Extraction of the 235U and 239Pu Antineutrino Spectra at Daya Bay

Abstract

This Letter reports the first extraction of individual antineutrino spectra from U235 and Pu239 fission and an improved measurement of the prompt energy spectrum of reactor antineutrinos at Daya Bay. The analysis uses 3.5×106 inverse beta-decay candidates in four near antineutrino detectors in 1958 days. The individual antineutrino spectra of the two dominant isotopes, U235 and Pu239, are extracted using the evolution of the prompt spectrum as a function of the isotope fission fractions. In the energy window of 4-6 MeV, a 7% (9%) excess of events is observed for the U235 (Pu239) spectrum compared with the normalized Huber-Mueller model prediction. The significance of discrepancy is 4.0σ for U235 spectral shape compared with the Huber-Mueller model prediction. The shape of the measured inverse beta-decay prompt energy spectrum disagrees with the prediction of the Huber-Mueller model at 5.3σ. In the energy range of 4-6 MeV, a maximal local discrepancy of 6.3σ is observed. © 2019 authors. Published by the American Physical Society.