The Reactor Experiment for Neutrino Oscillations (RENO) has performed a definitive measurement of the neutrino-oscillation mixing angle, θ13, by observing the disappearance of electron-antineutrinos emitted from a nuclear reactor, with a significance of 4.9 σ
RENO detects antineutrinos from six reactors, each with a thermal power output of 2.8 GWth, at Yonggwang Nuclear Power Plant in Korea. The reactors are almost equally spaced in a line about 1.3 km long and the experiment uses two identical detectors located at 294 m and 1383 m on either side of the centre of this line, beneath hills that provide, respectively, 120 and 450 m of water-equivalent of rock overburden to reduce the cosmic backgrounds. This symmetric arrangement of reactors and detectors is useful for minimizing the complexity of the measurement. RENO is the first experiment to measure θ13, the smallest neutrino-mixing angle and the last to be known, with two identical detectors.
In the 229-day data-taking period from 11 August 2011 to 26 March 2012, the far (near) detector observed 17,102 (154,088) electron-antineutrino candidate events with a background fraction of 5.5% (2.7%). During this period, all six reactors were operating mainly at full power, with two reactors being off for a month each for fuel replacement.
The two identical antineutrino detectors allow a relative measurement through a comparison of the observed neutrino rates. Measuring the far-to-near ratio of the reactor neutrinos in this way can considerably reduce several systematic errors. The relative measurement is independent of correlated uncertainties and helps in minimizing uncorrelated reactor uncertainties.
Each detector comprises four layers. At the core lies the target volume of 16.5 tonnes of liquid scintillator that is doped with gadolinium. An electron-antineutrino can interact with a free proton in the scintillator, ν + p → e+ + n. The positron from this inverse β-decay annihilates immediately giving a prompt signal. The neutron wanders into the target volume, eventually being captured by the gadolinium – giving a delayed signal. The delayed coincidence between the positron and neutron signals provides the distinctive signature of inverse β-decay.
The central target volume is surrounded by a 60 cm layer of liquid scintillator without gadolinium, which serves to catch γ-rays escaping from the target volume, thus increasing the detection efficiency. Outside this γ-catcher, a 70 cm buffer-layer of mineral oil shields the inner detectors from radioactivity in the surrounding rocks and in the 354 photomultiplier tubes (10-inch) that are installed on the inner wall of the buffer container. The outermost veto layer consists of 1.5 m of pure water, which serves to identify events coming from the outside through their Cherenkov radiation and to shield against ambient γ-rays and neutrons from the surrounding rocks. Both detectors are calibrated using radioactive sources and cosmic-ray induced background samples.
Based on the number of events at the near detector and assuming no oscillation, RENO finds a clear deficit, with a far-to-near ratio R = 0.920 ± 0.009 (stat.) ± 0.014 (syst.). The value of sin22θ13 is determined from a χ2 fit with pull terms on the uncorrelated systematic uncertainties. The number of events in each detector after the background subtraction has been compared with the expected number of events, based on the neutrino flux, detection efficiency, neutrino oscillations and contribution from the reactors to each detector determined by the baselines and reactor fluxes. The best-fit value obtained is sin22θ13 = 0.113 ± 0.013 (stat.) ± 0.019 (syst.), which excludes the no-oscillation hypothesis at 4.9 σ.
The RENO collaboration consists of about 35 researchers from Seoul National University, Chonbuk National University, Chonnam National University, Chung Ang University, Dongshin University, Gyeongsang National University, Kyungpook National University, Pusan National University, Sejong University, Seokyeong University, Seoyeong University and Sungkyunkwan University.
