CEREX Progress report July – December 2011. “Detection of Explosives Particles by Imaging Raman Spectroscopy”

FOI has in earlier reports showed that Raman spectroscopy is a method capable of detecting explosives at stand-off distances. The method has been evaluated for bulk- [1-3], vapor- [4] and particle-detection [5]. When searching for explosive threats e.g. home-made explosives, it can be very difficult to get visible access to the explosive material itself and therefore not practical to use. By instead searching for traces of explosives, that are distributed on the outside of the device and to the surroundings by e.g. fingerprints, it can be possible to find and identify the explosive device. In the CEREX project, detection equipment capable of finding single particles of explosives will be developed and demonstrated. To find these small amounts of explosives multispectral imaging Raman spectroscopy [6] is being used. This report describes the ongoing work on building an eye-safe explosive particle detection demonstrator that will be finished during 2012. The demonstrator is capable of detecting single particles of DNT, sulfur and ammonium nitrate in a fingerprint using 532 nm laser wavelength [7]. During the next six months the system will be rebuilt to use an UV-laser instead. This will make the system eye-safe and also make the laser beam will be invisible. Continued studies on UV-Raman have been made using laser wavelengths from 360 nm to 320 nm. These measurements show that the collected Raman signal decreases when the wavelength decreases. This is not completely investigated yet since the Raman cross section should increase in this range due to the 1/?4 dependence. Fingerprints on an aluminum plate and trace residues on a paper tissue have been scanned after handling sulfur. The result showed clear identification of sulfur on both targets. A new graphical user interface (GUI) has been developed for the demonstrator in order to make it easy to use. Furthermore, a joystick has been incorporated with the system to make it easier to aim at different targets. As reticle there are two cameras, one with a zoomed view and one showing an overview of the site, where the zoomed image has the same field of view as the Raman images.