRA OF NEUTRONS ACCOMPANYING THE NEUTRON-INDUCED EMISSION FISSION OF U-238

A satisfactory description of the shape of the experimentally observed spectrum of neutrons produced in the emission fission of U-238 by 14. 7-, 16.0-, and 17.7-MeV neutrons is obtained within a statistical theo ry and the exciton model of preequilibrium emission. An excess of soft neutrons (low-energy anomaly), which was previously observed in compa ring experimental spectra with the results of theoretical calculations , is now reproduced fairly well. This becomes possible under the assum ption that, at high excitation energies of a compound system, the emis sion of postfission neutrons is governed by a two-step mechanism. Acco rding to this mechanism, some part of postfission neutrons can be emit ted by nonaccelerated fragments, whereas the remaining neutrons are em itted by fully accelerated fragments. The low-energy anomaly in the sp ectra is associated with neutron emission from nonaccelerated fragment s.