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See the Evil, Hear the Evil, Smell the Evil

Explosives can be “seen” with imaging equipment that peers through clothing or baggage, “smelled” with instruments that react to vapors or particles, or “heard” with machines that pick up radio-frequency echoes of the material.

Although the densities of explosives-and plastic weapons, for that matter-are too low to be detected by conventional “transmission” x-rays, where a single beam of energy passes through an object, other types of x-ray machines can do the job. American Science and Engineering (AS&E) in Billerica, Mass., and Nicolet Imaging Systems in San Diego make machines that analyze “backscatter” x-rays. Different materials reflect x-rays in different ways. This backscatter is captured by sensors in the walls of a hatchway surrounding a bag or a person. It is then analyzed by software that looks for signatures of lower-density items (a plastic knife, for example) and those of substances with low atomic numbers, such as nitrogen, which are characteristic of explosives. These objects are shown on the operator’s screen. By isolating questionable items, this scheme can thwart the common terrorist tactic of concealing explosives amongst clutter.

AS&E’s machines are installed at numerous prisons, U.S. Customs checkpoints, airports in Europe, and the White House. The 101ZZ, which requires an operator, can scan up to 600 bags an hour, exceeding the FAA throughput standard. But its false-alarm rate is above the FAA limit; it’s difficult to distinguish a roll of plastic explosives from a rolled up magazine or a salami. The machine costs from $80,000 to $120,000; an automated version costs about $300,000.

The darling of the x-ray contenders for screening bags is the CTX-5000, made by InVision Technologies in Foster City, Calif. The computed-tomography machine, similar to the CT scanners used in hospitals, takes cross-sectional slices and combines them into a three-dimensional image. Although its false-alarm rate meets the FAA’s criteria, the system is slow; two machines must operate in parallel to process 450 bags an hour-an expensive configuration at about $1 million a machine. Nevertheless, the FAA has granted certification for systems comprising at least two CTX-5000 machines. In late December the agency signed a $52.2 million contract with InVision to install more than 50 machines at major airports, including Chicago’s O’Hare.

EG&G Astrophysics in Long Beach, Calif., makes a lower-cost variant that takes just two, orthogonal slices. While better than a standard transmission x-ray photo, the image is less refined than that of the CTX-5000.

Vivid Technologies in Woburn, Mass., uses x-rays of two different energies, each absorbed most strongly by materials of a different density. By comparing the relative attenuation of the two beams, the system can distinguish objects from background clutter, and also determine a mean atomic number for a given object delineated in the image. If, for example, the system spies a mysterious brick, it can give an average number for the entire object (which may consist of several different materials). It then compares this number to those of explosives. If there is a correlation, a guard opens the bag to see whether the brick is a chunk of explosive or a block of cheese. The difficulty is that the atomic numbers of the 12 or more common explosives, including TNT, nitroglycerin, and RDX and PETN plastics, are similar to those for common compounds, from foodstuffs to books, so again false alarms are high. Vivid’s machines cost from $250,000 to $400,000.

Another unit, made by Millitech Corp. in South Deerfield, Mass., exploits the fact that all objects not at absolute zero emit electromagnetic energy. The scanner differentiates objects by analyzing their emission patterns in the extremely high frequency range (near 100 gigahertz). Interpreting the rather grainy images requires highly trained operators, however. The benefit is that passengers are not exposed to x-rays.
Explosives that aren’t seen might be smelled with “trace detectors” that react to small quantities of tell-tale vapors or particles. In this approach, people or bags pass through a closed portal. Air is blown over them, and the vapors or particles are collected for chemical analysis. In some machines, passengers push open “saloon” doors, or walk through brushes, which pick up particles left on their clothing or hands. Alternatively, an operator can rub a wand along the person or bag to vacuum up vapors and particles. This technique is attractive for analyzing laptop computers, radios, and other electronics, which set off metal detectors but cannot be taken apart for inspection. Since x-ray machines can’t distinguish objects amid the cluttered circuitry, electronic devices are among terrorists’ favorite hiding places for explosives.

The most widely deployed trace detector is the wand-style EGIS system, made by Thermedics Detection in Woburn, Mass. The machines are used at 42 airports in 12 countries to scan bags and are also used at border crossings in Israel. At the end of November, the FAA placed a $1 million initial order for the machines-which sell for $150,000 to $200,000 apiece-under the October 9 appropriation.

