Evolution of the Reactor Antineutrino Flux and Spectrum at Daya Bay

The Daya Bay experiment has observed correlations between reactor core fuel evolution and changes in the reactor antineutrino flux and energy spectrum. Four antineutrino detectors in two experimental halls were used to identify 2.2 million inverse beta decays(IBDs) over 1230 days spanning multiple fuel cycles for each of six 2.9 GWth reactor cores at the Daya Bay and Ling Ao nuclear power plants. Using detector data spanning effective ^(239)Pu fissionfractions F_(239) from 0.25 to 0.35, Daya Bay measures an average IBD yield σ_f of (5.90±0.13)×10^(-43) cm^2/ fissionand a fuel-dependent variation in the IBD yield, dσ_f/dF_(239), of (-1.86±0.18)×10^(-43) cm^2/ fission. This observation rejects the hypothesis of a constant antineutrino flux as a function of the ^(239)Pu fissionfraction at 10 standard deviations. The variation in IBD yield is found to be energy dependent, rejecting the hypothesis of a constant antineutrino energy spectrum at 5.1 standard deviations. While measurements of the evolution in the IBD spectrum show general agreement with predictions from recent reactor models, the measured evolution in total IBD yield disagrees with recent predictions at 3.1σ. This discrepancy indicates that an overall deficit in the measured flux with respect to predictions does not result from equal fractional deficits from the primary fissionisotopes ^(235)U, ^(239)Pu, ^(238)U, and ^(241)Pu. Based on measured IBD yield variations, yields of (6.17±0.17) and (4.27±0.26)×10^(-43) cm^2/ fissionhave been determined for the two dominant fissionparent isotopes ^(235)U and ^(239)Pu. A 7.8% discrepancy between the observed and predicted ^(235)U yields suggests that this isotope may be the primary contributor to the reactor antineutrino anomaly.