ABSTRACT. In the wake of the recent terrorist attacks, such as the 2008 Mumbai hotel explosion or the. December 25th 2009 âunderwear bomberâ, our group ...
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Detection of Explosives by HyperSpectral Differential Reflectometry Thierry Dubroca and Rolf E. Hummel MRS Proceedings / Volume 1405 / 2012 DOI: 10.1557/opl.2012.225
Link to this article: http://journals.cambridge.org/abstract_S1946427412002254 How to cite this article: Thierry Dubroca and Rolf E. Hummel (2012). Detection of Explosives by HyperSpectral Differential Reflectometry. MRS Proceedings,1405, mrsf111405y0302 doi:10.1557/opl.2012.225 Request Permissions : Click here
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Mater. Res. Soc. Symp. Proc. Vol. 1405 © 2012 Materials Research Society DOI: 10.1557/opl.2012.225
Detection of Explosives by Hyper-Spectral Differential Reflectometry Thierry Dubroca1 and Rolf E. Hummel1 1
Department of Material Science and Engineering, University of Florida, Rhines Hall, Gainesville, Florida 32611-6400 ABSTRACT In the wake of the recent terrorist attacks, such as the 2008 Mumbai hotel explosion or the December 25th 2009 “underwear bomber”, our group has developed a technique (US patent #7368292) to apply differential reflective spectroscopy to the problem of detecting explosives in order to detect terrorist threats. Briefly, light (200-500 nm) is shone on a surface such as a piece of luggage at an airport or a parcel at a courier distribution center. Upon reflection, the light is collected with a spectrometer combined with a camera. A computer processes the data and produces in turn a differential reflection spectrum taken between two adjacent areas of the surface. This differential technique is highly sensitive and provides spectroscopic data of explosives. As an example, 2,4,6, trinitrotoluene (TNT) displays strong and distinct features in differential reflectograms near 420 nm. Similar, but distinctly different features are observed for other explosives such as RDX, PETN or ANFO. Our detection system uses a two dimension detector (CCD camera) which provide spatial and spectroscopic information in each of the two dimensions. By scanning (involving fixed optical equipment and scanning moving bags or parcels on a conveyor belt), the surface to be surveyed the system provide the spatial location of the potential threat. We present in this paper how our detector works and how it is applied to the problem of explosive screening for explosives at airports and mail sorting centers. Additionally, we will present the effect of the explosives morphology on the detection response. In particular we will evaluate the implication on the limit of detection of the instrument as well as discuss the sample morphology with respect to a realistic threat scenario. INTRODUCTION Differential reflectometry is a powerful optical technique, which is capable of detecting small amounts (that is, traces) of explosive materials[1-6]. This standoff technique can be used at airport baggage checking areas, mass transit facilities, and parcel screening centers at commercial shipping companies. One of our thrust areas is to investigate which parameters have an influence on the differential reflectograms of explosives, such as the electron configurations of the molecules, environmental influences, reproducibility, contamination of the substances, possible influences of taggants, differentiation between explosives and harmless substances, distinction between different types of explosives, possible common features of various explosives, sensitivity of the technique, possible variations of mixtures of different explosives, and influences of substrates (plastics, metals, canvas etc.). Moreover, one of our goals is to understand the mechanisms which cause the differential reflectograms of explosives by applying molecular model calculations. The second thrust area is geared towards the development of a fast and sensitive prototype differential reflectometer (DR) for practical applications at airport luggage screening stations, or parcel shipping centers, and similar places which allow us to arrive at practical results in a few seconds without the involvement of an operator and human judgment. Our optical screener is safe for human eyes (no laser beam), and the human body (no x-rays). Furthermore, the
screening does not invade the privacy of humans. We envision that our instrument would be eventually used in tandem with common x-ray machines (which detect predominantly metals) or as stand-alone detectors. The DR applies a conveyor belt, a broad-band light source, a spectrometer, and a chargecouple device (CCD) camera (Figure 1). Two reflectivities R 1 and R 2 are taken consecutively of the sample to be scanned as a consequence of the moving belt. The normalized difference between the two sample areas (ΔR/ R ) for a large range of wavelengths (200 to 500nm) is measured in 1 nm intervals, whereby ΔR=R 2 -R 1 is the difference of the reflectivities of the two sample parts, and R = (R 1 +R 2 )/2 is the average reflectivity. Measuring R 1 and R 2 at nearly the same time and forming the ratio ΔR/ R eliminates possible influences from fluctuations of the line voltage. It also eradicates spectral effects including intensity variations of the light source, the spectral sensitivity of the detector, the spectral reflectivities of the mirrors or the substrate, and it minimizes the influence of ambient light. ΔR/ R =f (λ), is essentially proportional to the optical absorption of light.
