On the determination of anti-neutrino spectra from nuclear reactors

TL;DR

This paper investigates how higher order corrections and theoretical uncertainties affect the determination of anti-neutrino spectra from nuclear reactors, highlighting potential limitations and confirming recent flux shifts.

Contribution

It provides a critical evaluation of the errors in anti-neutrino spectrum extraction and introduces a method using virtual beta branches with minimal bias.

Findings

Induced currents like weak magnetism may limit accuracy.

A fit with virtual beta branches reduces bias.

Confirmed a 3% upward shift in anti-neutrino fluxes.

Abstract

In this paper we study the effect of, well-known, higher order corrections to the allowed beta decay spectrum on the determination of anti-neutrino spectra resulting from the decays of fission fragments. In particular, we try to estimate the associated theory errors and find that induced currents like weak magnetism may ultimately limit our ability to improve the current accuracy and under certain circumstance could even largely increase the theoretical errors. We also perform a critical evaluation of the errors associated with our method to extract the anti-neutrino spectrum using synthetic beta spectra. It turns out, that a fit using only virtual beta branches with a judicious choice of the effective nuclear charge provides results with a minimal bias. We apply this method to actual data for U235, Pu239 and Pu241 and confirm, within errors, recent results, which indicate a net 3% upward shift in energy averaged anti-neutrino fluxes. However, we also find significant shape differences which can in principle be tested by high statistics anti-neutrino data samples.