Latest Results of Reactor Antineutrino Flux and Spectrum at Daya Bay - Antineutrino detected by inverse beta reaction (IBD) in Gd-loaded liquid scintillator • Prompt e+ signal : 1-10 MeV, determined by antineutrino energy • Delayed neutron capture signal: 8 MeV @ Gd • Time correlation: capture time is about 28 μs in 0.1% Gd-LS XXVIII International Conference on Neutrino Physics and Astrophysics, Heidelberg, 4-9 June 2018 Liang Zhan, Institute of High Energy Physics, Beijing on behalf of the Daya Bay Collaboration Reactor Antineutrinos at Daya Bay • Six reactors with a total thermal power of 17.4 GW • Antineutrino flux produced by fissions of isotopes: 235U, 238U, 239Pu , and 241Pu • Reactor operator provides generated thermal power (Wth) and fission fraction (fi /F) • ei : Energy release per fission for isotope i • Si(E): Antineutrino energy spectrum for each isotope • αi : fission fraction Abstract The latest measurement of the reactor antineutrino flux and energy spectrum by the Daya Bay reactor neutrino experiment is reported. The antineutrinos were generated by six 2.9 GWth nuclear reactors and detected by eight antineutrino detectors deployed in two near (500 m and 600 m flux-weighted baselines) and one far (1600 m flux-weighted baseline) underground experimental halls. An improvement on the neutron detection efficiency determination was performed using a new neutron calibration campaign and dedicated data-simulation comparison. With a 1230-day data set, the IBD yield was measured to be (5.91±0.09)x10-43cm2/fission. The ratio between the measured to predicted reactor antineutrino yield is 0.952±0.014(exp.)±0.023(model). The comparison of the measured IBD positron energy spectrum with the predictions is also reported with a previous 621-day data set. A reactor antineutrino spectrum weighted by the IBD cross section is extracted for model-independent predictions. Reactor Antineutrino Detection S: antineutrino flux from reactors σ: cross section of inverse beta decay. L: baseline, surveyed with a precision of 28 mm. Psur : antineutrino survival probability, including the fit parameter sin22θ13 Np: number of target protons, determined by target mass. Є: detection efficiency - Antineutrino spectrum of IBD reactions An improvement on the determination of the neutron detection efficiency was performed compared with previous publication Reactor Antineutrino Flux Measurement - Determination of the neutron detection efficiency was improved using a new neutron calibration campaign and dedicated data-simulation comparison. - An extensive neutron calibration campaign was carried out in Daya Bay at the end of 2016. Two types of neutron sources ( 241Am-13C and 241Am-9Be) were deployed vertically in three calibration axis and the data in 59 different combinations of sources and locations was collected. - A benchmark quantity was defined on the neutron capture energy spectrum. - Good agreement between calibration data and simulation on the energy spectrum and F. - The neutron detection efficiency was determined to be (81.48%±0.60%) with a reduction in the uncertainty by 56%. - A new measurement on the reactor antineutrino yield was performed using the 1230-day data which has average fission fractions of (0.571, 0.076, 0.299, and 0.054) for (235U, 238U, 239Pu , and 241Pu) Reactor Antineutrino Spectrum Measurement - Measured IBD prompt energy spectrum vs. prediction after normalization - Obvious discrepancy between data and prediction, significance was evaluated. - 3 σ for the whole spectrum - 4.4 σ for a 2-MeV energy window around 5 MeV - Uncertainty of the prompt energy spectrum - An antineutrino spectrum of IBD reactions is provided as an input for reactor neutrino experiments after unfolding the IBD prompt energy spectrum to antineutrino energy. - Consistent results by two unfolding methods: singular value decomposition (SVD) and Bayesian iteration Reactors: D1, D2, L1, L2, L3, and L4 Detectors: AD1, AD2, …, AD8 - A variety of simulati