Background radiation from fission pulses

Abstract

Extensive source terms for beta, gamma, and neutrons following fission pulses are presented in various tabular and graphical forms. Neutron results from a wide range of fissioning nuclides (42) are examined and detailed information is provided for four fuels: 235 U, 238u, 232Th (an(j 239p u; t h e se bracket the range of the delayed spectra. Results at several cooling (decay) times are presented. For p-and y spectra, only 235 U and 239 Pu results are given; fissionproduct data are currently inadequate for other fuels. The data base consists of all known measured data for individual fission products extensively supplemented with nuclear model results. The process is evolutionary, and therefore, the current base is summarized in sufficient detail for users to judge its quality. Comparisons with recent delayed neutron experiments and total p~ and y decay energies are included. I. A. Yields The fission-product yields in use are a preliminary, unpublished version for ENDF/B-VI, 2 described in Ref. 3; mass chain yields are listed in Ref. 3 as well. All yield data are being updated and reevaluated prior to issue in Version VI of ENDF/B; the data in use are the most recent available and are current to about mid-1983. For this study, we did alter the 238 U fast-fission yields primarily in their distribution along the mass chains. The method of evaluation and the distribution models are described in Ref. 3. (Only the 238JJ proton pairing parameter was changed from values listed in that reference. The change was based on recent unpublished measurements made at Grenoble, France, and it resulted in a significant alteration in the calculation of delayed neutrons; however, it is less important to other aggregate quantities.) Most high-energy neutron yields are based on nuclear models and systematics, not on measurements. B. Halflives and Decay Branching Except for neutron branching (denoted as Pn), these data are taken from ENDF/B-V 2 and are listed in the summary document of Ref. 4. As noted in a later section, we now have sufficient tests to believe in their validity and the yield data can be assessed as generally very good. C. Beta/Gamma Spectra and Their Decay Energies Earlier testing of these ENDF/B-V data 2 demonstrated that measured nuclide spectra were usually deficient for many high-Q transitions. 5 ' 6 An earlier version of ENDF/B that relied more on nuclear model generated spectra showed better agreement with benchmark experiments than Version V. Other countries have also discovered the deficiency and the only reasonable recourse in the near future for individual nuclides is an augmentation of spectra using nuclear models. 7 As discussed later, this has already been accomplished for total P~ and y decay energies, but not spectra, by the Japanese,8.9 with excellent results. As related to this report, the total ]3~ and y energies recommended by the Japanese are used in place of those in ENDF/B-V. Here, however, our primary interest is in aggregate spectra. For this purpose, we use 18-and 19-group functions fitted to ou...

β-decay half-lives andβ-delayed neutron emission probabilities of nuclei in the regionA≲110, relevant for the r process

“…199 keV for 87 Br, 450 keV for 98 Rb, and 579 keV for 137 I). This trend was also observed by the same authors in the total spectra of 235 U and 239 Pu, with average E βn of 575 keV and 525 keV, respectively, and with little neutron intensities at E βn 800 keV [80,83]. The reason for these "compressed" E βn spectra is the strong, often preferred population of the lowest excited states in the final nuclei [82].…”

The neutron long counter NERO for studies of neutron emission in the r-process

“…Neutron energies in the laboratory frame E n were found to be 199 keV for 87 Br, 450 keV for 98 Rb, and 579 keV for 137 I. This result was further supported by the measured average neutron energies of fission fragments from 235 U ( E n =575 keV) and 239 Pu ( E n =525 keV), where, in addition, very few neutrons were found at E n 800 keV [31,34]. According to these authors, the reason for the "compressed" E n spectra is the preferred population of the lowest excited states in the final nuclei [33].…”

JENDL-4.0: A New Library for Nuclear Science and Engineering

“…The values of the delayed -and -rays were calculated from JENDL/FPD-2000 and the presently evaluated FPYs. The delayed neutron energy was calculated using the formula given in a report by England et al, 66) i.e.,…”