The Water Cherenkov Monitor for Anti-Neutrinos (WATCHMAN) project is based on the idea that a gadolinium-loaded water Cherenkov can detect the few-MeV electron antineutrinos produced by an operating nuclear reactor. A sufficiently large water Cherenkov detector could therefore remotely detect nuclear reactor operation in another country, thus providing a way of monitoring compliance with non-proliferation treaties. Neutrinos are an unavoidable by-product of nuclear fission, and cannot be prevented from leaving the vicinity of the reactor, so this technique is not susceptible to preventive measures by the nation in question.
Remote detection of reactor antineutrinos has so far only been demonstrated using liquid scintillator based experiments, most notably KamLAND, which measured the θ12 (solar) neutrino oscillation using reactor antineutrinos from reactors at a mean distance of about 180 km. However, a Gd-loaded water Cherenkov should be able to do this with high efficiency, using the gadolinium to tag the neutron emitted as a result of inverse beta decay, ν̅e + p → e+ + n. One of the aims of the WATCHMAN project is to demonstrate the feasibility of this technique by placing a relatively small (kiloton scale) water Cherenkov ~10 km away from an operating nuclear reactor.
WATCHMAN in Sheffield
Much of the work needed for WATCHMAN overlaps with ANNIE, Super-K and Hyper-K: all involve Gd-loaded water Cherenkov detectors (although it is not yet clear whether the main tanks of Hyper-K will be Gd-loaded—this decision will doubtless be affected by the performance of Gd-loaded Super-K—the Hyper-K project includes a smaller intermediate water Cherenkov at ~1 km which will definitely be Gd-loaded). An exciting opportunity provided by WATCHMAN is the chance to host this experiment in the UK: the STFC Boulby Underground Laboratory at Boulby Mine, with which Sheffield is very closely associated, is at an appropriate distance from the Hartlepool nuclear power station and would be a suitable site.