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Devices Detect Explosive Residue

Photonics Spectra
Jul 1998
Kevin Robinson

To evolve low-cost, portable land mine detection technology, researchers at Duke University have developed microelectromechanical devices that could become a two-part detection system for explosives. The first device uses ultrasound to loosen the explosive particles; the second detects them. A research group led by electrical and computer engineering professor Richard Fair designed the devices, which the team hopes to test on explosives by the end of summer.

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"If you have mines in a field," Fair said, "TNT will be absorbed into the ecosystem, even on the leaves of trees." The surface of the ground above buried mines has concentrations of explosives in the parts-per-million or parts-per-billion range, he added.

Using a micromachined device with an array of 2000 silicon nitride membranes, the researchers built a device that focuses ultrasound energy onto the ground.

Fair plans to use the ultrasound energy to loosen the explosive particles from the ground. From there, the particles can be vacuumed for detection. This method is less time-consuming than sampling the air, where the concentrations can be as low as parts per trillion.

Once the particles are loosened, the second device detects them. This instrument, also micromachined out of silicon, consists of an array of cantilevered arms made of polycrystalline silicon and gold. Because the silicon and the gold have different thermal coefficients, the cantilevers bend as they absorb heat. The gold, however, makes them reflective to most visible wavelengths.

Fair plans to scan a laser tuned to the absorption of the explosive over the cantilever. If there are no particles present, the cantilever will not bend because it will reflect the laser's energy. If an explosive particle is present, the laser will cause it either to explode or decay, heating the cantilever and inducing it to bend. In addition, the researchers will detect how much the cantilever bends by using a HeNe laser and a position-sensitive detector. For the field version of the device, Fair plans to use electronics to detect changes in the cantilever's capacitance.

Fair said the technology would be used as part of a mine-detection system that would incorporate other technology such as ground-penetrating radar or acoustic methods. A portable bomb-detection system using the devices also could be used in airports. "If you have parts per billion on any surface, this technique could be useful," he added.

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