Sara Wallin

Laser-based standoff detection of explosives: a critical review

AbstractA review of standoff detection technologies for explosives has been made. The review is focused on trace detection methods (methods aiming to detect traces from handling explosives or the vapours surrounding an explosive charge due to the vapour pressure of the explosive) rather than bulk detection methods (methods aiming to detect the bulk explosive charge). The requirements for standoff detection technologies are discussed. The technologies discussed are mostly laser-based trace detection technologies, such as laser-induced-breakdown spectroscopy, Raman spectroscopy, laser-induced-fluorescence spectroscopy and IR spectroscopy but the bulk detection technologies millimetre wave imaging and terahertz spectroscopy are also discussed as a complement to the laser-based methods. The review includes novel techniques, not yet tested in realistic environments, more mature technologies which have been tested outdoors in realistic environments as well as the most mature millimetre wave imaging technique. FigureStandoff detection and identification is one of the most wanted capabilities

Explosives standoff detection using Raman spectroscopy: from bulk towards trace detection

This paper gives a brief overview on our latest progress in the area of standoff detection. Standoff Raman measurements from 200 m and 470 m distance have been performed on bulk amounts of TATP and AN respectively, the former through a double sided window, the latter under heavy rain. Resonance Raman measurements on TNT, DNT and NM vapors in the ppm concentration regime are presented, showing resonance enhancement in the range of 2 200 (NM) to 57 000 (TNT) as compared to 532 nm Raman cross sections. Finally, the application of hyper spectral Raman imaging is described, exemplified by the resolution of four different samples (sulphur, AN, DNT, and TNT) in the form of 5 mm wide discs in one single image.

Resonance-Enhanced Raman Spectroscopy on Explosives Vapor at Standoff Distances

Resonance-enhanced Raman spectroscopy has been used to perform standoff measurements on nitromethane (NM), 2,4-DNT, and 2,4,6-TNT in vapor phase. The Raman cross sections for NM, DNT, and TNT in vapor phase have been measured in the wavelength range 210–300 nm under laboratory conditions, in order to estimate how large resonance enhancement factors can be achieved for these explosives. The results show that the signal is enhanced up to 250,000 times for 2,4-DNT and up to 60,000 times for 2,4,6-TNT compared to the nonresonant signal at 532 nm. Realistic outdoor measurements on NM in vapor phase at 13 m distance were also performed, which indicate a potential for resonance Raman spectroscopy as a standoff technique for detection of vapor phase explosives. In addition, the Raman spectra of acetone, ethanol, and methanol were measured at the same wavelengths, and their influence on the spectrum from NM was investigated.

Classification of Raman Spectra to Detect Hidden Explosives

Raman spectroscopy is a laser-based vibrational technique that can provide spectral signatures unique to a multitude of compounds. The technique is gaining widespread interest as a method for detecting hidden explosives due to its sensitivity and ease of use. In this letter, we present a computationally efficient classification scheme for accurate standoff identification of several common explosives using visible-range Raman spectroscopy. Using real measurements, we evaluate and modify a recent correlation-based approach to classify Raman spectra from various harmful and commonplace substances. The results show that the proposed approach can, at a distance of 30 m, or more, successfully classify measured Raman spectra from several explosive substances, including nitromethane, trinitrotoluene, dinitrotoluene, hydrogen peroxide, triacetone triperoxide, and ammonium nitrate.

Raman spectra of P4 at low temperatures

Raman spectra of solid P4 have been recorded from 12 K up to room temperature using Fourier transform-Raman spectroscopy. The three Raman-active modes corresponding to Td symmetry have been resolved, and line shifts depending on temperature and matrix environment have been measured. Two phase transitions have been observed at T≈80 K (irreversible) and T≈195 K (reversible) corresponding to the γ→β and β↔α transitions, respectively. In the β phase, the ν2 mode is split into two lines, νsplit≈7.7 cm−1. The splitting of the ν2 mode is an indication of a breaking of Td symmetry for the β phase. A matrix shift of ≈3 cm−1 for P4 in a N2 matrix (1:5) was measured. Comparing experimental transitions of pure P4 with literature values on matrix isolated P4 in N2 (1:1000) we can conclude that there is a matrix shift in nitrogen of between 6 and 9 cm−1 depending on vibrational mode. The line positions found for pure P4 in the γ phase at 12 K are ν1=599.8 cm−1, ν2=361.6 cm−1, and ν3=459.0 cm−1. The corresponding values...

