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- Acoustic trickery can spot dangerous chemicals in sealed
drums
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- It isn't 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.
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- 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.
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- 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. See diagram
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- 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.
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- 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.
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- 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.
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- "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.
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- 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.
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- 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.
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- 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.
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- 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," he warns.
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