分类： 物理学 >> 核物理学 提交时间： 2016-09-10
摘要： This Letter reports a measurement of the flux and energy spectrum of electron antineutrinos from six 2.9~GWthnuclear reactors with six detectors deployed in two near (effective baselines 512~m and 561~m) and one far (1,579~m) underground experimental halls in the Daya Bay experiment. Using 217 days of data, 296,721 and 41,589 inverse beta decay (IBD) candidates were detected in the near and far halls, respectively. The measured IBD yield is (1.55 ± 0.04) × 10−18~cm2/GW/day or (5.92 ± 0.14) × 10−43~cm2/fission. This flux measurement is consistent with previous short-baseline reactor antineutrino experiments and is 0.946±0.022 (0.991±0.023) relative to the flux predicted with the Huber+Mueller (ILL+Vogel) fissile antineutrino model. The measured IBD positron energy spectrum deviates from both spectral predictions by more than 2σ over the full energy range with a local significance of up to ∼4σ between 4-6 MeV. A reactor antineutrino spectrum of IBD reactions is extracted from the measured positron energy spectrum for model-independent predictions.
分类： 物理学 >> 核物理学 提交时间： 2016-09-10
摘要： The Daya Bay experiment was the first to report simultaneous measurements of reactor antineutrinos at multiple baselines leading to the discovery of ν¯e oscillations over km-baselines. Subsequent data has provided the world's most precise measurement of sin22θ13 and the effective mass splitting Δm2ee. The experiment is located in Daya Bay, China where the cluster of six nuclear reactors is among the world's most prolific sources of electron antineutrinos. Multiple antineutrino detectors are deployed in three underground water pools at different distances from the reactor cores to search for deviations in the antineutrino rate and energy spectrum due to neutrino mixing. Instrumented with photomultiplier tubes (PMTs), the water pools serve as shielding against natural radioactivity from the surrounding rock and provide efficient muon tagging. Arrays of resistive plate chambers over the top of each pool provide additional muon detection. The antineutrino detectors were specifically designed for measurements of the antineutrino flux with minimal systematic uncertainty. Relative detector efficiencies between the near and far detectors are known to better than 0.2%. With the unblinding of the final two detectors' baselines and target masses, a complete description and comparison of the eight antineutrino detectors can now be presented. This paper describes the Daya Bay detector systems, consisting of eight antineutrino detectors in three instrumented water pools in three underground halls, and their operation through the first year of eight detector data-taking.
分类： 物理学 >> 核物理学 提交时间： 2016-09-10
摘要： The Daya Bay reactor neutrino experiment observed electron antineutrino disappearance associated with θ13 with a significance better than 5σ in 2012. The final two of eight antineutrino detectors were installed in the summer of 2012. Including the 404 days of data collected with the full detector configuration resulted in a 3.6 times increase of statistics over the previous result with the 6-AD configuration. With improvements of the systematic uncertainties and better estimation of backgrounds, Daya Bay has measured sin22θ13=0.084±0.005 and |Δm2ee|=2.42+0.10−0.11×10−3~eV2. This is the most precise measurement of sin22θ13 to date and the most precise measurement of of |Δm2ee| via electron antineutrino disappearance. Several other analysis results are presented, including an independent measurement of θ13 using inverse-beta decays associated with neutron capture on hydrogen, a measurement of reactor antineutrino flux and spectrum, and a search for light sterile neutrino mixing.
分类： 物理学 >> 核物理学 提交时间： 2016-09-09
摘要： We report a new measurement of electron antineutrino disappearance using the fully-constructed Daya Bay Reactor Neutrino Experiment. The final two of eight antineutrino detectors were installed in the summer of 2012. Including the 404 days of data collected from October 2012 to November 2013 resulted in a total exposure of 6.9×105 GWth-ton-days, a 3.6 times increase over our previous results. Improvements in energy calibration limited variations between detectors to 0.2%. Removal of six 241Am-13C radioactive calibration sources reduced the background by a factor of two for the detectors in the experimental hall furthest from the reactors. Direct prediction of the antineutrino signal in the far detectors based on the measurements in the near detectors explicitly minimized the dependence of the measurement on models of reactor antineutrino emission. The uncertainties in our estimates of sin22θ13 and |Δm2ee| were halved as a result of these improvements. Analysis of the relative antineutrino rates and energy spectra between detectors gave sin22θ13=0.084±0.005 and |Δm2ee|=(2.42±0.11)×10−3 eV2 in the three-neutrino framework.
分类： 物理学 >> 核物理学 提交时间： 2016-09-09
摘要： In the Daya Bay Reactor Neutrino Experiment 960 20-cm-diameter waterproof photomultiplier tubes are used to instrument three water pools as Cherenkov detectors for detecting cosmic-ray muons. Of these 960 photomultiplier tubes, 341 are recycled from the MACRO experiment. A systematic program was undertaken to refurbish them as waterproof assemblies. In the context of passing the water leakage check, a success rate better than 97% was achieved. Details of the design, fabrication, testing, operation, and performance of these waterproofed photomultiplier-tube assemblies are presented.
分类： 物理学 >> 核物理学 提交时间： 2016-09-07
摘要： The Daya Bay Reactor Antineutrino Experiment reported the first observation of the non-zero neutrino mixing angle θ13 using the first 55 days of data. It has also provided the most precise measurement of θ13 with the extended data to 621 days. Daya Bay will keep running for another 3 years or so. There is about 100 TB raw data produced per year, as well as several copies of reconstruction data with similar volume to the raw data for each copy. The raw data is transferred to Daya Bay onsite and two offsite clusters: IHEP in Beijing and LBNL in California, with a short latency. There is quasi-real-time data processing at both onsite and offsite clusters, for the purpose of data quality monitoring, detector calibration and preliminary data analyses. The physics data production took place a couple of times per year according to the physics analysis plan. This paper will introduce the data movement and storage, data processing and monitoring, and the automation of the calibration.
分类： 物理学 >> 核物理学 提交时间： 2016-08-23
摘要： This article reports an improved independent measurement of neutrino mixing angle θ13 at the Daya Bay Reactor Neutrino Experiment. Electron antineutrinos were identified by inverse β-decays with the emitted neutron captured by hydrogen, yielding a data-set with principally distinct uncertainties from that with neutrons captured by gadolinium. With the final two of eight antineutrino detectors installed, this study used 621 days of data including the previously reported 217-day data set with six detectors. The dominant statistical uncertainty was reduced by 49%. Intensive studies of the cosmogenic muon-induced 9Li and fast neutron backgrounds and the neutron-capture energy selection efficiency, resulted in a reduction of the systematic uncertainty by 26%. The deficit in the detected number of antineutrinos at the far detectors relative to the expected number based on the near detectors yielded sin22θ13=0.071±0.011 in the three-neutrino-oscillation framework. The combination of this result with the gadolinium-capture result is also reported.
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