Scientists have developed a new method to detect chemicals inside a container over distances that top the 100-metre mark. This will help people assess explosive substances from a distance, making such tasks safer. The study was funded in part by the OPTIX ('Optical technologies for the identification of explosives') project, which is backed with EUR 2.49 million under the Security Theme of the EU's Seventh Framework Programme (FP7).
Part of the OPTIX consortium, researchers from the Vienna University of Technology (TU Vienna) in Austria say the laser light that makes up this new technique is scattered in a very specific way by various substances. This light is used to help researchers assess the contents of a closed, non-transparent container.
'The method we are using is Raman spectroscopy,' explains TU Vienna's Professor Bernhard Lendl. The team irradiated the sample with a laser beam. When the sample's molecules scatters the light, the energy changes. Photons, for instance, can transfer energy to the molecules by exciting molecular vibrations. The light's wavelength changes, as does its colour. Scientists evaluate the colour spectrum of the scattered light to find out which molecules have scattered.
'Until now, the sample had to be placed very close to the laser and the light detector for this kind of Raman spectroscopy,' explains Dr Bernard Zachhuber of TU Vienna. Measurements can now be made over long distances because of this sophisticated technology. 'Among hundreds of millions of photons, only a few trigger a Raman-scattering process in the sample,' Dr Zachhuber says.
According to the team, these scattered particles of light are scattered uniformly in all directions. Only a small fraction returns 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, they say.
The researchers tested used explosives, such as trinitrotoluene (TNT), ammonium nitrate/fuel oil (ANFO) or Research Department Explosive (RDX), in many instances, generating very positive results. 'Even at a distance of more than 100 meters, the substances could be detected reliably,' says Engelene Chrysostom from TU Vienna.
The team says Raman spectroscopy works over long distances even if the sample is inside a non-transparent container. The laser beam is scattered by the container wall, but a tiny portion of the beam penetrates the box. According to the researchers, it can still excite Raman-scattering processes. 'The challenge is to distinguish the container’s light signal from the sample signal,' says Professor Lendl.
An easy, geometric trick makes this possible. The laser beam hits the container at a specific spot, and the light signal emitted by the container stems from a tiny region. The team says this innovative method could help airports with their security checks. But other areas would benefit as well, especially those that need to investigate closely and securely.
The OPTIX consortium is led by Indra Sistemas from Spain, and is made up of experts from Germany, Spain, Lithuania, the Netherlands, Austria and Sweden.
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