By Michael Lucibella



Global map of reactor neutrino emission.

Photo courtesy of Glenn Jocher and John Learned, University of Hawaii

APS March Meeting, Denver — The Department of Energy (DOE) is funding the WATer CHerenkov Monitor of AntiNeutrinos (WATCHMAN), a prototype neutrino detector that can monitor whether a nuclear reactor 400 kilometers away is enriching the raw material for nuclear weapons. If successful, the WATCHMAN Collaboration’s research could make it nearly impossible for countries to hide their illicit nuclear enrichment. It also marks the start of the neutrino’s transformation into a practical tool for uses outside of basic research.

“This would be the first really applied use of neutrinos,” said Mark Vagins of the University of California Irvine and member of the collaboration.

All nuclear reactors emit radiation and antineutrinos. While radiation can be blocked with a few feet of soil or concrete, antineutrinos pass unimpeded through hundreds of miles of solid Earth.

“That’s the beauty of this signal — you won’t be able to stop it or shield it,” said Adam Bernstein of Lawrence Livermore National Labs and head of the Watchman Collaboration. “Neutrinos have no nationalities.”

Giant neutrino detectors could be built within the borders of a country to keep tabs on the reactors of a nearby nation hostile to inspections. Based on the detected neutrino signature, monitors can discern what is happening inside of that nation’s reactors.

“If an unfriendly nation is producing plutonium, they will typically run their reactor in a specific cycle,” Vagins said. “What we want to really determine is how well we can see this on-off cycle.”

Because neutrinos only weakly interact with matter, nonproliferation monitors will need massive water detectors to analyze reactors at long distances. When an antineutrino strikes an atomic nucleus, it emits a tiny flash of light, which propagates through the clear water and can be picked up by sensitive photodetectors. But this is rare, so large volumes of water are needed to see the effect at all.

The prototype the collaboration hopes to complete by 2016 will use a few thousand tons of water spiked with gadolinium to observe reactors a dozen kilometers away. They predict that detectors engaged in nonproliferation monitoring would need about a million tons of water to observe reactors 400 kilometers away.

The collaboration hopes to build the prototype at the Fairport Salt Mine outside of Cleveland, Ohio, once the home to the world’s first water neutrino detector.

“The mine is still active. The area used by scientists, the instrumented cavern, has remained unused for many years,” Vagins said.

The Perry Nuclear Power Plant 12 kilometers up the road offers the ideal source of nearby reactor neutrinos. The mine also extends under Lake Erie, keeping atmospheric backgrounds to a minimum.

WATCHMAN builds on a series of recent experiments at the San Onofre nuclear power plant in California. There, smaller detectors adjacent to the reactor units proved it was possible to see inside a reactor core from its neutrino emissions.

Though primarily backed by the National Nuclear Security Administration, the collaboration hopes DOE̕s Office of Science will pick up between about 25 to 45 percent of the roughly $40 million bill.

“If we build a detector of this scale, at a kiloton, you can do science with that,” Vagins said. “We don’t really have a supernova detector in America, and this would be a very capable one.”

The collaboration also hopes to use it for basic particle physics, in particular the hunt for hypothesized sterile neutrinos that do not interact with matter in the usual ways, except gravitationally. So far, administrators from the Office of Science haven’t committed any funds, but have expressed some interest in the Watchman science program.

“Right now we’re exploring it. Our interest isn’t exuberant, but we’re certainly looking,” said Alan Stone at the Office of Science. “We’re looking at all opportunities and looking to see what we can do with limited budgets.”

A final decision from the Office of Science will likely come sometime after May when the Particle Physics Project Prioritization Panel delivers its final recommendations for the department’s future plans.