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X-Rays Mark the Spot for Mines

Photonics Spectra
Nov 2000
Dan Drollette

Finding land mines is slow, tedious and dangerous. Because today's mines are mostly plastic, metal detectors cannot find them. The most reliable detection method requires "de-miners" to get on their hands and knees to probe the ground with a metal rod. Often, they work with mines only 4 to 5 inches from their faces.

Consequently, one person can safely clear only about 5 to 80 square meters per day, depending upon vegetation density, wrote Eddie Banks, a mine-removal expert working for Bosnian authorities. Trained dogs can sniff out mines faster, increasing the pace to between 300 and 1000 square meters daily. But the dogs cost $10,000 each, and can tire or become distracted. Even at that rate, it will take at least 10 years to clear the highest-priority minefields out of an estimated 30,000 minefields in the Balkans. Some Croatian authorities say that removing all mines from their country will take decades.

This may change with a photonic device called Eliminate Landmines Forever, which can clear 500 to 2000 square meters per hour and locate land mines 2 to 3 meters away, diminishing the risk to de-miners.

Chemical fingerprints

Developed by Henry M. Blair of Chemical Detection Technologies Inc. of Tucson, Ariz., it works by searching for the distinctive chemical fingerprints of explosives.

All mines emit nitrates into the soil directly above them. When his device stimulates these nitrates with low-energy x-rays, they characteristically fluoresce in a way that is captured by a specially tweaked, cooled, solid-state germanium crystal detector.

By comparing this spectral fingerprint to the known spectroscopic signatures of common explosives with a fast multichannel analyzer, the technology can find a mine and immediately determine its type.

And because the chemical signatures of the nitrates found in mines are completely different from those found naturally in the soil or in chemical fertilizers, there is no possibility of the device being confused.

The discovery that low-energy x-rays can make nitrates fluoresce upset some theories. What's more, the commonly used figures for absorption spectra were incorrect. "The classic physics textbooks were wrong," Blair said.

He said that there appear to be few limits to the depth at which the system can detect mines, and few time limitations. So far, it has detected mines that have been buried for as long as five years and as recently as 10 minutes. The system was demonstrated in a public showing on the US Capitol grounds Sept. 13, and it has been through preliminary field trials in Croatia.

He's hoping to get more field data and to refine the system's capabilities.

Said Blair, "We need perfect data, because one mistake is a dead person.

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