A RUSTY warhead can conceal all manner of hazards. Does it contain
explosives? Nerve gas? If the label is damaged or missing, it鈥檚 hard to know
what鈥檚 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鈥檚 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. 鈥淏ack then, disposing of chemical weapons meant digging a hole in the
ground and throwing old munitions in,鈥� says Gus Caffrey, a physicist at Idaho
National Engineering and Environmental Laboratory near Idaho Falls.
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Nowadays, chemical weapons have to be incinerated or destroyed chemically.
鈥淭he safe and lawful disposal of a munition in the US requires knowledge of its
contents,鈥� 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 鈥渇ill鈥�.
鈥淭he neutrons easily penetrate the steel wall of a munition and excite the
atomic nuclei inside,鈥� Caffrey says. The energies of the gamma rays emitted by
these excited nuclei reveal which elements are in the warhead. 鈥淧INS allows the
presence of chemical elements to be determined and thus chemical fills
identified,鈥� says Margaret Tout of Britain鈥檚 chemical and biological weapons
research unit at Porton Down, Hampshire, which has recently bought a PINS
instrument. 鈥淔or example, high explosives contain large amounts of nitrogen
whereas mustard gas contains chlorine and sulphur.鈥�
Analysing a warhead takes just 18 minutes on average. The only precautions
needed are to protect the operators against radioactivity. 鈥淵ou minimise your
time near the instrument,鈥� Caffrey says. 鈥淵ou set up the computer and
electronics say 20 metres away from the radioactive source and detector.鈥� Each
PINS instrument costs around $120 000 and is simple to operate. Caffrey鈥檚
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.
鈥淓xplosives do not have an obvious sinister shape,鈥� he says. 鈥淏y adding the
neutron interrogation, you can recognise the chemicals that are specific to
explosives.鈥� It also could be used to determine the contents of drums of
chemical waste or to search for the explosive signatures of landmines.