Spectrum of neutrons accompanying fission of 232Th, 235U, and 238U by 2.9-MeV and 14.7-MeV neutrons (below and above the threshold of emission fission)

The time-of-flight method has been used to determined the spectra N(E,En) of neutrons accompanying fission of the nuclei 232Th, 235U, and 238U by neutrons with energies En = 2.9 and 14.7 MeV - below and above the threshold of emission fission, respectively. The results of the experiment and their analysis have established two new features of low-energy emission fission: the presence, in the spectra, of prefission neutrons of the hard part with a fairly sharp upper limit Emax≅8.5 MeV associated with the nonequilibrium mechanism of neutron emission, and an excess of neutrons in the soft part, E < 2 MeV, compared with the description based on the standard statistical model. The first feature can be given a satisfactory quantitative description, but the nature of the second, the existence of which is confirmed by analysis of the available data on the doubly differential yields of fission neutrons, remains open (a suggestion is made concerning its relation to the dynamics of the fission process)