Possible origin of shoulder in the reactor antineutrino spectrum

TL;DR

This paper suggests that the shoulder observed in reactor antineutrino spectra may originate from the electron spectra of uranium and plutonium isotopes, influenced by the reactor neutron spectrum, especially in the 6-8 MeV range.

Contribution

It proposes a possible explanation for the reactor antineutrino shoulder involving contributions from uranium and plutonium isotopes affected by the neutron spectrum.

Findings

Re-analysis shows a shape mismatch in $^{238}U$ electron spectrum.

The bump resembles the reactor antineutrino shoulder around 5 MeV.

Additional contributions from the neutron spectrum may explain the anomaly.

Abstract

The re-analysis of measured $^{238}U$ integrated electron spectrum reveals a shape mismatch with respect to the so called ab initio calculation and the ratio resembles well with the recently observed reactor antineutrino shoulder or bump around 5 MeV prompt energy when plotted in a reduced scale where the first two moments (area and average) of the spectrum are made identical (KNO type scaling). However, such bumps are not seen in case of other actinides like $^{235}U$ and $^{239,241}Pu$ isotopes. Since the magnitude of the bump in $^{238}U$ is comparable with the experimental observations, $^{238}U$ can not be the only source of anomaly unless similar contributions come from other uranium and plutonium isotopes as well. Therefore, it is strongly conjectured that the integrated electron spectra of uranium and plutonium isotopes particularly in the $6$ MeV to $8$ MeV energy range may have additional contributions coming from the harder part of the reactor neutron spectrum which may not be present in the thermal measurements.