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  • Laser detects hidden explosives

Jun 2012
Ashley N. Paddock,

VIENNA ­ — Common explosives inside a container can now be detected from more than 100 m using laser light.

Specific substances scatter light in a variety of specific ways. This is the basis of Raman spectroscopy and can be used to analyze the contents of a nontransparent container without opening it.

Scientists at Vienna University of Technology (TU Vienna) used Raman spectroscopy to discover what was in certain containers, irradiating samples with a laser beam. When the light is scattered by the molecules of the sample, it changes its energy. For example, the photons transfer energy to the molecules by exciting molecular vibrations, changing the wavelength of light and, thus, its color. By analyzing the color spectrum of the scattered light, the researchers determined what kind of molecules scattered it.

"A variety of substances were analyzed, ranging from substances relevant for improvised explosive devices (IEDs) such as chlorates, ammonium nitrate and triacetone triperoxide, to military explosives such as PETN, RDX, HMX and TNT, and a war agent stimulant (DIMP)," Bernhard Zachhuber of TU Vienna told EuroPhotonics. "These could be detected at a distance of 100 meters."

Bernhard Zachhuber mounts optical elements of the spectrometer. Courtesy of TU Vienna.

Until now, samples had to be close to the laser and light detector to achieve this kind of Raman spectroscopy, Zachhuber said. With his technological advancements, measurements can now be made over long distances.

"We succeeded in analyzing a variety of substances in small glass vials at 100 meters' distance," he said. "The content in nontransparent containers was measured at distances up to 40 meters." Time constraints on the experiment made it difficult to test the method at longer distances.

"It should, however, be noted that, for longer distances, it becomes increasingly difficult to position the laser precisely on the sample due to atmospheric turbulences," Zachhuber said. "The precision of the laser positioning on the sample is limited, which could reduce the accuracy of the method."

The light particles of a sample are scattered uniformly in all directions. Only a tiny fraction of them travel back to the light detector, and from this very weak signal, as much information as possible must be extracted. This can be done using a highly efficient telescope and extremely sensitive light detectors.

The TU Vienna scientists collaborated with private companies and with partners in public safety, including the Spanish Guardia Civil, to put their method to the extreme. With the help of the Austrian military, they tested common military explosives on their testing grounds.

"To evaluate the method, containers ranged from shampoo containers and colored glass bottles to bags of different color and material," Zachhuber said. "For example, the detection of NaClO3 in a 1.5-mm-thick white plastic container was possible."

Detecting explosives in nontransparent containers can be challenging. Although the laser beam is scattered by the container wall, a small portion of the beam can penetrate the box. Inside the sample, it can still excite Raman scattering processes. The difficulty lies in distinguishing the container's light signal from the sample's signal.

This can be achieved using a simple geometric trick: The laser beam hits the container on a small, well-defined spot. The light signal emitted by the container stems from a very small region, while the light that enters the container is scattered into a much larger region. If the detector telescope is not aimed exactly at the point at which the laser hits the container, but rather just a few centimeters away, the characteristic light signal of the contents can be measured instead of the signal coming from the container.

The new method could make airport security checks a lot easier, but the scientists say that the area of application is broader than that. It also could be used wherever it is hard to get close to the subject of investigation­ for studying icebergs or for geological analysis on a Mars mission, for example.

"A possible application is online process control, where it is beneficial when a measurement technique does not interfere with the production process," Zachhuber said. "Considerable research is also being performed to implement the technique for planetary geological application."

Next, Zachhuber plans to improve existing methods for explosives detection and to develop new concepts to meet the challenging requirements of threat scenarios.

"Using tunable UV lasers, the gas phase can be probed on purpose for specific volatile target molecules, further expanding the field of stand-off Raman applications for counterterrorism," he said.

In future work, he also plans to use lasers with a higher pulse repetition rate to broaden the applicability of the technique.

Application of radiation to an object.
raman spectroscopy
That branch of spectroscopy concerned with Raman spectra and used to provide a means of studying pure rotational, pure vibrational and rotation-vibration energy changes in the ground level of molecules. Raman spectroscopy is dependent on the collision of incident light quanta with the molecule, inducing the molecule to undergo the change.  
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