Explosives detectionDifferent technologies aim to replace dogs as explosives detectors

Bomb-sniffing dogs are the best and most popular way for airport security quickly to detect anyone planning to bring explosives to an airport. Denis Spitzer is trying to change that.

Dr. Spitzer and his colleagues are working on a sensor that will detect vapors of TNT and other explosives in very faint amounts. The device Spitzer is trying to create would replace dogs as the top bomb detecting method in the field.

The New York Times reports that inventing a machine to replace dogs will not be easy, however. A bomb-sniffing device must be extremely sensitive, able to develop a signal from a relative handful of molecules. It must also be highly selective, able to distinguish an explosive from the “noise” of other compounds.

At this time there are some vapor detectors on the market, but dogs are still the best when it comes to bomb detection.

“Dogs are awesome,” Aimee Rose, a product sales director at the sensor manufacturer Flir Systems, which markets a line of explosives detectors called Fido, told the New York Times. “They have by far the most developed ability to detect concealed threats,” she said.

When it comes to using dogs in the field, there are some drawbacks. Dogs can get distracted, they cannot work around the clock, and they require expensive and extensive training and handling.

Flir’s bomb detectors are lightweight, compact, and considered to be on par with dogs. The military is currently using Flir’s devices, which are capable of detecting TNT at a few parts per quadrillion. Rose does not consider the devices an alternative to dogs, but instead “we see our technology as complementary to dogs.”

The devices use a fluorescent polymer technology developed by Timothy Swager, a chemist at MIT, under whom Rose studied. Thin films of the polymers emit visible light when exposed to ultraviolet rays, but molecules of TNT stop the fluorescence. A single TNT molecule can quench many thousands of fluorescence reactions, greatly increasing sensitivity.

The Times notes that Spitzer has a different approach with his sensors. The slivers of silicon are fixed at one end of the device and are made to vibrate. As molecules of explosives are captured by the cantilever, the added mass changes the vibration rate, which can be measured by a laser.

Microcantilevers are used in many sensor applications today, but are not sensitive enough to detect explosives, so they are used in conjunction with nanotubes — hollow cylinders of titanium oxide which allow more molecules to be captured.

The sensitivity of an explosive detector is critical because many explosive compounds, such as RDX and PETN, are not very volatile at normal temperatures, so very few particles vaporize. As a result, bomb detection is now focused more on finding explosive particles on surfaces of one’s clothing or luggage than on detecting molecules in the air.

David Atkinson, chief scientist for explosives research at the Pacific Northwest Laboratory in Richland, Washington, knows this method is the best way to detect explosives. “We’ve had a particle-based detection paradigm for the past two decades.” Even today, when a laptop or other object is deemed suspicious after being X-rayed, an agent wipes the surface and puts the wipe in a spectrometer that ionizes any explosive compounds present, allowing them to be quickly identified.

“The key is you’ve got to get that particle,” Atkinson told the Times. “The whole ability to detect explosives is whether the operator correctly samples.”

No one knows exactly why dogs are so good at detecting explosive material. Dogs have roughly thirty times as many olfactory cells as people, and the brain region devoted to smell is proportionally much larger, but size does not explain the entire situation.

“The black box that is a dog that we don’t really understand makes a lot of scientists uneasy,” Paul Waggoner, senior scientist at the canine detection research institute at Auburn University told the Times.

Spitzer is still working on making his machines more sensitive to explosive materials. Right now, the experiments his staff is conducting suggest they can detect TNT at concentrations at less than one part per trillion, which is still not as good as dogs. The next step for his team is to determine ways to make the nanotubes used in their devices more selective so the sensors do not pick up on water or other molecules which could affect the measurements.

Eventually, Spitzer will test the sensors in simulated airport rooms, and under other conditions that simulate an active airport. “When you have real conditions, sometimes you can have surprises,” Spitzer said.