Let us spray: low-cost explosive sensor

     In Christmas Day 2009, a 23-year-old Nigerian student named Umar Farouk Abdulmutallab boarded Detroit-bound Northwest Airlines flight 253 in Nigeria with more than 80 grams of the plastic explosive PETN sewn into his underwear — enough to blow a hole in the fuselage. PETN, or pentaerythritol tetranitrate, is the same explosive used by the shoe bomber Richard Reid. The intent of the Christmas Bomber was to cause the aircraft to crash, but the attempt failed due to incompetence. Attempts by airport security to discover the explosive also failed.

     The incident illustrates not just how obsolete airport metal detectors have become in preventing explosive devices from finding their way on board airplanes, but also the evolving threat explosives present.

     Explosives may appear in virtually any form, from solid to liquid to gas. They can also be made to resemble anything from an iPhone to a baby bottle.

     One of the problems for law enforcement and airport security personnel is, explosive detection technologies do not adapt quite so fast. No single explosive detection technology can reliably and rapidly screen all people, cargo and vehicles, although new technologies continue to emerge that may help close the threat gap.

     Researchers at the University of California-San Diego have one solution.

     Chemists there have devised a simple new spray-on film that could one day allow airport security, police and hazardous material teams to quickly and confidently screen vehicles, passengers, luggage and cargo for traces of nitrogen-based explosive residue.

     The basic idea is, airport screeners would apply an extremely thin spray of fluorescent polymer film on a suspect surface to reveal the presence of dangerous chemicals, such as nitroglycerin. Contaminated fingerprints leave dark shadows on the film, which glow blue under ultraviolet light.

     Incriminating traces are revealed as soon as the solution dries, typically within 30 seconds.

Spray it again, Sam

     "No special instruments or training are needed to interpret the results because the polymers fluoresce brightly when exposed to explosive residue," says William Trogler, a UCSD professor of chemistry and biochemistry.

     Trogler says only a minute amount of film is necessary to provoke a chemical reaction. A single one-thousandth of a gram layer of the polymer is enough to detect as little as a few trillionths of a gram of residue on a palm-sized surface. Any surface, including fingers, that has come in contact with nitrogen-based explosives will have 1,000 times that quantity or more stuck to it.

     "It's a simple visual test for explosives that doesn't take a scientist to understand how to use or interpret it," Trogler says.

     One of the films can distinguish between classes of explosive chemicals, a property that could provide evidence to help solve a crime, or prevent one, Trogler says.

     Other explosive residue-sensing technologies exist, such as handheld electronic "sniffers" that sample the air; but Trogler says his technology is different because the UCSD films adhere directly to potentially contaminated surfaces.

     "This makes the films more sensitive than previous methods, which rely on capturing molecules that escape into the air," he says.

     Trogler explains the technology could also possibly be used to scan tickets or boarding passes to see if a person had recently handled explosives.

     Trogler's technology starred in an episode of the television series "CSI: Miami," where it was used to connect fingerprints left on a video camera to a bomb used in a bank heist. In reality, the security systems company RedXDefense has licensed the technology, and has developed a portable kit called XPAC.

Miniature explosive sensors

     Another new way to detect explosives has appeared at Oak Ridge National Lab. Researchers there have come up with what they believe to be a more reliable miniature explosive sensor technology. This one provides a way to mass produce miniature explosive sensors less likely to alarm falsely.

     The development may one day make it possible to more effectively protect infrastructure assets and public assembly venues from terrorist bombings.

     Miniature sensor technology already exists. Most use surface chemistry for recognition, but are susceptible to high numbers of false positives.

     The Oak Ridge research found a different way to detect explosives, this one based on the physical properties of their vapors.

     "Our technology shows that different explosives have unique thermal characteristics that can be used for identification," says Thomas Thundat, a scientist with Oak Ridge National Labs and the University of Tennessee.

     Current miniature sensors have a chemical layer attached to their surface designed to bind specifically to explosives, but they are often unable to discriminate between chemicals of similar nature, one of which may be dangerous and the other benign. This tends to create false positives.

     "They may detect a trace amount of TNT, for instance, but they may not be able to distinguish that from a trace amount of gasoline," Thundat says.

     Instead, the Oak Ridge sensor uses a micromechanical concept called a microfabricated bridge that can be electronically heated from room temperature to 500 degrees Celsius in 50 milliseconds. During this heating process, absorbed explosive molecules are burned, melted and evaporated.

     "The process creates a signature that is unique to explosives," Thundat says.

     The Oak Ridge method is therefore capable not only of differentiating individual explosive vapors such as trinitrotoluene (TNT), pentaerythritol tetranitrate (PETN), and cyclotrimethylene-trinitramine (RDX), but also of differentiating explosive vapors from non-explosives. Ergo, fewer false positives.

