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The steep decrease in the delayed neutron yield (DNY) in the 4- to 7-MeV region has not yet been fully understood. In the present study, an attempt was made to understand the variation in terms of increased neutron emission from fission fragments for U-235. As the incident energy increases, the excitation energy of the primary fragments increases, thus the fragments have a chance to emit more prompt neutrons. This results in decreased yields of precursors, since some of the "would-be precursor" fragments fail to be actual precursors after neutron emission. This process was followed nuclide-wise to obtain the final precursor yields. The initial total excitation energies (TXE) of the would-be precursor fragments were calculated by two methods; Case A: energy balance between total energy release and total kinetic energy (TKE) of fragments, and case B: prompt neutron multiplicity and kinetic energy. The DNYs were calculated for the two cases, using three different data for neutron separation (binding) energies: the empirical data of Audi et al., FRDM and TUYY atomic massa formulas. The results indicate that the variation of DNY is dependent on the assumtion used to estimate the initial excitation energy as well as the data set of the neutron separation energy.

An interpretation of energy-dependence of the delayed neutron yields in the MeV-region