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High-Tech Detection

While near-term technological improvements offer hope for better demining efficiency, technologies undergoing vigorous research and development for use against airline terrorism offer even more promise for the future. Portable, rugged versions of these technologies, which detect small amounts of explosives, would be required for use in demining, but the task is certainly not beyond the capabilities of high-tech firms in the United States and elsewhere.

These technologies could take advantage of the fact that landmines use characteristic materials in well-defined shapes and sizes, giving them mechanical, acoustic, electromagnetic, and nuclear absorption and reflection properties potentially detectable from a modest distance. All mines contain high explosives, substances otherwise rare in the soil, and are thus open to many means of detection based on their chemical composition.

Such chemical sensing is perhaps the most advanced of these avenues. Since all mines contain 10 grams or more of explosives, one way to avoid the time-consuming step of discriminating mines from false alarms, and to detect plastic as well as metallic mines, is to devise detectors sensitive to the presence of explosives, either in their condensed or vapor phase. We know that mines carry traces of their explosives because dogs trained to scent high explosives can detect buried mines under field conditions in a short time, with a 95 percent success rate and a false alarm rate of around two to one. Unfortunately, though, dogs tire easily and are expensive to train and keep. Arrays of sensors, each with some specificity to a particular molecule or compound, are quite commonly used in the food and perfume industries to identify products’ constituent compounds. The U.S. Defense Advanced Research Projects Agency is actively pursuing an array of such sensors intended for explosives detection at airports that could well be adaptable for humanitarian demining.

One detector already in trial use at airports pulls an air sample through to a collector that transfers any minute traces of explosives to a separation device. There, an instrument called a high-speed gas chromatograph separates explosives from one another and from non-explosive compounds by the length of time it takes each compound to go through the instrument. Each compound yields a reliable and characteristic signature. Noting both this signature time and its amplitude, the detector can determine the type of explosive and the level of its concentration in the air sample. The manufacturer, Thermetics Detection based in Woburn, Mass., claims that its system can detect the presence of 10 to 20 picograms of TNT-a grain twice the size of a speck of dust-with a thousand times the sensitivity of a dog. The system is capable of detecting picogram levels of explosives in less than a minute, and has worked well in the presence of potentially interfering compounds in the air or the soil.

Company representatives believe that a single portable, battery-powered detector could detect mines with greater than 90 percent accuracy while scanning ten square yards per minute. What is not known yet is to what degree high-explosive vapor and particles deposited by past weapons firing in the areas where mines are buried might generate an unmanageably high level of background noise. Detailed field measurements at the sites of past combat, as well as of background levels in battle-free and mine-free areas, must be conducted before the practicality of this potential mine detector can be fully determined.

At least two other technologies could potentially be used to detect mines by sensing their main charges. The first is based on nuclear quadrupole resonance (NQR), an electrostatic relative of magnetic resonance imaging now familiar in the medical world. NQR is an effect displayed by atomic nuclei that are not spherically symmetrical but slightly squashed or elongated at the poles. Nitrogen atoms, a near-universal primary ingredient of high explosives, possess just such nuclear asymmetry. Depending on what kind of crystalline structure the nitrogen nuclei find themselves in, their non-sphericity produces a unique set of very narrowly spaced energy levels that is characteristic of the crystalline solid itself. An explosive compound can therefore be identified by the subtle resonance of its constituent nitrogen atoms.

NQR detectors have already been tested in airports, where they have managed, within six seconds, to detect the military explosive RDX in quantities comparable to those in a mine. Tests at the Naval Research Laboratory based in Alexandria, Va., have shown that NQR detectors, unaffected by soil contaminants like metals and magnets, can reliably discern explosives from other nitrogenous materials in the soil such as fertilizer or living organisms. A field NQR detector would operate much like a hand-held metal detector but would require a backpack to accommodate its larger battery. NQR commercial developer Quantum Magnetics of San Diego estimates that a prototype mine detector based on NQR could be developed within two years at a cost of about $1 million. The price of such detectors, once produced in quantities of several thousand, they believe, would probably be under $10,000 each-some two to three times more than the cost of high-quality metal detectors. With an adequate level of development funding, it is quite possible that NQR could become an effective tool for discriminating mines from metal clutter within 3 to 5 years, reducing the false alarm rate to negligible levels.

The technology does pose some difficulties at present, however. The dominant obstacle is the efficient detection of TNT, the explosive ingredient of 80 percent of landmines. TNT has an intrinsically weak NQR signal, requiring a longer integration time in the detector. An NQR mine detector that had to linger over each spot on the ground for minutes at a time would clearly be too slow, although it could still presumably prove useful in distinguishing mines from scrap metal.

A second way to detect plastic mines by their explosive content is to “illuminate” the ground with a beam of low-energy x-rays. Because of the difference in their average atomic numbers, soil will absorb low-energy x-rays impinging upon it, while the lighter mine will “backscatter” a large fraction of the incoming radiation. When imaged, the mine thus appears as a luminous spot on a dark background of soil. Experiments conducted as early as 1975 by the U.S. Army Mobility Equipment Research and Development Center showed that, while awkward, clumsy, and dangerous at the time, the method does in fact work, unambiguously detecting small (six centimeters in diameter) plastic mines buried under two centimeters of soil.

Although x-ray backscattering detectors perform well in detecting explosives and other materials with low atomic numbers at airports and customs stations, they have shortcomings for detecting plastic mines: they cannot reliably discriminate explosives from other materials with similar atomic numbers (such as roots and water), they detect only shallow-buried mines, and they require an intense source of ionizing radiation that could cause health hazards to the operator. A hand-held detector may therefore not prove practical, but x-ray backscattering detectors might eventually be used on remotely controlled demining vehicles to detect plastic mines in conjunction with a metal detector such as the meandering winding magnetometer.

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