WASHINGTON, D.C.--A prototype detector akin to magnetic resonance imaging (MRI) machines used in medicine has successfully ferreted out buried landmines, Pentagon researchers announced today at the meeting of the American Association for the Advancement of Science (ScienceNOW's publisher). Such detectors could slash the time, expense, and danger required to find and remove the 100 million landmines strewn across countries such as Cambodia and Bosnia.
A typical "de-miner" walks softly and carries a metal detector and a big stick. Because modern landmines contain only a few small pieces of metal, the detector must be turned up to maximum sensitivity. But then the detector sounds false alarms when it finds bottle caps, nails, shell casings, and other bits of metal. The de-miner must check every alarm by probing the ground gingerly with the stick. "Finding landmines is easy," says physicist Allen Garroway of the U.S. Naval Research Laboratory in Washington, D.C. "Separating the landmines from the false alarms is the problem."
To reduce the number of false alarms, Garroway and his colleagues developed a detector that searches directly for the high explosives packed inside the mines. The detector exploits a phenomenon called nuclear quadrupole resonance, and works a bit like an MRI machine. A metallic coil roughly the size of a Frisbee sends out an electromagnetic pulse tuned to twirl the nuclei of nitrogen atoms in chemical groups that appear in TNT and most other high explosives. The twirling nuclei then produce an electromagnetic echo that can be detected with the same coil.
In tests last fall at Fort Leonard Wood in Missouri, a prototype detector found all 23 antitank mines buried in a test plot with no false alarms, reported Regina Dugan, a program officer with the Defense Advanced Research Projects Administration, which funded the research. The detector also found all seven smaller antipersonnel mines in a separate plot; however, two passes with the device were required to eliminate a handful of false positives.
Both the U.S. Army and the U.S. Marine Corps are working to refine these detectors, which could be deployed in 3 to 5 years, says Dugan. The technique clearly leads the race to detect buried explosives, but it ultimately may prove more cumbersome than other emerging technologies, says Nathan Lewis, a chemist at the California Institute of Technology who is developing an artificial "nose" to sniff out landmines. The detector needs five times as much power as a laptop computer, he says, "So just imagine the battery pack you have to carry on your back."