Abstract
This Letter reports the first extraction of individual antineutrino spectra from U-235 and Pu-239 fission and an improved measurement of the prompt energy spectrum of reactor antineutrinos at Daya Bay. The analysis uses 3.5 x 10(6) inverse beta-decay candidates in four near antineutrino detectors in 1958 days.
The individual antineutrino spectra of the two dominant isotopes, U-235 and Pu-239, 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 U-235 (Pu-239) spectrum compared with the normalized Huber-Mueller model prediction.
The significance of discrepancy is 4.0 sigma for U-235 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 5.3 sigma.
In the energy range of 4-6 MeV, a maximal local discrepancy of 6.3 sigma is observed.