With a laser, a telescope, and modest computer hardware, an Alabama A&M team was able to identify explosives from more than two football fields away.
It’s a classic catch-22. You’ve spotted a suspicious package in the distance. To tell if it’s a bomb, you need to take a closer look. But if it is indeed a bomb, there’s a chance that it could, you know, explode.
If you’re ever unlucky enough to face that scenario in real life, a new device developed by Anup Sharma and coworkers at Alabama A&M University might come in handy. Known as a Standoff Raman Spectrometer, it can detect tiny amounts of explosive from more than a football field away. The detector sniffs out incendiary substances not with swabs or samples but with a laser beam.
If laser light is tuned to just the right frequency—the so-called resonant frequency—it can cause an explosive material’s molecules to jiggle and emit a little light of their own. So if the researchers shine laser light at a target and see a little light of a particular wavelength shining back at them, they know they’ve potentially found a bomb—or at least the makings of one. Granted, the returning light is too faint to see with the naked eye. To detect it, the team equipped a 110-inch telescope with an electronic light sensor.
To be sure, the Alabama A&M group isn’t the first to use the technique. It was first demonstrated about two decades ago at Lawrence Livermore National Laboratory in Berkeley, California. In addition to detecting explosives, it can be used to detect toxins and narcotics. The US Army Research Laboratory has developed a variation, dubbed LIBS, that can quickly identify just about any substance one could think of.
But Standoff Raman Spectroscopy has typically been performed using green lasers. Because green light is relatively energetic, there’s a certain risk that it will jiggle explosive molecules vigorously enough to permanently alter them or, worse, detonate them. Sharma and company’s innovation was to show that you can apply the same technique using gentler, infrared lasers.
The group used their detector to identify ammonium, sodium, and magnesium nitrates from distances of up to 250 meters. Ammonium nitrate, a common fertilizer, was the explosive material used in the 1995 Oklahoma City bombing. Sodium nitrate, also known as Chile saltpeter, is a common ingredient in fireworks.
In indoor trials, the various nitrates could be easily detected at distances up to 60 meters—the length of the longest available straightaway in the building where the group performed their tests. Outdoors and at night, the detector could sniff out explosives from up to about 250 meters away. But the faint signal produced by the explosive material was too easily drowned out by sunlight to achieve long-distance detection during the daytime.
With stronger lasers and a few refinements to the system’s optics, however, the researchers think they can improve their detector’s sensitivity. For bomb-squad agents who regularly find themselves in one of life’s most precarious catch-22s, that can only be good news.
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