Constructing a Nuclear Bomb Detector On a Budget

There are several ways to avert a nuclear terrorist attack. One is diplomacy. Last week, for example, the US and several other countries wrapped up negotiations with Iran—long suspected of being a state sponsor of terrorism—to prevent the country from developing weapons-grade nuclear material. But signed promises between diplomats can’t stop a group or individual with enough horrific determination. That’s why the government wants a backup plan: nuclear-sensing networks around our most valuable assets. Since 9/11, the US has been keenly interested in building nuclear sensors. Problem is, the best material for sniffing out bomb-grade radioactive material is helium-3, an isotope too rare to build enough detectors to guard every port, every border, and all the roads leading into every city. But one company thinks the fix is to build larger detectors out of other, less sensitive materials, on the principle that a bigger detector would capture more of the telltale signs of radioactivity. That company, Silverside Detectors, is building a detector for neutrons—the best way to sniff out a nuke. (Radioactive material also emits gamma rays, but gamma ray detectors have a tendency to cry wolf. The potassium in a large shipment of bananas might be enough to set off a gamma ray detector; likewise a person who has recently had radiation therapy.) Neutron detectors are tricky to build, though, because even the most radioactive substances only emit a trickle of neutrons. And without any charge, the particles are incredibly difficult to detect. The only way to detect the sprinkling of neutrons from a bomb is to look for the reactions that happen when they collide with other particles. Helium-3 is the best for this because certain properties of the atom give it a higher probability of interacting with a neutron. But helium-3 is incredibly rare in nature. In fact, the only reason scientists were able to get enough to build the few existing helium-3 detectors is because the stuff happens to be a byproduct of regular maintenance on the warheads in hydrogen bombs. Because of the spike in demand for helium-3 detectors after 9/11, “the world basically bottomed out its main neutron detection material,” says Sarah Haig, the co-founder of Silverside Detectors. That’s why her company is trying to make detectors with a less expensive material. That material is lithium-6. “It’s going to convert fewer of the neutrons into a signal, but because the sensor is bigger it will catch more neutrons,” says Haig. Their prototype, called the Lithium Large Area Neutron Detector (LLAND), is built like a double-paned window. Instead of glass, neutrons first encounter a proprietary metallic material that slows neutrons down—a crucial step before they reach the lithium-6 foil stuck to the inside of each pane. Lithium-6 atoms then intercept some of the neutrons, creating a reaction that sends out two other particles. Finally, argon gas between the panes carries the charge from those released particles to wires that are attached to a readout device (it’s Android compatible!), alerting security that somebody is trying to smuggle a nuke. “The neat thing is that the whole thing takes less than a thousandth of a second from when the neutron leaves a vehicle to when it is detected,” says Andrew Inglis, Silverside’s other founder and resident nuclear physicist. At least, that’s the hope. Using lithium-6 to detect neutrons isn’t a new idea—the basic detector concept was developed in the early 1980s—but it’s still an idea that anybody has yet to develop at scale. “The question when you move from research to practical applications is whether anyone can afford to buy it,” says Herschel Workman, whose company ParkTec, Ltd. worked for years on developing lithium-6 detectors for the Department of Energy and Department of Homeland Security. He says by the time his detector was ready for mass production, his company had gone bankrupt and neutron detectors had fallen out of vogue at the federal agencies once poised to be customers. Workman says he’s worried that a similar fate might befall Silverside. Currently, Silverside—currently being funded through a DARPA grant—is still in R&D. Haig says that through the summer they’ll be refining their prototype. “In some ways, the science is done,” she says. “We’re just perfecting the manufacturing.” Like diplomacy, building a nuclear bomb detector takes patience, time, and many trips back to the drawing board.

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