First-forbidden transitions in reactor antineutrino spectra

Abstract

We study the dominant forbidden transitions in the antineutrino spectra of the fission actinides from 4 MeV onward using the nuclear shell model. Through explicit calculation of the shape factor, we show the expected changes in cumulative electron and antineutrino spectra. Relative to the allowed approximation this results in a minor decrease of electron spectra above 4 MeV, whereas an increase of several percent is observed in antineutrino spectra. We show that forbidden transitions dominate the spectral flux for most of the experimentally accessible range. Based on the shell model calculations we attempt a parametrization of forbidden transitions and propose a spectral correction for all first-forbidden transitions. We enforce correspondence with the Institut Laue-Langevin data set using a summation+conversion approach. When compared against modern reactor neutrino experiments, the resultant spectral change is observed to be of comparable magnitude and shape as the reported spectral shoulder.

First-forbidden transitions in the reactor anomaly

“…The results shown in Fig. 5 were obtained for g A = 0.9 and MEC = 1.4, for which good agreement was reached with experimental lifetimes for almost all transitions [18,54]. In order to get a measure for the uncertainty on our results, we vary the coupling constants within a window as described below.…”

“…Each shape factor was normalized to its value at E = 0. Results correspond to gA = 0.9 and MEC = 1.4, where applicable [18,54]. Note the difference in scales on the y-axis.…”

“…[45], we calculated 36 dominant forbidden transitions with the nuclear shell model, all of which are first forbidden. Note that we have included here more transitions than the 29 that were included in our previously published work [18]. A summary of their properties is shown in Table I.…”

“…The excellent agreement using the experimental data with the usual assumptions is strong evidence that our calculations are indeed accurate for the majority of the decays, especially the most important ones. Uncertainties due to g A quenching and meson exchange currents (MEC) in pseudoscalar transitions in the fission fragment region have been previously reported on [18,[53][54][55][56][57], and will be discussed in the total uncertainty estimation of Sec. V B.…”

“…Due to the larger sample of numerical shape factors presented here compared to our previous work, the parametrization procedure has evolved to better reflect the internal structure of the shape factor distribution. For completeness then, we outline both the procedure used in the previous work [18] and its current state.…”