When this collision occurs, the proton transforms into a neutron and ejects a positron, the antimatter version of an electron. Because the positron moves so quickly through the water, it emits the optical equivalent of a sonic boom, called Cherenkov radiation. (Watchman is a shortening of Water Cherenkov Monitor of Antineutrinos.) Mixed in the water will be the element gadolinium, which will absorb the neutron generated in the collision, emitting a second flash of Cherenkov light. The demonstration will scale up previous work that was able to detect antineutrinos at a distance of about 80 feet from a reactor core. Detectors as large as the one at Boulby could be placed near the nuclear infrastructure of a state that had agreed to shut down its nuclear reactors, allowing international authorities to verify compliance. Potentially, much larger detectors could monitor sites hundreds of miles away in hostile nations that do not allow inspections. The same apparatus would also assist astronomers studying supernovas, the explosions of distant stars. In 1987, several large neutrino detectors detected a few handfuls of neutrinos and antineutrinos from the explosion of a star more than 160,000 light-years away. Watchman would similarly detect such cosmic explosions, but with improved acuity. The presence of gadolinium in the new detector would make it possible to differentiate neutrinos, which would generate just one flash of Cherenkov light, from antineutrinos, which would generate two. “We couldn’t do that in 1987,” said Robert Svoboda, a professor of physics at the University of California, Davis and a member of the Watchman team. For neutrino scientists, Watchman will also finance the development of improved technology to record the Cherenkov flashes. These are to be deployed in a second phase of Watchman and then could be used in other neutrino experiments. “There’s this nice duality between basic science research and nonproliferation,” Dr. Bernstein said.
British and American scientists will build a test antineutrino detector at Hartlepool in England, some 15 miles from the the Boulby Mine nuclear power plants, to demonstrate the capability of sensors to detect fission reactions and verify compliance to shutting down nuclear reactors. The apparatus is named Watchman and is a 50 ft cylinder that is filled with 7.7 million pounds of water located in the mine 3,600 feet underground. The project is sponsored by the US Department of Energy's National Nuclear Security Administration and will contribute $30m over six years.