The biggest major science project in China that has been built through a genuine international collaboration has begun operation. Once fully complete next year, the Daya Bay Reactor Neutrino Experiment – a partnership lead by 19 Chinese and 16 US universities – will begin searching for the final undetermined neutrino “mixing angle”, known as θ13.
Neutrinos are difficult to detect because they interact weakly with matter. They come in three “flavours” – electron, muon and tau – that change or “oscillate” from one to another as they travel in space. The oscillation strength between different types of neutrino is characterized by three “mixing angles” – known as θ12, θ23 and θ13 – with Daya Bay designed to determine θ13 by measuring the disappearance of electron antineutrinos.
The US Department of Energy is providing about half of the cost of the $68m facility, with China paying for the other half and all of the civil-engineering costs. The Daya Bay experiment detects electron antineutrinos produced via nuclear beta decay at two neighbouring nuclear reactors – the Daya Bay and Ling Ao power plants, which are around 55 km north-east of Hong Kong.
The new neutrino facility will consist of three experimental halls that contain identical neutrino detectors, each filled with 20 tonnes of gadolinium-doped liquid scintillator. When a neutrino strikes the liquid, a flash of light is produced that is then picked up by a bank of photomultiplier tubes around the liquid.