Journal
PHYSICAL REVIEW LETTERS
Volume 118, Issue 25, Pages -Publisher
AMER PHYSICAL SOC
DOI: 10.1103/PhysRevLett.118.251801
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Funding
- Ministry of Science and Technology of China
- U.S. Department of Energy
- Chinese Academy of Sciences
- CAS Center for Excellence in Particle Physics
- National Natural Science Foundation of China
- Guangdong provincial government
- Shenzhen municipal government
- China General Nuclear Power Group
- Ministry of Education, Key Laboratory of Particle Physics and Particle Irradiation (Shandong University)
- Ministry of Education, Shanghai Laboratory for Particle Physics and Cosmology
- Research Grants Council of Hong Kong Special Administrative Region of China
- University of Hong Kong
- MOE program for Research of Excellence at National Taiwan University
- National Chiao-Tung University
- NSC fund support from Taiwan
- U.S. National Science Foundation
- Alfred P. Sloan Foundation
- Ministry of Education, Youth, and Sports of the Czech Republic
- Joint Institute of Nuclear Research in Dubna, Russia
- National Commission of Scientific and Technological Research of Chile
- Tsinghua University Initiative Scientific Research Program
- Key Laboratory of Particle and Radiation Imaging (Tsinghua University)
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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 GW(th) reactor cores at the Daya Bay and Ling Ao nuclear power plants. Using detector data spanning effective Pu-239 fission fractions F-239 from 0.25 to 0.35, Daya Bay measures an average IBD yield (sigma) over bar (f) of (5.90 +/- 0.13) x 10(-43) cm(2)/fission and a fuel-dependent variation in the IBD yield, d sigma(f)/dF(239), of (-1.86 +/- 0.18) x 10(-43) cm(2)/fission. This observation rejects the hypothesis of a constant antineutrino flux as a function of the Pu-239 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 3.1 sigma. 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 U-235, Pu-239, U-238, and Pu-241. Based on measured IBD yield variations, yields of (6.17 +/- 0.17) and (4.27 +/- 0.26) x 10(-43) cm(2)/fission have been determined for the two dominant fission parent isotopes U-235 and Pu-239. A 7.8% discrepancy between the observed and predicted U-235 yields suggests that this isotope may be the primary contributor to the reactor antineutrino anomaly.
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