Sound gun to track down dangerous chemicals
how does one identify the contents of an unmarked drum? Several researchers have tried to do so, but it has never been easy. Either the process is cumbersome, unaffordable or unreliable. Now a team at the Los Alamos National Laboratory in New Mexico has come up with a "sound gun" that will easily tell the difference between a fuel drum and a barrel of nerve gas.
The team says the 'gun' can tell the difference between more than 100 chemicals in about 30 seconds from a distance of three metres. The 'gun' exploits the ability of ultrasound to form narrow beams. As the frequency of sound increases, the angle it spreads over decreases allowing the beam to be "fired" at a particular drum. But the ultrasound does not actually reveal the contents of the drum.
Says Dipen Sinha, lead researcher of the team, "The trick 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 resonance has been measured, the device checks the values against a library to see if it can find a match.
The resonance of the container depends on what is stored inside. 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 liquid inside," says Sinha ( New Scientist , Vol 164, No 2210).
The next step for the team is to miniaturise the 'gun'. And if that can be done, the device can also be used by fire-fighters to identify potentially hazardous chemicals in blazing buildings.
However, 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.
At the moment, the most reliable way of identifying contents of unmarked steel containers is to use a portable but cumbersome instrument called pins to fire a stream of neutrons into them. These pass through the steel easily and excite the atoms of the chemicals inside it, producing gamma rays characteristic of those chemicals.
The researchers reckon that the new system has several advantages. It is hand-held, cheaper than pins ; can identify the contents of a drum in a stack and no special protection is needed when using it -- unlike pins whose operators must be protected from the radioactive neutron source.
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