     Eventually, the homeland security benefit of cheap, tiny sensors is that they can be deployed almost anywhere. Presumably, they could be networked and GPS-located. Homeland protection authorities could sprinkle them liberally in and around strategic buildings, ports and other critical infrastructure components. Transportation Security Administration (TSA) officials could finally saturate airport luggage and cargo handling areas, as well as passenger lounges and parking garages with explosive detection sensors.

Backscatter systems

     The UCSD and Oak Ridge technologies are just two of a number of airport explosive detection technologies to have surfaced in the past decade, most of them since the 2001 terrorist attacks. Each has distinct applications and advantages. Each also has its limits.

     Most sensing technologies used in airports search luggage for signs of explosive equipment such as detonators. But detonators can be concealed inside electronic equipment, so chemical analysis is also necessary.

     Chemical analysis involves taking a swab or sensing the air around objects to obtain spectrographic analysis of any explosive vapors present. Sniffer dogs can also detect traces of explosives on baggage, but neither method is altogether practical for monitoring every passenger and all baggage.

     X-ray technology is more proficient at scanning everything that moves past it. Some newer types of X-ray machines can detect specific compounds by measuring reflected X-ray photons. X-ray scattering effects reveal materials composed of low atomic number densities, such as the components in common explosives.

     But cargo and vehicle scanning devices produce extremely high X-ray doses, necessary to penetrate metal hides of containers. High dose radiation is unsafe for any humans who happen to be vehicle occupants.

     Instead, ultra-low dose X-ray applications safe for humans have appeared in two forms: backscatter X-ray and transmission X-ray.

     Backscatter X-ray is useful when the primary application is scanning humans. Backscatter X-rays penetrate the body only a fraction of an inch before bouncing back. Internal organs are not visible in the image, eliminating clutter and making it much easier for the operator to analyze the image for potential threats.

     "All explosive detection done by X-ray units is measured by algorithms that judge density against particular atomic weights of known explosives," says Keith James, director for CBRNE Programs for SoBran, a technical and professional services company. X-ray technology is proficient at finding new threats such as liquid explosives, ceramic knives and guns made of plastic. As useful as the X-ray technology may be, it also has its limits, particularly when lethal objects and common objects have similar chemical densities.

     "Unfortunately, backscatter X-ray can't distinguish between a bar of explosive nitrate and a bar of chocolate," James says. Frequent false positives are the result.

     Another problem with the technology is the issue of personal privacy.

     While backscatter X-ray systems, when used as an alternative to personal pat-down secondary searches at airports and other security checkpoints, can easily penetrate clothing to reveal concealed contraband, they also reveal intimate body contours. Essentially, backscatter technology undresses human subjects.

     Some consider this a violation of privacy, the appearance of security achieved by further erosion of personal liberty.

     The American Civil Liberties Union and the Electronic Privacy Information Center call backscatter scans "virtual strip searches." The issue is, the image that appears on the monitor used by airport screeners is a nude picture of the subject. It not only reveals hidden explosives or weapons, by also otherwise private, confidential medical information, such as the fact a passenger may be fitted with a colostomy bag.

     Nevertheless, backscatter technology is being used in several U.S. airports and courthouses, although a possible replacement technology is in the wings. Millimeter wave technology, another way to obtain full-body imaging, uses non-ionizing electromagnetic waves to generate images based on the energy reflected from the body.

     The images generated through millimeter waves are lower resolution than those of X-ray backscatter and supposedly less intrusive. The government is still looking under everyone's clothing, but with millimeter wave technology, "facial images are blurred," according to a TSA press release.

     Additionally, the officer that does view the image will be remotely located and unable to associate the image with the passenger being screened. Once viewed remotely, the image will not be stored, transmitted or printed.

     As of December 2009, 40 passenger imaging systems had been installed at some of the nation's largest airports, used for primary or secondary (random) screening. TSA reports it has plans to purchase 150 more.

Transmission X-ray

     Backscatter X-ray is also not as useful when scanning objects such as vehicles or freight containers. Since the X-rays penetrate only a fraction of an inch, only a surface scan is performed. Many blind spots will be evident in the image. Items buried in an object will not be seen at all, such as the inside of a vehicle gas tank, one popular location to smuggle contraband.

     Instead, first single-energy, then dual-energy, transmission X-rays were developed to screen vehicles and shipping containers. Since these X-rays pass completely through, blind spots are minimized. However, clutter can become an issue. Single-energy systems produce black and white images, and operators often have difficulty distinguishing between metal and non-metal objects.

     Dual-energy systems create color-coded images, allowing the operator to easily distinguish metal from organic material and eliminate the clutter issue. A faster and more thorough examination of the vehicle is accomplished compared to visual inspection and under-car cameras or mirrors.

     Ultra-low dose, dual energy transmission X-ray systems have also evolved to screen occupied vehicles. Throughput is therefore not sacrificed, since passengers are not required to exit the vehicle.

     Douglas Page writes about science, technology and medicine from Pine Mountain, Calif. He can be reached at douglaspage@earthlink.net.

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