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Daya Bay on the Brink

Within the past few weeks, the Daya Bay Reactor Neutrino Experiment in China has made rapid strides toward completion of the first of three underground experimental halls for collecting data on the last unknown neutrino “mixing angle,” denoted θ13 (pronounced theta one three). Mixing angles are needed to understand how the three kinds of neutrinos – electron neutrino, muon neutrino, and tau neutrino – continually change “flavors,” or oscillate, as they streak through space or matter.

Two “near halls,” one near the pair of Daya Bay nuclear power-plant reactors and another near the four Ling Ao power-plant reactors a kilometer away, each house two antineutrino detectors (ADs). (Reactors emit antineutrinos, distinguished from ordinary neutrinos mainly by the kind of interactions in which they participate.) A third “far hall,” at a distance where the antineutrinos reach the first maximum amplitude of their oscillations into other flavors, will house four ADs. The unknown mixing angle will be determined with great precision by comparing the numbers of neutrinos collected at the various halls.


By the end of June, two ADs had been installed in the water pool in the Daya Bay Near Hall (Hall 1). The hall is now being filled with pure water.


A roof containing detectors to track cosmic rays will be rolled over the pool, whose walls are lined with photomultiplier tubes that also help trace evidence of cosmic rays. In mid-August Hall 1 is expected to begin building the dataset that will eventually be used to establish the unknown mixing angle. The immediate focus, however, is to measure any slight differences between the side-by-side detectors in Hall 1.


The two ADs destined for Hall 2 were also completed in late June. The cylindrical antineutrino detectors are designed to be filled with clear liquid scintillator, which reveals antineutrino interactions by the very faint flashes of light they emit. Sensitive photomultiplier tubes line the detector walls, ready to amplify and record the telltale flashes. A relatively small number of these interactions, over a thousand a day out of the millions of quadrillions of antineutrinos produced by the reactors every second, will be captured by the twin detectors in each near hall.


Once finished, the ADs are transported to the mixing hall inside the tunnels, to be filled. Two acrylic vessels nest inside the outer steel hull. The innermost is filled with liquid scintillator and a small amount of gadolinium. The second is filled with the scintillator alone, and the surrounding stainless steel vessel is filled with mineral oil.


Hall 2, the Ling Ao Near Hall (left), is shown almost ready for installation of the next two ADs, and will begin taking data not long after Hall 1. The Far Hall, Hall 3 (right), is still under construction and will begin taking data when it is finished in 2012.

The thumbnails shown here are a small sample of the over 300 photographs taken by Berkeley Lab photographer Roy Kaltschmidt on his visit to Daya Bay in June. The entire collection can be seen at

For a slide show of the Daya Bay complex, visit

For more information on the scientific goals of the Daya Bay experiment, visit

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