Inside story

By Lila Guterman A RUSTY warhead can conceal all manner of hazards. Does it contain explosives? Nerve gas? If the label is damaged or missing, it’s hard to know what’s inside without taking the dangerous step of drilling a hole in the warhead. Now American researchers have developed a portable device that can quickly reveal the contents of a warhead without anyone having to open it up or move it to a lab. The system will be used in the US’s chemical weapons disposal programme. It could also form the basis of new landmine detectors or airport baggage scanners. In 1988, people living in the Solomon Islands found American warheads dating back to the Second World War. Whatever was inside the projectiles sloshed around, suggesting it might be a chemical warfare agent rather than high explosive. “Back then, disposing of chemical weapons meant digging a hole in the ground and throwing old munitions in,” says Gus Caffrey, a physicist at Idaho National Engineering and Environmental Laboratory near Idaho Falls. Nowadays, chemical weapons have to be incinerated or destroyed chemically. “The safe and lawful disposal of a munition in the US requires knowledge of its contents,” Caffrey says. An explosive warhead can damage an incinerator, while arsenic-containing munitions are not permitted in incinerators. To find out what warheads contain, Caffrey and his colleagues developed a system they call portable isotopic neutron spectroscopy (PINS). It sprays neutrons from a lump of the radioactive element californium into a warhead and records the signature gamma-ray emission from the contents or “fill”. “The neutrons easily penetrate the steel wall of a munition and excite the atomic nuclei inside,” Caffrey says. The energies of the gamma rays emitted by these excited nuclei reveal which elements are in the warhead. “PINS allows the presence of chemical elements to be determined and thus chemical fills identified,” says Margaret Tout of Britain’s chemical and biological weapons research unit at Porton Down, Hampshire, which has recently bought a PINS instrument. “For example, high explosives contain large amounts of nitrogen whereas mustard gas contains chlorine and sulphur.” Analysing a warhead takes just 18 minutes on average. The only precautions needed are to protect the operators against radioactivity. “You minimise your time near the instrument,” Caffrey says. “You set up the computer and electronics say 20 metres away from the radioactive source and detector.” Each PINS instrument costs around $120 000 and is simple to operate. Caffrey’s team has just used PINS to analyse the 109 warheads from the Solomon Islands and discovered that most of them contained mustard gas. Future plans for the PINS technology include making it faster and smaller, says Caffrey. It may be used by police forces or airport security to detect explosives in suspicious packages, which are hard to detect in X-ray imagers. “Explosives do not have an obvious sinister shape,” he says. “By adding the neutron interrogation,
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