Impact of fission neutron energies on reactor antineutrino spectra

Abstract

Recent measurements of reactor-produced antineutrino fluxes and energy spectra are inconsistent with models based on measured thermal fission beta spectra. In this paper, we examine the dependence of antineutrino production on fission neutron energy. In particular, the variation of fission product yields with neutron energy has been considered as a possible source of the discrepancies between antineutrino observations and models. In simulations of low-enriched and highly-enriched reactor core designs, we find a substantial fraction of fissions (from 5% to more than 40%) are caused by non-thermal neutrons. Using tabulated evaluations of nuclear fission and decay, we estimate the variation in antineutrino emission by the prominent fission parents 235 U, 239 Pu, and 241 Pu versus neutron energy. The differences in fission neutron energy are found to produce less than 1% variation in detected antineutrino rate per fission of 235 U, 239 Pu, and 241 Pu. Corresponding variations in the antineutrino spectrum are found to be less than 10% below 7 MeV antineutrino energy, smaller than current model uncertainties. We conclude that insufficient modeling of fission neutron energy is unlikely to be the cause of the various reactor anomalies. Our results also suggest that comparisons of antineutrino measurements at low-enriched and highly-enriched reactors can safely neglect the differences in the distributions of their fission neutron energies.

First Search for Short-Baseline Neutrino Oscillations at HFIR with PROSPECT

“…Of these, the sterile neutrino hypothesis has received particular attention due to its broad potential impact and to existing supporting experimental indications from accelerator and radioactive source neutrino experiments [14][15][16][17][18]. A range of experimental [19][20][21][22][23], theoretical [24][25][26][27][28], and global analysis [29][30][31][32] efforts have sought to understand the origin of these discrepancies.…”

Colloquium : Neutrino detectors as tools for nuclear security

“…Details of the neutrino flux vary according to the nature of the fission source. Most importantly, the neutrino flux depends on which nuclides undergo fission, while the energy of the fission-inducing neutrons has a smaller impact (Littlejohn et al, 2018). The dominant nuclides in most reactors and explosions are 235 U, 239 Pu, 238 U, and 241 Pu.…”

Ab initio calculations of reactor antineutrino fluxes with exact lepton wave functions

“…There are two different methods to obtain the theoretical calculation of the reactor antineutrino flux and spectrum. The first one employs the ab initio method [3][4][5][6][7][8][9][10][11][12][13][14] by a direct summation of all the beta decay branches using the available information from the latest nuclear databases. The other method uses an effective conversion procedure of virtual branches [15][16][17][18][19][20][21][22][23] based on the measurements of the integral electron energy spectra of the fissionable isotopes.…”

“…In the first method of the ab initio calculation, the fission yield, the endpoints and branching ratios of each fission fragment can be accessed from the nuclear databases. The reactor antineutrino flux can be obtained by a direct summation of all the beta decay branches using an analytical description of the single beta decay spectrum [3][4][5][6][7][8][9][10][11][12][13][14]. Therefore, the calculating accuracy depends on the uncertainties of the fission yields and the beta decay information.…”