The Possible Origin and Implications of the Shoulder in Reactor Neutrino Spectra

TL;DR

This paper investigates the origin of the shoulder observed in reactor antineutrino spectra, analyzing nuclear databases and considering various physical explanations, highlighting the need for new experiments to clarify the anomaly.

Contribution

It compares predictions from different nuclear databases and explores potential causes of the spectral shoulder, including isotope contributions and neutron spectrum effects.

Findings

ENDF/B-VII.1 predicts a shoulder from aggregate beta spectra.

JEFF-3.1.1 predicts a shoulder mainly from $^{238}$U.

Possible explanations include measurement issues and neutron spectrum effects.

Abstract

We analyze within a nuclear database framework the shoulder observed in the antineutrino spectra in current reactor experiments. We find that the ENDF/B-VII.1 database predicts that the antineutrino shoulder arises from an analogous shoulder in the aggregate fission beta spectra. In contrast, the JEFF-3.1.1 database does not predict a shoulder for two out of three of the modern reactor neutrino experiments, and the shoulder that is predicted by JEFF-3.1.1 arises from $^{238}$U. We consider several possible origins of the shoulder, and find possible explanations. For example, there could be a problem with the measured aggregate beta spectra, or the harder neutron spectrum at a light-water power reactor could affect the distribution of beta-decaying isotopes. In addition to the fissile actinides, we find that $^{238}$U could also play a significant role in distorting the total antineutrino spectrum. Distinguishing these and quantifying whether there is an anomaly associated with measured reactor neutrino signals will require new short-baseline experiments, both at thermal reactors and at reactors with a sizable epithermal neutron component.