Portland, Ore. Experience in photoluminescent spectroscopy has helped university researchers bring to light a method by which explosives can be detected at a distance.
In contrast, to detect the presence of explosives today, airport and other screeners must swab an object like a suitcase or clothing, use a dog to sniff it, or blast puffs of air across a filter that traps airborne explosive particles.
The advantages of the photoluminescence-based explosive detection method are that it can be remotely applied and that it requires neither time-consuming and expensive machines nor trained dogs. All you do is shine a laser on an object and watch for the photoluminescent "signature" distinctive of explosives. Whether it's an approaching ship, a suitcase, a moving car or a pile of trash half a mile up the road, if you can shine a green laser on it, professor Rolf Hummel at the University of Florida (Gainesville) says he can tell you whether it's an explosive device.
Hummel, an engineering professor emeritus specializing in materials science, was intrigued by a U.S. Army Research Office challenge to universities to find an inexpensive, quick and reliable explosive-detection system. With his lab's high-resolution photoluminescent meter, one of his students performed a simple test that no major lab had thought to perform before: Shine a green laser on an explosive and see if it glows like a white shirt under a black light, but in the infrared. When Hummel's assistant at the Nanoscale Devices and Novel Engineered Materials center at the University of Florida performed the test, it worked the first time, the second time and every time thereafter. No false alarms yet.
Hummel performed the work with University of Florida professor Paul Holloway and doctoral candidates Anna Fuller and Kwanghoon Kim.
The novelty of Hummel's discovery stems from the fact that no one knew explosives were photoluminescent in the infrared let alone that the vapors forced out of them by their very reactive nature could be made to glow from a distance with a laser.
No false positives
Although photoluminescence is a well-known field that's how fluorescent tubes work no one thought to check to see if explosives glowed in the infrared when zapped with a green laser. Hummel said his years of experience suggested this combination when he started thinking about what a lifesaver such a device might be.
Chuck Schau, a scientist at Raytheon Missile System's Radiation Technology Laboratory, actually had conducted experiments on detecting explosives using photoluminescence before Hummel's discovery. He did not have the specialized detectors present in the Nanoscale Devices and Novel Engineered Materials center at the University of Florida, however, so he missed observing the distinctive infrared peak that enables Hummel's claim of freedom from false positives. Raytheon is now considering licensing the technology for military use.
"This discovery is very exciting for the field of remote detection," Schau said.
The researchers have proven that the effect works with TNT, nitroglycerin and the plastic explosives trinitrofenil-Nmetilnitramina (Tetryl), hexahydro-trinitro-triazine (RDX), octahydro-tetranitrotetrazocine (HMX) and pentaerythrol tetranitrate (PETN).
Photoluminescence works by adding energy to a molecule with a high-energy wavelength of light, then allowing that molecule to fall back into its ground state by emitting a photon of a different, distinctive wavelength. For instance, the ultraviolet rays from the mercury vapor in a fluorescent bulb bump the phosphor coating on the inside of the tube into a higher energy state. When the coating molecules drop back into their ground state, they emit a visible photon, thereby making a light bulb.
Similarly, in Hummel's approach a high-energy green laser (325- to 532-nanometer wavelength) adds energy to molecules floating in the air and on the surface of anything, including people, luggage and approaching vehicles. If explosives are involved in any way, Hummel claims, his detector will set off an alarm triggered by a glow that has a unique signature.
Over the past year Hummel has tested his technique on everything he can image might end up in a suitcase, from benzene, naphthalene, nail polish, perfume, lighter fluid, rubber cement, acetone, toluene, fertilizer on golf shoes, sugar, baking soda, polyvinyl pyrrolidone (PVP), paint thinner and more with no false positives yet.
"It turns out that all explosives use groups of nitrogen molecules bonded to two oxygen molecules, which makes them very reactive they can't help but seep out into the air around anything concealing them. When our green laser hits these molecules they photoluminesce at the 705-nm wavelength. Anything else that radiates at that frequency also radiates at nearby frequencies. Only explosives emit in this very narrow band," said Hummel.
As a consequence, Hummel's detector could theoretically sense a single molecule from an explosive's "aroma" without fear of false positives, by using two sensors one for explosives and one to prevent false positives. The primary sensor has a filter so it only senses exactly 705-nm wavelengths, thus "arming" the alarm. But the alarm is only set off if a second, "fail-safe" sensor verifies that there is no equally strong radiation at wavelengths adjacent to 705 nm, thus preventing false positives, according to Hummel.
"This invention is almost a year old now, because at first people here did not believe us that no one had thought of this before. But when they checked the literature and found we were right, they told us we couldn't tell anybody until they got a patent on it," said Hummel.
In the ensuing year, Hummel's group at the University at Florida built several successful prototypes. Even without special equipment, the researchers were able to trigger the effect by merely donning a soldier's night-vision goggles and shining a green presentation laser on an explosive.
Eventually, Hummel envisions sensors mounted outside doors at airports, to scan people and vehicles before they even get close to the terminal. By doing so at a distance, automatic door closers, blast barriers and police units could be automatically deployed before bombers get close to secure areas.