|Daya Bay nuclear power complex (Guangdong, China)|
At 2.9 GW per reactor, the complex produces ~3 x 1021 anti-neutrinos every second.
It is well established that the flavor of a neutrino oscillates with time. Neutrino oscillations can be described by the three mixing angles (θ12, θ23, and θ13) and phase of the Pontecorvo-Maki-Nakagawa-Sakata matrix, and two mass-squared differences (Δm231 and Δm221). Of the mixing angles, θ13 is the least known. The Daya Bay Reactor Neutrino Experiment is designed to precisely determine θ13 by building eight functionally identical anti-neutrino detectors (ADs) to measure the anti-neutrino flux from six nuclear reactors. Access the technical design report here.
The Daya Bay nuclear power complex consists of three nuclear power plants. Three experimental halls (EHs) are connected with horizontal tunnels and located underground in the mountains adjacent to the nuclear power plants. The effect of neutrino oscillation is observed by comparing the events at the near and far detectors. Fewer events than expected at the far detectors indicates that anti-neutrinos from the reactors are oscillating into another flavor of neutrino, which is not detected.
The EHs are located underground to shield from cosmic ray muons (coming from several km above the Earth's surface). Such muons produce spallation neutrons and other backgrounds, which can cause events that appear to be neutrino events in the anti-neutrino detectors (ADs).
|Experimental Hall 3 (December 2011)|
|Schematic of Experimental Hall 1|
In each EH, either two or four ADs will be installed inside a water pool, which shields the ADs from ambient radiation (neutrons and gammas) in all directions. In addition to providing shielding, each water pool is instrumented with photomultiplier tubes (PMTs) which enables the identification of muons by detection of Cherenkov radiation. Modules containing four layers of resistive plate chambers (RPCs) cover the top of the pool as an independent detector of muons. By efficiently detecting muons, muon-induced backgrounds can be rejected.
|An electron anti-neutrino (νe) is detected via the inverse β-decay reaction, νe + p --> e+ + n (in a Gadolinium-doped liquid scintillator). The time-coincidence of the prompt scintillation from the positron (e+) and that from the delayed neutron (n) capture on Gd provides a distinctive νe signature. For detailed information, see A side-by-side comparison of Daya Bay antineutrino detectors. |
With 55 days of data, 10416 νe candidates were detected in the far hall (EH3), and the Daya Bay Reactor Neutrino Experiment announced a measurement of a non-zero value for the neutrino mixing angle θ13 with a significance of 5.2 standard deviations, on March 8, 2012. For the details of the study, see Observation of electron-antineutrino disappearance at Daya Bay. For the most recently reported result (October 23, 2012), see Improved Measurement of Electron Antineutrino Disappearance at Daya Bay.
This discovery will help lead to a better understanding of neutrino oscillation, which is not currently part of the standard model of particle physics. It is the gateway to studying leptonic CP violation, a fundamental symmetry breaking related to why matter, as opposed to anti-matter, is the predominant constituent of the universe.