Identifying Explosives at a Distance

The random Raman laser is the lastest technology to detect explosives and other nasty stuff from a safe vantage

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Being standoffish is usually frowned upon—that is, unless what you’re standing off from might be an explosive or a cloud of anthrax spores. That’s why efforts have accelerated to develop standoff detection techniques that use lasers to identify chemicals and biological substances from a safe distance.

The newest entry in the field is called random Raman spectroscopy. Shine a laser beam into a loose material—say, a powder—and if the density is right, the photons will bounce around among the powder’s particles until they stimulate a new laser emission. Such a random laser, as it is known, works much the same way as a more traditional laser cavity, only without mirrors.

Normally, about 1 in 10 million photons undergoes a process called spontaneous Raman scattering, in which it drops to a lower frequency determined by the particular molecule it’s bouncing off. The random laser enhances this Raman scattering, producing a signal strong enough for a detector to pick up at a distance. By measuring the shift in frequency, scientists can tell the chemical makeup of the powder.

Marlan Scully and Vladislav Yakovlev of Texas A&M University, in College Station, demonstrated such a setup. Scully says they can perform spectroscopic analysis of a material at a distance of a kilometer, and that 10 kilometers should be possible. That would be useful for, say, a drone flying over an area where explosives might be hidden, or an airplane measuring the quality of soil on a farm.

Another approach to spectroscopy uses terahertz waves, or T-rays. T-rays have the advantage of being able to penetrate many substances, without the ionizing radiation of X-rays. But they have a downside. “The terahertz wave does not travel easily through the atmosphere because of water absorption,” says Xi-Cheng Zhang, head of the University of Rochester’s Institute of Optics, in New York.

One way around the problem is what he calls terahertz-radiation-enhanced emission of fluorescence, which is designed to detect trace gases emitted by an explosive. He focuses laser beams at two wavelengths on a point in the air, where they interact to create a plasma filament that fluoresces in the ultraviolet. He also emits a terahertz pulse. The T-rays interact with the material being studied to provide the spectroscopic information and then interact with the plasma field to alter its fluorescence. That encodes the spectroscopic information onto the ultraviolet radiation, which is easily picked up by a photodetector. Zhang says the challenge of doing this increases with distance, but he’s already demonstrated detection at 10 meters.

Fow-Sen Choa, a professor of computer science and electrical engineering at the University of Maryland, Baltimore County, uses a quantum cascade laser to do photoacoustic spectroscopy. Heating a material with a modulated laser beam causes it to expand and create a pressure wave, as if it were a tiny audio speaker. Microphones pick up the sound wave and identify the material based on its frequency. “Whether it’s TNT or fertilizer, you can tell pretty easily,” Choa says.

Most of the development of this technique is focused on the accurate detection of the sound and elimination of noise, Choa says. “Distance is not yet the focus,” he says. “The issue is how accurate you will be.”

There’s no ideal distance for how far the detector should stand off, says James Kelly, senior scientist at Pacific Northwest National Laboratory, in Richland, Wash., other than far enough to be safe. The distance, and the most effective technology, really depends on the particular requirements of an application.

Kelly’s team is working on being able to measure a substance at a dosage of 1 microgram per square centimeter on a surface. That’s about what you’d get if someone handling explosives had then touched something and left a fingerprint. The team would like to be able to use an eye-safe system such as hyperspectral imaging to scan vehicles coming to a checkpoint or parking at a stadium, for instance, to see if there are any traces of explosives on them. Because it can be challenging to tease out such a signal from those given off by the paint and other coatings on the surface of a car, researchers at PNNL and other teams are using an eye-safe tunable laser to do reflectance-based hyperspectral imaging, in which multiple images of the surface are taken at different wavelengths under the laser’s illumination. Two substances that might be indistinguishable at one wavelength can look very different at another.

For that application, which could be used by the United States’ Transportation Security Administration or border patrol, a distance of 50 to 100 meters might be desirable, Kelly says. A drone surveying a war area would probably require detection distances in the kilometer range.

For a lot of the techniques being developed, it’s not so much the detection technology itself that’s the bottleneck but the analysis of the signal, Kelly says. Finding trace amounts of explosives does little good at a checkpoint if it takes several minutes of computer processing to identify them.

In the end, no one technology is likely to win out, researchers say. More probably, the one that is used will be the one best suited to a particular need. “There’s a whole gamut of techniques people are looking at,” Kelly says.

A correction to this article was made on 26 September 2014.

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