28 February 2000

Itty Bitty Bang Bang

by Kate Melville

The US Department of Energy's Oak Ridge National Laboratory (ORNL) has built a new tool that can detect tiny explosions. While this may sound pretty boring it may lead to new hand-held devices that could be used to check people and luggage at airports and or to detect land mines.

The breakthrough came through the development at ORNL of miniature micro-machined silicon cantilevers that can detect very small forces caused by a heat-induced nano-explosion. At only ten percent of the width of a human hair, the instrument is sensitive to parts-per-trillion and works by soaking up TNT molecules given off by explosives. The semiconductor absorbs TNT which is then heated by a small battery; the TNT molecules then produce miniature explosions which are identified by an optical beam.

The exceptional sensitivity of this device comes from its sensor which is micro-machined from single-crystal silicon and can be heated quickly to a very high temperature. What happens is that when the temperature cantilever is increased (with battery current) to 575 degrees Celsius, the adsorbed TNT molecules explode.

According to lead researcher Thomas Thundat, of the ORNL Life Sciences Division, this new tool could make a huge difference to locating and remove unexploded land mines (an issue given international prominence after the involvement of the late Princess Dianna). "These dangerous remnants of often forgotten wars kill 26,000 innocent civilians a year. Countless more people are maimed each year by the more than 110 million mines scattered over 64 countries."

While are many other technologies and techniques already exist to detect land mines and explosives, nano-explosions have several advantages, including high sensitivity and low cost and.

"Because the thermal mass of cantilevers is small, we can heat them to more than 700 degrees Celsius in a millisecond with just a micro-watt of power," Thundat said.

Measurement of the explosion is done via a diode laser beam a bit like those found in a CD player only about 1,000 times more sensitive. This high sensitivity laser detects vibrations caused by the tiny explosions. By then scanning an appropriate temperature scale, researchers have used this device to detect explosive molecules, including those used in plastic explosives.

So what's next? For those of us who suffer airport security it may mean marginally greater safety, but the real gain will be if this breakthrough can help rid the world of land mines that infest so many 3rd World countries.