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 fission fractions 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/fission and 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 fission fraction 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 fission isotopes ^(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/fission have been determined for the two dominant fission parent 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.
© 2017 American Physical Society. Received 6 April 2017; published 19 June 2017. Daya Bay is supported in part by the Ministry of Science and Technology of China, the U.S. Department of Energy, the Chinese Academy of Sciences, the CAS Center for Excellence in Particle Physics, the National Natural Science Foundation of China, the Guangdong provincial government, the Shenzhen municipal government, the China General Nuclear Power Group, Key Laboratory of Particle and Radiation Imaging (Tsinghua University), the Ministry of Education, Key Laboratory of Particle Physics and Particle Irradiation (Shandong University), the Ministry of Education, Shanghai Laboratory for Particle Physics and Cosmology, the Research Grants Council of the Hong Kong Special Administrative Region of China, the University Development Fund of The University of Hong Kong, the MOE program for Research of Excellence at National Taiwan University, National Chiao-Tung University, and NSC fund support from Taiwan, the U.S. National Science Foundation, the Alfred P. Sloan Foundation, the Ministry of Education, Youth, and Sports of the Czech Republic, the Joint Institute of Nuclear Research in Dubna, Russia, the National Commission of Scientific and Technological Research of Chile, and the Tsinghua University Initiative Scientific Research Program. We acknowledge Yellow River Engineering Consulting Co., Ltd., and China Railway 15th Bureau Group Co., Ltd., for building the underground laboratory. We are grateful for the ongoing cooperation from the China General Nuclear Power Group and China Light and Power Company.
Submitted - 1704.01082.pdf
Published - PhysRevLett.118.251801.pdf
Supplemental Material - cov_stat.txt
Supplemental Material - cov_syst.txt
Supplemental Material - flux_data.txt
Supplemental Material - spec_data.txt
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