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Locating explosives with laser beams

Scientists have found a way to detect chemicals over long distances, even when they are enclosed in containers

Most of us like to keep a safe distance from explosive substances, but in order to analyse them, close contact is usually inevitable.

Now, scientists at the Vienna University of Technology (TU Vienna) have developed a new way to detect chemicals inside a container over a distance of more than a hundred metres.

Laser light is scattered in a very specific way by different substances. Using this light, the contents of a non-transparent container can be analysed without opening it.




A bright laser beam hits the sample, the scattered light is collected by a telescope

“The method we are using is Raman-spectroscopy,” says Bernhard Lendl, a Professor at TU Vienna. The sample is irradiated with a laser beam. When the light is scattered by the molecules of the sample, it can change its energy. For example, the photons can transfer energy to the molecules by exciting molecular vibrations. This changes the wavelength of the light, and thus its colour. By analysing the colour spectrum of the scattered light, scientists can determine by what kind of molecules it must have been scattered."

“Until now, the sample had to be placed very close to the laser and the light detector for this kind of Raman-spectroscopy”, says Bernard Zachhuber, one of the researchers. Due to his technological advancements, measurements can now be made over long distances. “Among hundreds of millions of photons, only a few trigger a Raman-scattering process in the sample”, continues Zachhuber.

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

In this project, funded by the EU, the researchers at TU Vienna collaborated with private companies and with partners in public safety, including The Spanish Guardia Civil who are extremely interested in the new technology.

During the project, the Austrian military was also involved. On their testing grounds the researchers from TU Vienna could put their method to the extreme. They tested frequently used explosives, such as TNT, ANFO or RDX. The tests were highly successful: “Even at a distance of more than a hundred metres, the substances could be detected reliably,” concludes Engelene Chrysostom of TU Vienna.



Bernhard Zachhuber, adjusting the spectrometer

Raman spectroscopy over long distances even works if the sample is hidden in a non-transparent container. The laser beam is scattered by the container wall, but a small portion of the beam penetrates the box. There, in the sample, it can still excite Raman-scattering processes.

“The challenge is to distinguish the container’s light signal from the sample signal”, says Bernhard Lendl. This can be done using a simple geometric trick: The laser beam hits the container on a small, well-defined spot. Therefore, the light signal emitted by the container stems from a very small region. The light which enters the container, on the other hand, is scattered into a much larger region. If the detector telescope is not exactly aimed at the point at which the laser hits the container but at a region just a few centimetres away, the characteristic light signal of the contents can be measured instead of the signal coming from the container.

The new method could make security checks at the airport a lot easier – but the area of application is much wider. The method could be used wherever it is hard to get close to the subject of investigation. It could be just as useful for studying icebergs as for geological analysis on a Mars mission. In the chemical industry, a broad range of possible applications could be opened up.

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