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Keep your distance

作者:曹胭    发布时间:2019-03-07 02:02:01    

By Ian Sample IT IS not easy to tell the difference between an ordinary fuel drum and a barrel of nerve gas. But a “sound gun” built by a team at the Los Alamos National Laboratory in New Mexico could make the job of weapons inspectors much easier. The team says the gun can tell the difference between more than a hundred chemicals in about 30 seconds, from a distance of 3 metres. They will reveal the details of their invention at a meeting of the Acoustical Society of America in Columbus, Ohio, next week. At the moment, the most reliable way of identifying the contents of unmarked steel containers is to use a portable but cumbersome instrument called PINS to fire a stream of neutrons into them (New Scientist, 31 October 1998, p 16). These pass through the steel easily and excite the atoms of the chemicals inside it, producing gamma rays characteristic of those chemicals. A cheaper method, which was also devised by researchers at Los Alamos, uses a special drill bit that can extract a sample of the chemical and seal the container without any of the contents leaking out (New Scientist, 19/26 December 1998, p 10). But both methods have disadvantages. The PINS source must be placed within a centimetre of the target drum, while the drill must of course touch it—and if noxious substances do leak out, the drill operator could be in trouble. The researchers reckon that the new system has several advantages: it is handheld, it is cheaper than the PINS machine, it can identify the contents of a drum in a stack and no special protection is needed when using it—unlike the PINS, whose operators must be protected from the radioactive neutron source. The sound gun exploits the ability of ultrasound to form narrow beams: as the frequency of sound increases, the angle it spreads out over decreases, allowing the beam to be “fired” at a particular drum. But the ultrasound does not actually reveal what is in the drum. “The trick in this case,” says Dipen Sinha, lead researcher of the team, “is to send a low-frequency signal that rides on a high-frequency carrier wave.” He superimposed a low-frequency 15-kilohertz sound on top of a 217-kilohertz ultrasound beam. This low-frequency sound makes a container resonate. The resonance is measured by bouncing a laser beam off the vibrating drum. The beam’s frequency is slightly shifted depending on the vibrational frequency of the drum. Once the resonance has been measured, the device checks the values against a library to see if it can find a match. Sinha has shown that the resonance of a container depends on what is stored inside it. The speed of the sound, and the way the sound is weakened in strength by a material of a given density and viscosity, all affect the laser spectrum in specific ways. “When all these factors are taken into account, one has a very good idea of the liquid inside,” he explained. The next step is to miniaturise the device. If that can be done, the sound gun could also be used by firefighters to identify potentially hazardous chemicals in blazing buildings. But Nicholas Davies of Britain’s chemical weapons centre at Porton Down, Wiltshire, has doubts about the accuracy of the sound gun. “Containers vary in their construction and material, and this will lead to some confusion during the interpretation,

 

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