In an uncertain world, how do you make sure that weapons-grade plutonium is not smuggled across borders? Or that rogue nations are complying with nuclear weapons treaties? Or that nuclear waste is not dumped in an unsuspecting country? Geiger counters can be used at border crossings to detect the neutrons emitted by the decay of plutonium 239 and 240, but the devices are bulky and fragile. Transporting them to the world's hot spots can be a problem, as the gas cylinders they require are under too much pressure to be safely carried onboard a plane. To solve this problem, researchers at the Pacific Northwest National Laboratories developed the Puma scintillating detector. This lightweight device contains a sandwich of neutron-sensitive cerium glass fibers layered with lithium. When a neutron strikes this combination, it collides with the lithium and releases a shower of electrons that excite the cerium atoms, causing them to fluoresce and emit photons that can be detected. By using optical fibers (left) instead of high-pressure gas cylinders, the lightweight Puma radiation detector can be safely flown to any of the world's trouble spots. A prototype installed at a test site on the Austro-Hungarian border (right) searches for smuggled plutonium. This approach had been attempted previously, said project leader Mary Bliss, but lithium-doped glass was difficult to work with. Lithium and silica don't want to mix -- what she described as "the old oil-and-water problem." Termed "liquid-liquid immiscibility," this phenomenon results in glass that looks like mayonnaise because of the difference in light refraction between the two elements. But by using a cerium-lithium combination, researchers produced glass of sufficient quality and consistency to be drawn into fibers. Scientists also had to come up with a manufacturing process that would ensure that the vital photon-emitting version of cerium, Ce3+, did not oxidize into the photon-absorbing ion Ce4+ -- which would prevent the detector from working. A prototype of the technology is currently in use at the Austro-Hungarian border by the International Atomic Energy Agency. The detector, licensed for commercial development by Nucsafe of Clinton, Tenn., has not found any plutonium yet, but it did pick up the rays emitted by a bus passenger who had recently undergone radiation therapy. "There aren't many options available to people looking for neutrons," Bliss said, citing the need for instruments that can detect emissions other than gamma rays. "If a sophomore at Princeton can put together the rudiments of a nuclear weapon, then anyone with a background in physics can figure out some way to make a nasty bomb. This is just about the only way to prevent these devices from getting out."