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Spectroscopy Offers Hope in Land-Mine Detection

Photonics Spectra
Apr 2001
Michael D. Wheeler

From Bosnia to Angola to North Korea, the threat to civilians of injury or death by unexploded land mines is very real. Yet for all of the sophistication of 21st-century warfare, land-mine detection continues to depend on a relatively primitive tool: the metal detector. Now there may be an alternative, a portable surface-enhanced Raman spectroscopy unit that detects the explosives themselves rather than their metal casings.

Metal detectors effectively locate mines, but are equally good at ferreting out shrapnel and discarded metal, which they cannot differentiate from the ordnance. This results in needless excavations. The appeal of vibrational spectroscopy is its ability to reduce the number of false positives, since it investigates the bond structures unique to particular molecules. The challenge to Raman spectroscopy is that it lacks sensitivity.

A portable surface-enhanced Raman spectroscopy system developed at EIC Laboratories Inc. promises to refine the detection of unexploded land mines. Courtesy of Kevin M. Spencer.

In surface-enhanced Raman spectroscopy, however, when an analyte adsorbs to a roughened metal surface, the surface electromagnetic wave overlaps with that of the interrogating laser, amplifying the Raman signal by 100 to 1014. Trace amounts of explosives escape from their casings into the surrounding soil. Therefore, researchers at EIC Laboratories Inc. theorized that, by using a substrate that selectively adsorbs these molecules, they could create a spectrometer that would sniff out land mines.

Portability is key

The first step in putting the theory into practice was to assemble components that would provide accurate and reproducible results and that also would be portable. "Part of this program is to create something that can be used by a Marine for several consecutive hours in the field," explained lead researcher Kevin M. Spencer, co-author of a paper that appeared in the Dec. 1, 2000, issue of Analytical Chemistry, reporting this improved detection technique. "Realistically, the system could not weigh more than 20 lb, exclusive of batteries."

A near-IR diode laser was the best candidate for the Raman source, and the team selected a 785-nm laser to maximize the response of the CCD monitoring the spectrograph. Another design consideration was incorporating fans that would provide rapid cooling for the laser and CCD but that would not compromise the system's integrity against the elements and background stray light.

In blind field tests, the unit accurately identified the spectra of TNT and its impurities 2,4-DNT and 1,3-DNB, but further refinements are necessary before it can be deployed. "As a land-mine sensor for use in Bosnia, Angola, etc., the sensor is still a ways off," Spencer said. "Considering the landmass that needs to be swept and the desired sweep rates, the sensor would need to collect samples in less than a second."

In the meantime, the technique is being evaluated for use in other applications, such as monitoring water supplies for chemical or biological warfare agents, in which samples can be taken once every 10 minutes.

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