28 April 2011—The neutrino, or "ghost particle," is strikingly aloof. On Earth, tens of billions of the sun’s neutrinos pass through an area the size of a thumbtack every second. But most of these particles zip straight through Earth without a single interaction with another bit of matter.
Neutrino detectors, which aim to catch such ghosts, are traditionally big affairs, sometimes employing large vats of water or oil, a deep patch of the Mediterranean, or even a cubic kilometer of Antarctic ice to boost the chance of seeing the specters.
But physicists have been working to adapt the technology to make detectors small enough to be installed inside nuclear power plants. If their prototypes are proved, such detectors could continuously monitor nuclear reactors and provide a new way to safeguard against nuclear proliferation.
The detectors in development hunt for the neutrino’s antimatter sibling, the antineutrino, which shares most of its properties. Nuclear reactors are the largest man-made source of antineutrinos. A typical 1-gigawatt nuclear reactor pumps out some 100 billion billion antineutrinos per second. Because these particles pass through any shielding pretty much unimpeded, their signal can’t be masked.
A joint group of physicists based in California at Lawrence Livermore National Laboratory and Sandia National Laboratories, along with researchers at Atomic Energy of Canada Limited's Chalk River Laboratories, aims to capture reactor-born antineutrinos with a detector they plan to install next year at the Point Lepreau Generating Station, a CANDU-type nuclear reactor in New Brunswick, Canada. They will present an update on tests of the detector next week at the American Physical Society April Meeting in Anaheim, Calif.
To catch antineutrinos, the detector employs hundreds of liters of organic solvent mixed with gadolinium atoms. Incoming antineutrinos occasionally collide with protons in the mixture, creating a neutron and a positron, the antimatter partner of the electron. The positron creates a flash of light when it meets with and annihilates an electron, and the neutron releases light when it is captured by a gadolinium atom. The detectors are studded with photosensitive tubes that convert these flashes of light into electronic signals.
The two precisely timed flashes are unique to antineutrinos, but physicists must still contend with confounding signals, particularly those created by charged particles that originate in Earth’s atmosphere. They can insulate detectors from this background noise by placing them underground.
The Point Lepreau detector, which will measure about 3 meters on each side, is the fourth in a series developed by the Livermore-Sandia team. The previous three detectors were installed over the course of the past decade at a 1.1-GW pressurized water reactor at California’s San Onofre Nuclear Generating Station.
The team’s first demonstration detector at San Onofre picked up about 400 antineutrinos per day during a 600-day test period. That was good enough to provide near real-time data on the state of the reactor. The detector could "tell when the reactor has been turned off within a few hours, which is important, because if you wanted to remove any fissile material, you would have to turn the reactor off," says team member Timothy Classen, of Lawrence Livermore.
But the team hopes it can mine antineutrino data to give even more information about what’s going on inside the reactor. An operator that runs a reactor at a higher power or uses fuel with more uranium-238 can boost the production rate of plutonium that could be used for nuclear weapons. Uranium releases more detectable antineutrinos than plutonium does, so monitoring the rate of antineutrino emission could potentially indicate whether a reactor is being run as intended or if material for weapons has been removed.