Final Measurement of the U235 Antineutrino Energy Spectrum with the PROSPECT-I Detector at HFIR

TL;DR

This paper presents a highly precise measurement of the U235 antineutrino spectrum using the PROSPECT-I detector, revealing an excess in the 5-7 MeV range and providing new insights into reactor antineutrino anomalies.

Contribution

It provides the most precise U235 antineutrino spectrum measurement to date, utilizing previously unused detector data to double the statistical power and compare with models and commercial reactor data.

Findings

Detection of a 5-7 MeV excess over models

Constraints on U235's role in the spectral anomaly

Enhanced statistical precision in antineutrino spectrum measurement

Abstract

This Letter reports one of the most precise measurements to date of the antineutrino spectrum from a purely U235-fueled reactor, made with the final dataset from the PROSPECT-I detector at the High Flux Isotope Reactor. By extracting information from previously unused detector segments, this analysis effectively doubles the statistics of the previous PROSPECT measurement. The reconstructed energy spectrum is unfolded into antineutrino energy and compared with both the Huber-Mueller model and a spectrum from a commercial reactor burning multiple fuel isotopes. A local excess over the model is observed in the 5MeV to 7MeV energy region. Comparison of the PROSPECT results with those from commercial reactors provides new constraints on the origin of this excess, disfavoring at 2.2 and 3.2 standard deviations the hypotheses that antineutrinos from U235 are solely responsible and non-contributors to the excess observed at commercial reactors respectively.