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CAR BOMB DETECTOR, under development at HiEnergy Technologies, Inc., irradiates the trunk of a car with fast neutrons, revealing the chemical signatures of its contents.
COURTESY OF HiENERGY TECHNOLOGIES, INC.

The drive into the parking lot is an all-too-familar ritual for this regular visitor to this rather nondescript government building. The driver rolls down the window and presses a red button that instantly spews out a parking ticket. But the red button activates more than just a ticket printer. Underneath the car a device quickly bathes the car's trunk in invisible neutrons, a procedure that makes materials inside the trunk emit gamma-rays. Certain gamma ray frequencies correspond to the chemical signature of explosives. If there is a match, security forces will intercept the vehicle.

Our visitor parks the car and walks through the building's doors, oblivious to the sudden blast of air that propels molecules from his clothes and skin to an overhead array of tiny electronic sniffers that can identify trace amounts of explosives from even a tiny bomb concealed on his person. Ahead are the x-ray machine and metal detectors that have become fixtures at airports and other key facilities. His briefcase is scanned, and, accustomed to the high rate of false positives, he waits for its inevitable opening. But this x-ray machine has been augmented with x-ray diffraction technology that increases the machine's accuracy. The briefcase thus moves on through, untouched by human hands. That's not the end of the line, however. Our visitor subsequently passes through a portal that employs a technology called quadruple resonance (QR). The QR device transmits low-intensity radio waves that momentarily disturb the nuclei of various materials carried in his pockets. As the nuclei right themselves, they emit a radio signal of their own--call it an echo. That echo is picked up by a receiver, and its signature is instantly compared against a database for explosives. Cleared, our visitor continues to his appointment after being delayed only a few minutes by the security check.

In the post-9/11 world and especially in the wake of the March 11 terrorist train bombings in Madrid, Spain, bomb detection has a higher-than-ever priority. Airport screening with x-ray machines and specially trained dogs is common; now other transportation modes are also being examined for their vulnerability. A train station in suburban Maryland, for example, is currently serving as a testbed for the Transportation Security Administration (TSA), which will research what bomb detection methods and technologies are most effective for railroads.

But as the scenario described above suggests, there is no one silver bullet that can be used to find bombs. People will be scanned, sniffed and zapped as they pass through a variety of detectors. "All techniques have weakness," says John Parmeter, a bomb detection specialist at Sandia National Laboratories in Albuquerque, N.M. "The ideal system of the future is probably going to include multiple technologies for detection."

Whatever the technology employed, bomb detection is an intimate and dangerous business, primarily because explosive vapors are hard to discern and the devices have a limited range of sensitivity. These technologies also have another thing common: they are expensive. X-ray machines currently used at airports can cost as much as $1.5 million each, and busy airports, such as San Francisco International, can employ as many as 40, according to Sergio Magistri, chief executive officer of InVision Technologies, which along with L3 Communications, manufactures the TSA-certified devices. These machines can generate a 3-D image--using a process called computed tomography--of a bag's contents that allows an operator to identify suspicious objects. Unfortunately, harmless items such as a jar of peanut butter or a fruitcake appear suspicious as well, generating double-digit rates of false positives and contributing to the long delays travelers experience. (InVision was recently acquired by General Electric in a deal that will position GE as a security juggernaut, thanks to related acquisitions in the field.)