Possibilities for standoff Raman detection applications for explosives

This paper provides a brief overview of the Raman-based standoff detection methods developed at FOI for the purpose of standoff explosives detection. The methods concerned are Raman imaging for particle detection and Resonance Enhanced Raman Spectroscopy for vapor detection. These methods are today reaching a maturity level that makes it possible to consider applications such as trace residue field measurements, on site post blast analysis and other security of explosives related applications. The paper will look into future possible applications of these technologies. Our group has extensive activities in applications of the technology, among others in projects for the Seventh Framework Program of the European Union. Some of these possible applications will be described and a look into future development needs will be made. As far as possible, applicability will be discussed with a view on realistic explosives trace availability for detection. Necessary data to make such realistic applicability assessment is not always available and a brief discussion on the applicability of using the developed Raman technology to obtain this kind of data will also be made. The aspects of transitioning from research to practical applications, considering also eye-safety of the system, will be discussed as well.

Stand-off detection of vapor phase explosives by resonance enhanced Raman spectroscopy

Stand-off measurements on nitromethane (NM), 2,4-DNT and 2,4,6-TNT in vapor phase using resonance Raman spectroscopy have been performed. The Raman cross sections for NM, DNT and TNT in vapor phase have been measured in the wavelength range 210-300 nm under laboratory conditions, in order to estimate how large resonance enhancement factors can be achieved for these explosives. The measurements show that the signal is greatly enhanced, up to 250.000 times for 2,4-DNT and 60.000 times for 2,4,6-TNT compared to the non-resonant signal at 532 nm. For NM the resonance enhancement enabled realistic outdoor measurements in vapor phase at 13 m distance. This all indicate a potential for resonance Raman spectroscopy as a stand-off technique for detection of vapor phase explosives.

Detection limit of imaging Raman spectroscopy

Multispectral imaging Raman spectroscopy is a novel technique for detecting and identifying explosive residues, e.g. explosives particles which are left on surfaces after handling or manufacturing of explosives. By imaging a suspect surface using the imaging Raman technique, explosives particles at stand-off distances can be identified and displayed using color coding1. In this paper we present an attempt to determine a limit of detection for imaging Raman spectroscopy by analyzing holes of various sizes in aluminum plates filled with four different substances; 2,4-dinitrotoulene (DNT), ammonium nitrate (AN), sulfur, and 2,4,6-trinitrotoulene (TNT). The detection time in the presented experiments has not been optimized, instead more effort has been invested in order to reduce false alarms. The detection system used is equipped with a green second harmonic Nd:YAG laser with an average power of 2 W, a 200 mm telescope and a liquid crystal tunable filter to scan the wavenumbers. The distance to the target was 10 m and the imaged area was 28 mm × 28 mm. The measured multi-spectral data cubes were evaluated using least square fitting to distinguish between DNT, AN,S, TNT and the background. The detection limit has been determined to be sub microgram using the current setup.

Localisation of threat substances in urban society - LOTUS: a viable tool for finding illegal bomb factories in cities

Results of dispersion experiments and dispersion modelling of explosives, drugs, and their precursors will be presented. The dispersion of chemicals evolving during preparation of home made explosives and a drug produced in an improvised manner in an ordinary kitchen has been measured. Experiments with concentration of hydrogen peroxide have been performed during spring and summer of 2009 and 2010 and further experiments with concentration of hydrogen peroxide, synthesis and drying of TATP and Methamphetamine are planned for the spring and summer of 2011. Results from the experiments are compared to dispersion modelling to achieve a better understanding of the dispersion processes and the resulting substances and amounts available for detection outside the kitchen at distances of 10-30 m and longer. Typical concentration levels have been determined as a function of environmental conditions. The experiments and modelling are made as a part of the LOTUS project aimed at detecting and locating the illicit production of explosives and drugs in an urban environment. It can be concluded that the proposed LOTUS system concept, using mobile automatic sensors, data transfer, location via GSM/GPS for on-line detection of illicit production of explosive or precursors to explosives and drugs is a viable approach and is in accordance with historical and todays illicit bomb manufacturing. The overall objective and approach of the LOTUS project will also be presented together with two more projects called PREVAIL and EMPHASIS both aiming at hindering or finding illicit production of home made explosives.

Time-of-flight mass spectrometry for explosives trace detection

This paper presents the ongoing development of a laser ionization mass spectrometric system to be applied for screening for security related threat substances, specifically explosives. The system will be part of a larger security checkpoint system developed and demonstrated within the FP7 project EFFISEC to aid border police and customs at outer border checks. The laser ionization method of choice is SPI (single photon ionization), but the system also incorporates optional functionalities such as a cold trap and/or a particle concentrator to facilitate detection of minute amounts of explosives. The possibility of using jet-REMPI as a verification means is being scrutinized. Automated functionality and user friendliness is also considered in the demo system development.