Let us spray: low-cost explosive sensor

A low-power, realtime explosive trace sensor has emerged that is both highly selective and highly sensitive


     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.

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