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

Abstract

Abstract. 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.

Neutron energy dependence of delayed neutron yields and its assessments

“…In this work, the mass yield Y (A) are calculated by the means of five components of the Gaussian function, in which Katakura's systematics and parameters are used [23]. This systematics takes into account the energy dependence of prompt neutrons, and substantially includes Ohsawa's idea that a highly excited state of primary fission fragments facilitates prompt neutron emissions [4,12]. We use the evaluated values of England and Rider for σ A [24] and assume that the most probable charge Z p can be expressed by the following equations;…”

“…They considered a highly excited state of primary fission fragments, which facilitates multiple neutron emission, in order to explain the energy dependence of delayed neutron yields between 4 to 7 MeV. Ohsawa and Hambsch have also discussed delayed neutron yields around the resonance region for 235 U and 239 Pu by using the multimodal random neck rupture model [13].…”

“…However, they did not investigate other actinides and higher energy than E n = 15 MeV. In a recent work, a similar calculation for 235 U was performed up to E n = 8 MeV by Ohsawa and Miura [4], and Ohsawa and Fukuda [12]. They considered a highly excited state of primary fission fragments, which facilitates multiple neutron emission, in order to explain the energy dependence of delayed neutron yields between 4 to 7 MeV.…”

“…Fortunately, several developments have been achieved in theory as well as experiment since Alexander and Krick had studied the energy dependence of delayed neutron yields. Those developments, including Ohsawa's idea [12], are able to improve the quality of the summation calculation and the reliability of fitting. We discuss the incident neutron energy dependence of the delayed neutron yield of uranium (A = 233, 235, 236, 238) and plutonium isotopes (A = 239 − 242) in a similar way to Alexander and Krick's approach.…”

Analysis of incident-energy dependence of delayed neutron yields in actinides

“…Thus, overall yields of precursors are decrease as a function of incident energy. The steep decrease at 4 to 7 MeV, as observed in JENDL-4.0, could be reproduced if the initial total excitations energies of the 'would-be precursor' fragments are calculate as proposed by Ohsawa et al 9 . 040002-4…”