You can hold it between your thumb and forefinger. It's grayish-green, made of plastic and rubber, and shaped like a small jar of facial cream. It's buried in such areas as Angola, Southeast Asia, and Bosnia, and if you step over this tiny object, it will literally knock your socks off, as well as all of your toes.
Type 72, commonly called a toe-popper, is an antipersonnel land mine. Because the only metal it contains is a small firing pin, it's virtually undetectable by current technologies. Made in China, Type 72 costs $3 a pop.
The toe-popper is only one type of plastic land mine. Of the 675 varieties (totaling 110 million mines) known to be buried around the world, 290 are plastic—and so invisible to metal detectors. A good portion of the remaining 385 contain only small amounts of metal, are comparable in size to Type 72, and are as difficult to detect. With support from the U.S. Army, Jeffrey Schiano, an assistant professor of electrical engineering at Penn State, is working on a device that could detect such bombs.
In the late 1960s, Robert Marino, a molecular physicist, was contacted by the Army. The Vietnamese were taking U.S. land mines, removing them from their metal casings, and reburying them in plastic bags. The Army had no way of finding the explosives. Marino had been researching the unique shape and behavior of the nitrogen nucleus in sodium nitrite, a hot dog preservative. As nitrogen is also found in TNT, the Army wondered if Marino's research could apply to explosives. It did. But the war ended, funding dwindled, and Marino set this application aside. Then in 1988, Pan Am Flight 103 exploded above Lockerbie, Scotland. Marino's research suddenly became important again. The bomb had passed through airport metal detectors and x-ray machines. "The explosive probably had no case," says Schiano. "It was simply put into a bag or a briefcase."
By the early '90s, a commercial device based on Marino's research was put to work searching luggage in airports, but the machine produced a lot of false alarms. Since 1996, Schiano has been working on this glitch and other problems of such bomb-finding devices. "Marino's research relied on the idea that nitrogen has a football-shaped nucleus with a uniformly distributed positive charge centered within the atom," Schiano explains. In some nitrogen-containing materials, the negatively charged electron cloud that surrounds the positively charged nucleus is spherical, like a basketball. In a spherical electron cloud, the nucleus has no "preferred state." It can somersault, spin around like a top, or rotate like the hands on a watch. But in explosive materials (and hot dog preservatives) the electron cloud is not spherical. "The positive charges in the nucleus want to get as close as they can to the negative charges in the electron cloud," says Schiano. So the nucleus orients itself to minimize that distance.
The nitrogen nucleus also has a north and south magnetic pole. If you create a magnetic field (by passing a current through an electric coil) near the nitrogen atom, the nucleus will rotate toward whichever direction has now become magnetic north. "Like the needle on a compass, the nucleus wants to face north," explains Schiano. If you remove that magnetic field, the nucleus will flop back to its preferred state. But like a pendulum, it will sway back and forth a while before coming to rest.
Listen. This swaying motion creates a sound called a nuclear quadrupole resonance (NQR) signal. Though short-lived, lasting perhaps a thousandth of a second, it can be heard. And when the magnetic field is turned on and off repeatedly, the weak signal put out by the nitrogen atoms in the explosive can be averaged and measured. With the use of this signal, detecting a nonmetallic explosive is possible, even when the bomb is as small as a toe-popper.
But there are problems. Schiano offers a rectangular plastic box, containing a whitish powder. "If this were TNT, which is what most bombs are made of," he says, "it would blow up this lab, along with a good fraction of the building." Schiano has never been able to test TNT. Though he may soon get a small sample, his research has relied on the nitrogen compound found in hot dog preservatives instead of the one in explosives. That's minor, however, compared to the other hurdles he's facing.
First, the signals emitted by nitrogen compounds, especially TNT, decay very rapidly. Add the fact that the strength of the signal depends on the distance from the detector, and listening and averaging can be very difficult. Problem number two, which explains all those false alarms, is piezoelectricity. When an electrical field is applied to certain materials like quartz, the material expands and contracts in different directions, much in the same way a balloon does when pressure is applied and released from both sides. The mechanical compression causes the material to produce a voltage, like the one made by a phonograph needle.
This same property causes piezo-electric materials to produce a burst of noise when exposed to a strong electromagnetic pulse. The signal is like the one produced by the swaying nitrogen nucleus, only stronger. And it can come from the bomb itself, or, as Schiano has discovered, from the soil, especially if the bomb is buried in sand, which contains quartz.
Imagine you're trying to tune into a radio station. You hear your favorite song, but it's being interrupted by another song. You move the tuner a hairs-breadth of a turn, first left, then right. When you go one way, it seems your song is tuning in, but as you try to escape the static, the other tune claims the radio waves. Piezoelectricity affects TNT's signal the same way the interference ruins your favorite song—by overlapping it. The result? A false alarm.
With the help of a three-by-two-foot plastic sandbox and a custom-built spectrometer, Schiano is trying to overcome these problems and perfect the device Marino envisioned 30 years ago. Using the sandbox as a simulated mine site, he buries plastic-encased sodium nitrite (hot dog preservatives). While he holds the device over the buried "bomb," the spectrometer automatically adjusts the applied magnetic field in order to obtain the strongest signal.
The detection device itself consists of a snaking silver coil imbedded in a yellow fiberglass base, two blue metallic boxes, and a length of thick plastic that connects the boxes to the base and serves as a handle. A current coursing through the coil creates the magnetic field. By altering the duration, repetition rate, and frequency of the applied signals, Schiano is hoping to come up with a set of algorithms, or mathematical rules, to compensate for the unknown depth of the bomb and limit the piezo-electric effect.
When he is finished, the components of Schiano's laboratory will fit in a single box. Explosives hunters will carry the miniature lab in a bag slung over their shoulders. Turn on the box, and a magnetic field is created. Pass the box over the soil and that magnetic energy tugs and releases the nitrogen nuclei in the TNT. Maybe the signal is very small because the bomb is embedded deep within the soil. Or perhaps the signal is being masked by piezoelectricity. Schiano's feedback algorithms will compensate automatically. The device will either quicken or relax the time that elapses between the creation and collapse of the magnetic field, as well as increase or decrease the repetition rate of the signal. In this way, the device will be able to obtain the clearest response from the explosive and, at the same time, establish its location.
That is, if indeed there's a bomb to signal back.
Jeffrey L. Schiano, Ph.D., is assistant professor of electrical engineering in the College of Engineering, 227D EE West, University Park, PA 16802; 814-865- 5422; schiano@steinmetz.ee.psu.edu. His work is supported by the United States Army Construction Engineering Research Laboratories.