WASHINGTON—Government and private researchers have developed a wide variety of technologies to detect explosives used by terrorists. But the systems are imperfect and can't guarantee that deadly devices won't slip through.
The drastic step of banning liquids from airline carry-on baggage after the exposure of the alleged London-based bomb plot provides vivid evidence of the weaknesses of existing detection methods.
The systems in use or being developed include more powerful X-ray and laser screeners, chemical "sniffers" that sense molecules given off by explosives, and machines that use extremely high frequency radiation or subatomic particles to identify suspicious substances.
Even state-of-the-art systems suffer from false positives—detecting nonexistent explosives—or false negatives, which miss real threats. They can be fooled by background clutter or strong odors, such as garlic and mint. They're subject to human error. What works well in a laboratory may fail miserably in the field.
"All systems have weaknesses," said John Parmeter, a bomb-detector expert at Sandia National Laboratories in Albuquerque, N.M.
One of the most reliable systems remains the sensitive nose of a dog that's trained to react to certain chemicals. However, dogs require special handling and tire easily. So a decade ago, the Defense Advanced Research Projects Agency at the Pentagon launched a "Dog's Nose" program to support research into devices that could equal or surpass a dog's abilities.
One such effort, the MicroHound sniffer, which Sandia developed, is a 12-pound box that pulls in air and traps chemical particles on a filter, where they can be analyzed.
Washington police used a similar hand-held device—the Fido Explosives Detector, from ICX/Nomadics, of Stillwater, Okla.—to protect visitors to the National Mall on July Fourth.
A system being deployed at many airports is the "puffer machine," a walk-through portal that squirts gentle puffs of air at passengers. The air rises and carries microscopic particles from clothing or skin to an overhead collection plate. Then an Ion Mobility Spectrometer device zaps the particles with a electric current, driving them toward a detector. Some particles travel faster than others, and their relative speed tells operators whether an explosive element is present.
GE Security, of Bradenton, Fla., sells an EntryScan IMS system that it claims can detect vapors from one of the most difficult-to-uncover explosives: the hydrogen peroxide-based liquid that suicide bombers favor and that was used in the London subway bombings in July 2005.
Implant Sciences Corp., of Wakefield, Mass., uses ion technology in a hand-held explosives detector it calls Quantum Sniffer. The 40-pound device is held close to an object to identify its contents. Machines have been sold to China and Europe as well as to the U.S. government.
A process called Nuclear Quadrupole Resonance scans people or objects with low-frequency radio waves to identify the molecular structure of a suspect substance. The waves produce an echo that gives a unique signal for each chemical element.
A suspicious target also can be bombarded with subatomic particles called neutrons. When a neutron strikes an atom, it gives off a distinctive gamma ray, a stream of high-energy radiation that identifies the atom.
HiEnergy Technologies, of Irvine, Calif., is developing such a neutron-gamma ray detector for the Army. It could identity a bomb hidden in the trunk of a car or an improvised explosive device along a highway in Iraq.
Terahertz imaging, a more powerful form of the X-ray screening that's standard at airport terminals, is being developed for the Defense Advanced Research Projects Agency by the New Jersey Institute of Technology, in Newark.
Terahertz is very high frequency radiation—trillions of cycles per second—that penetrates clothing, shoes, backpacks and suitcases to spot explosives and other chemical agents. It can't see through metal, however.
A far-out device, sometimes dubbed an "artificial nose," is being developed at the Oak Ridge National Laboratory in Tennessee. It uses an array of miniature springboards, called cantilevers, to measure the weight of a bunch of molecules. Different sets of molecules bend the cantilever in an identifiable pattern, which a computer matches against a database of known explosives.
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(c) 2006, McClatchy-Tribune Information Services.
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