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

Abstract

This Letter reports the first extraction of individual antineutrino spectra from ^(235)U and ^(239)Pu fission and an improved measurement of the prompt energy spectrum of reactor antineutrinos at Daya Bay. The analysis uses 3.5×10^6 inverse beta-decay candidates in four near antineutrino detectors in 1958 days. The individual antineutrino spectra of the two dominant isotopes, ^(235)U and ^(239)Pu, 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 ^(235)U (^(239)Pu) spectrum compared with the normalized Huber-Mueller model prediction. The significance of discrepancy is 4.0σ for ^(235)U 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.