With EGIS, an operator passes a vacuum wand the size of a compact umbrella over the bag. The wand is then inserted into an analysis unit, which uses gas chromatography to separate the elements in the sample and determine their concentrations. This enables the system to distinguish between nitrogen compounds in plastic explosives and those in foodstuffs. The unit displays a red light if explosives are found, and indicates whether it is TNT, nitroglycerine, or plastic. According to Thermedics, EGIS has a “false positive” rate of less than 1 percent, better than other sniffers. The rate has been verified at the Frankfurt/Main Airport in Germany, where 2,500 EGIS screenings are performed each day.

Throughput is the problem. The average analysis takes 18 seconds, and the entire procedure can take several minutes, far below the FAA’s threshold.

In November, Thermedics conducted a two-week field test of its first passenger sniffer, the SecurScan, at Boston’s Logan International Airport. More than 2,000 travelers volunteered to walk through a portal where 10 wands brushed over their clothing, drawing in an air sample. Three people were stopped: a bomb squad employee, a traveler who had been on a firing range, and an FAA official who had earlier cleaned his gun. SecurScan costs about $300,000.

Meticulous terrorists who shower, change clothes, and seal explosives in airtight containers might fool a trace detector. But a unique signature of the explosive might still be “heard” using a novel technique called quadropole magnetic resonance, similar to the magnetic resonance imaging used in medicine.

Once inside the QScan-1000, made by Quantum Magnetics in San Diego, a bag is zapped with a pulse of low-intensity radio waves. Nuclei in the bag and its contents are momentarily tipped out of alignment. As each material realigns, it reradiates a characteristic radio signal, which is picked up by a receiver and compared with the echoes for explosives. If a match is found, a red “fail” light goes on. The company says the process will not damage magnetic media such as computer disks or credit cards.

QScan costs about $300,000 and can inspect some 600 bags an hour. During a week-long trial at Los Angeles International Airport in November, only four false alarms occurred among 4,000 bags, an error rate of 0.1 percent. This newest of the detection technologies works well for plastic explosives, but not for all other types (the echo is not as strong), and the FAA has no certification procedures in place for it yet.

The only other detection technology the FAA has investigated is a scanner that bombards bags with neutrons to sense the density of specific elements, such as oxygen, nitrogen, carbon, and hydrogen. After $20 million in research, the agency has found the hardware to be far too costly, large, and heavy to be practical.

Each of the technologies under consideration can detect explosives, but what matters is how fast and how accurately. Conventional x-ray scanners process 600 bags an hour-six seconds each, on average. The 450-bag-an-hour mark was chosen because anything slower would discourage people from flying, according to Lyle Malotky, science adviser for civil aviation security at the FAA.

While the new x-ray scanners can meet the throughput limit, false alarms are high. Trace detectors are more accurate but need time to analyze samples. And there’s the rub: the technologies trade speed for accuracy, or vice versa. The FAA demands both.

Real-life trials point out unforeseen weaknesses. The FAA checks new equipment at its William J. Hughes Technical Center in Atlantic City, N.J., with a standard “test-bag set.” Although the CTX-5000 computed-tomography system passed at the Tech Center, its false-alarm rate rose above the FAA limit when used on actual passenger bags at San Francisco Airport. The rate was between 20 and 30 percent, according to Malotky, who is generally regarded as the country’s most prominent expert in aviation security. Among the culprits, says Malotky, were certain food products that are similar in shape and density to a ball of plastic explosive and were not included in the FAA’s test-bag set.

“False alarms are the bottom line,” says Lee Grodzins, professor of physics at MIT and vice-president of AS&E, which makes backscatter x-ray scanners. “Only one bag in a billion might contain a bomb. So in a real airport, every alarm will be a false alarm.” Each alarm will require a security guard to search the person or bag, shifting the detection burden to people, which slows the overall movement of passengers and bags. Metal detectors have a 10 percent false-alarm rate, Malotky notes, but most alarms are easy to resolve: just empty your pockets.

Experts acknowledge, too, that each of the new detection methods has an Achilles’ heel-some way in which a knowing person can fool the system. Any of the machines might catch a careless terrorist, Grodzins says, “but none of them will stop the Unabomber.”

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