As far as useful discoveries go, the neutrino didn’t look promising. These ghostly particles are produced by the sun, by radioactive elements, and by nuclear reactors, from which they speed outward with zero charge, almost no mass, and a nearly complete indifference to matter.
Last week, scientists released a map showing what the world would look like if we could see all the billions upon billions of neutrinos that emanate from the surface of the planet each second. It turns out that neutrinos’ uncontainable nature is potentially bad if you’re trying to hide something going on at a nuclear plant, but good if you want to monitor other people’s nuclear activities. Dark spots on the map indicate nuclear reactors and parts of the earth’s crust rich with radioactive uranium and thorium, which emit neutrinos when they decay.
The technology to pick up the tracks of elusive neutrinos has continued to improve since the first official detection in 1956 at the Savannah River nuclear facility in South Carolina. While most detectors have been built with the goal of studying the nature and behavior of neutrinos, scientists are starting to consider using neutrino detectors to probe the earth’s interior and monitor nuclear activities.
The map, published in Nature Scientific Reports, was created using neutrino signals captured in two detectors, one in Italy and one in Japan, says William McDonough, a geophysicist at the University of Maryland and a coauthor on the paper. The rest of the map was constructed using data about the composition and density of the earth’s crust and the location of the world’s reactors.
Dark patches appear around mountain ranges where there’s a lot of naturally occurring radioactive decay, he says. The Himalayas are responsible for a massive dark patch over southern Asia. Some of the dark spots are reactors, especially around France. (Technically, the ones from nuclear power plants are called antineutrinos—the antimatter counterpart to neutrinos—but the differences between the two types of particles are still being worked out.)
What renders neutrinos detectable at all is the fact that there are a lot of them. The detectors employ apartment-building-size tanks of mineral oil, through which trillions of neutrinos pass unobstructed each second. But every once in a while, one of them hits the nucleus of a hydrogen atom just so, annihilating a proton and leaving behind other particles—a positron and a neutron—that will register a signal.
Finding clandestine reactors is not as important as monitoring known ones, says physicist Patrick Huber of Virginia Tech. Reactors give off heat that can be detected easily with infrared sensitive satellites. We knew where all the Soviet reactors are, and now we know where they are in Russia and North Korea. What we don’t always know is how and when they are being used.
In any country where international treaties allow access, Huber says, small, refrigerator-sized neutrino detectors could be placed nearby to reveal whether reactors were unexpectedly turned on or off. Moreover, he says, neutrinos from different sources have a distinctive energy signature, and that can be used to distinguish plutonium from uranium, and possibly to reveal if someone diverted plutonium from a nuclear reactor.
An enormous neutrino detector might also prove useful for global monitoring, says Lindley Winslow, a neutrino physicist at MIT who was not part of the map group. There’s a mega-detector called Juno planned to start up in China in 2020, she says, though that’s primarily aimed at answering fundamental questions about the nature of the universe. The difference between neutrinos and antineutrinos may hold the answer to why the universe produced more matter than antimatter, thus allowing the world to exist, she says.
Those big questions about the nature of existence have driven the advance of neutrino detection technology, Huber says, but he’s happy that the effort could advance nuclear security. “When you become a particle physicist you assume what you do will all be in an ivory tower … that it will all be basic science with no practical applications,” he says. “But as you can see, that’s not quite true.”
This story was updated on September 15 to correct the year and the state of the first observation of the neutrino.