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Proceedings Paper

Stand-off spectroscopy for the detection of chemical warfare agents
Author(s): Rhea J. Clewes; Chris R. Howle; David J. M. Stothard; Malcolm H. Dunn; Gordon Robertson; William Miller; Graeme Malcolm; Gareth Maker; Rick Cox; Brad Williams; Matt Russell
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Paper Abstract

The most desirable configuration for detection of toxic chemicals utilises the maximum distance between detector and hazard. This approach minimises the contamination of equipment or personnel. Where the target chemical is an involatile liquid, indirect detection of the liquid contamination is made difficult by inherently low vapour pressure. In this instance, direct detection of the chemical hazard is the best approach. Recent technology developments have allowed spectroscopic systems to provide multiple options for the stand-off detection of involatile chemical warfare agents (CWAs). Two different stand-off spectroscopic systems, based upon IR absorption and Raman spectroscopic techniques are described here. The Negative Contrast Imager (NCI) is based upon an optical parametric oscillator (OPO) source comprising a Q-switched intracavity MgO:PPLN crystal. This crystal has a fanned grating design and wavelength tuning is achieved by translating the PPLN crystal within the 1064 nm pump beam. This approach enables the production of shortwave and midwave IR radiation (1.5 – 1.8 μm and 2.6 – 3.8 μm, respectively), which is scanned across the scene of interest. Target materials that have an absorption feature commensurate with the wavelength of incoming radiation reduce the intensity of returned signal, resulting in dark pixels in the acquired image. This method enables location and classification of the target material. Stand-off Raman spectroscopy allows target chemicals to be identified at range through comparison of the acquired signature relative to a spectral database. In this work, we used a Raman system based upon a 1047 nm Nd:YLF laser source and a proprietary InGaAsP camera system. Utilisation of a longer excitation wavelength than most conventional stand-off detection systems (e.g. 532 or 785 nm) enables reduction of fluorescence from both the surface and the deposited chemicals, thereby revealing the Raman spectrum. NCI and Raman spectroscopy are able to detect CWAs on surfaces at distances of 2 – 10 metres and have potential to detect over longer ranges. We report the successful identification of at least 60 μl of nitrogen mustard at a distance of a 2 m and 10 m using NCI and Raman spectroscopy.

Paper Details

Date Published: 30 October 2012
PDF: 8 pages
Proc. SPIE 8546, Optics and Photonics for Counterterrorism, Crime Fighting, and Defence VIII, 85460X (30 October 2012); doi: 10.1117/12.974574
Show Author Affiliations
Rhea J. Clewes, Defence Science and Technology Lab. (United Kingdom)
Chris R. Howle, Defence Science and Technology Lab. (United Kingdom)
David J. M. Stothard, Univ. of St. Andrews (United Kingdom)
Malcolm H. Dunn, Univ. of St. Andrews (United Kingdom)
Gordon Robertson, M Squared Lasers Ltd. (United Kingdom)
William Miller, M Squared Lasers Ltd. (United Kingdom)
Graeme Malcolm, M Squared Lasers Ltd. (United Kingdom)
Gareth Maker, M Squared Lasers Ltd. (United Kingdom)
Rick Cox, DeltaNu, Inc. (United States)
Brad Williams, DeltaNu, Inc. (United States)
Matt Russell, DeltaNu, Inc. (United States)

Published in SPIE Proceedings Vol. 8546:
Optics and Photonics for Counterterrorism, Crime Fighting, and Defence VIII
Colin Lewis; Douglas Burgess, Editor(s)

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