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.)

Smoother inspections may be on the horizon: U.S. airports are adopting a supplemental x-ray diffraction technology already being used in Europe that is more accurate and can detect smaller amounts of potentially explosive material. Together with the existing x-ray devices, it should reduce false alarms to a single-digit rate so that less intervention is required. "Humans are the weakest part of the security chain," Magistri observes. "We want to get humans out of it." Although each additional x-ray diffraction machine costs $1 million, Magistri says the machine will pay for itself within two years because fewer operators are required. With a low rate of false positives, he envisions these x-ray baggage checkers linked in a network manned by a single operator. The first x-ray diffraction units will start appearing in U.S. airports by the end of the year, he reports. Elsewhere, especially at border crossings, security experts are experimenting with a higher-powered backscatter x-ray for examining cargo.

But x-ray technologies borrowed from medical imaging research are not the ultimate answer, says Bogdan Maglich, CEO and chief scientist of HiEnergy Technologies, in Irvine, Calif. "X-ray detectors are chemically blind," Maglich comments. "They are unable to detect explosives. They are not chemical specific. They only detect possible explosives."

HiEnergy is developing a car bomb detector that it hopes to make available to customers like Spain and the U.S. Army by year's end at a cost of about $165,000 each. When installed at a checkpoint--say, the entrance to a parking garage--the device irradiates the trunk of the car with fast neutrons, which cause objects within the trunk to emit gamma rays. The signatures of certain gamma rays correspond to the chemical signatures of various explosives. Gamma-ray detectors have become technically practical to develop only in the past few years, Maglich says. Even so, the range of the HiEnergy CarBomb Finder is limited to just a few feet, and its level of sensitivity makes it most effective against large bombs transported in vehicles rather than the smaller explosive packages that suicide bombers might carry, for example. Further constraining the utility of this device is the fact that water can slow the fast neutrons, lengthening the time needed for analysis from seconds to minutes. HiEnergy also makes a van-mounted version of its car bomb detector that can be used to examine suspect vehicles on an as-needed basis.

Another more chemically astute alternative to x-rays is the quadruple resonance technology that zaps people with low-frequency radio waves. Still in development, most notably by an Invision subsidiary, QR would be used mostly to scan passengers and carry-on luggage. But like other technologies, QR has its limitations: it may be more sensitive to some explosives, such as those used by the military, than others.

The restricted range of bomb detection technologies of any stripe is the driving force behind research into electronic sniffers at Oak Ridge National Laboratory (ORNL) in Tennessee. Existing trace detection is effectively limited to ion mobility spectrometers (IMS) that isolate and identify negative ion particles associated with explosives that may be emanating from a person or object as they pass through a portal. (The Transportation Security Administration is now testing a "ticket licker" that would do the same to airline tickets.) ORNL scientists are developing a MEMS-based electronic nose smaller than the head of a pin that would work much like a dog's nose to sniff out explosives. According to Panos Datskos, one of the ORNL scientists working on the project, the sensor is actually an array of tiny cantilevers, each made of different materials and each uniquely coated. When as few as a half-dozen molecules come into contact with the cantilevers, each cantilever deforms differently to create a recognizable pattern while also producing a unique resonant frequency. A computer analyzes the resulting data, matching it against a database of explosives for identification. This methodology can also sense nonexplosive chemical, biological or radioactive weapons, all of which could be combined on a single, inexpensive device. Additionally, the cantilevers can respond to physical stimuli, such as temperature and sound, so they could be used for fire alarms or intruder alerts. The current range for an array of 20 to 30 cantilevers is three to 10 feet, but that range may increase if larger arrays are developed, Datskos reports. ORNL hopes to have a prototype ready in a year's time as work proceeds on such "difficult" issues as false positives.

Another prototype that may be ready within a year is a device that uses terahertz imaging to detect a centimeter-size object from as far away as 50 meters. Under development at the New Jersey Institute of Technology (NJIT), terahertz imaging, which utilizes frequencies just below the visible spectrum, is safer than x-rays for scanning people because it doesn't damage tissue, says NJIT physics professor John Federici, and it lends itself to use as a portable device. It can also assign a color to different types of explosives for quick identification. Terahertz imaging has its limitations, however, in that it cannot see through metal or water and needs to be used in a brightly lit environment.

But even Federici concedes that "the magic bullet doesn't exist" when it comes to bomb detection. All Federici and scientists like him can hope to do is to interject enough layers of detection to make it as difficult as possible for a bomb to slip through.