Figure 1. Schematic Representation of the Differential Reflectometry (DR) explosives detection system (left) and actual prototype (right). Note the light band on the suitcase. RESULTS AND INTERPRETATIONS Common Shape of Differential Reflectograms of Explosive Substances. Figure 2 displays differential reflectograms of various, selected explosives. It is observed that characteristic shoulders are present. For example, TNT (2,4,6 trinitrotoluene) is distinguished by a shoulder, having a mid-point near 420 nm, whereas other explosives display shoulders which are blue shifted into the ultraviolet (UV) region. The different structures are suspected to be due to different molecular structures of the pertinent explosives.
Nitromethane Diesel ANFO Ammonium nitrate Urea nitrate Kerosene
Figure 2. Selected Differential reflectograms of various explosive materials. The individual curves have been vertically shifted for clarity. Influence of spatial differential reflectance parameters on TNT absorption spectra Investigations were conducted to elucidate possible influences of the angle of incidence and other spatial parameters on the differential reflectograms. It was found that the 420 nm feature used to identify TNT in a differential reflectogram stems mainly from the diffuse component of the reflectivity. The specular component is of less importance for the explosives detection; indeed, it just decreases the amplitude of the feature and thus, the sensitivity of the device. Furthermore, the spectral "finger-print" does not change by varying these parameters. The maximum sensitivity of differential reflectometry is obtained for samples having a strong diffusivity and weak specularity. This condition is, as a rule, predominant in actual field applications for explosives detection. Reproducibility of differential reflectograms for TNT and harmless substances Figure 3 demonstrates the good reproducibility of differential reflectograms utilizing four independent measurements. Figure 3 also shows that some harmless substances (flour, carbon pad, sweetener, and salt) which look like TNT do not display the characteristic shoulder which distinguishes TNT from inert materials. More results on harmless substances can be found in reference[1].
Figure 3. Differential reflectograms of TNT and some harmless substances. Four independent measurements were taken to demonstrate reproducibility of the curves. The curves of the individual substances have been vertically shifted for clarity. Limit of detection (LOD) for explosives in spectroscopic differential reflectometry The LOD is defined as being the mass which provides a detection signal equal to three times the standard deviation of the noise level of the system being used to detect a substance. Using a differential reflection spectrum of TNT we define the detection signal being the height of the 420 nm structure (Figures 2 and 3). In order to vary the sample mass and therefore the detection signal, a TNT sample was prepared in a triangular shape (see Figure 4, inset) such that, when scanned with the DR, a series of spectra was acquired simultaneously while the spatial position across the sample was recorded. Note, that the amount of TNT decreases in a linear fashion as a function of the spatial position. Figure 4 displays the recorded spectra of TNT for the different sample masses. It is observed that the signal of the 420 nm structure height decreases with decreasing sample mass. It was found that the LOD is in the hundreds of nano-grams range. We anticipate that this result can be improved to reach single digit nano-gram range.
Figure 4. Differential reflectograms of TNT as a function of sample mass. Inset: Sample configuration displaying decreasing sample masses in the vertical direction.
In another approach, DR-data were collected utilizing a "sliding window" technique, (Figure 5), which facilitates the measurement of DR spectra by scanning the light across two samplehalves in various proportions. Specifically, the largest signal is obtained when the two sample halves (consisting for example of TNT and a black carbon pad, respectively) have identical sizes. By varying the proportions (X) between explosive and non-explosive areas, the smallest area can be determined at which a signal is still observable. From there, the smallest discernable mass can be calculated. A LOD, similar as above (hundreds of nano-grams range) was obtained. Y = 2.1 mm TNT X Carbon Pad
Scanned area (top part)
A B
Scanned area (bottom part)
C
Figure 5. Sliding window method for measuring the Limit of Detection. The Differential Reflectometer beam positions on the test sample are indicated by dashed, blue lines. Sketch A displays the DR beam for which the beam is covering TNT (R 1 ) then the beam covers a carbon pad (R 2 ). Sketch B displays a case where the light beam covers mostly the carbon pad. Sketch C displays the case where the light beam covers the entire carbon pad. X is the length of the beam which covers the TNT area. Diffuse Component In order to elucidate the effect of the surface structure, we used five otherwise identical TNT samples on aluminum substrates (H1, ..., H5) having five different degrees of surface roughness. The latter is accomplished by polishing TNT, utilizing polishing cloths of different grit sizes. Figure 6 depicts the diffuse reflectivity ( ) as a function of wavelength for these samples (whereby the angle of incidence is set >20° to eliminate specular reflection). It is observed that all curves display the characteristic absorption structure near 420 nm as known for TNT (Figure 2). It is further observed that the shape of the diffuse components as a function of wavelength is an inherent property of TNT and does not depend on the morphology, that is, on the surface roughness of the sample. In other words, the spectral "finger-print" which uniquely identifies TNT in a differential reflectogram is morphology- independent. For comparison, polished aluminum is also shown to demonstrate the absence of the 420 nm structure for this case.
Figure 6. Reflectivity of TNT taken at a large angle of incidence (>20 degrees) for various degrees of surface roughness. (The larger the grit sizes of the polishing cloth, the smaller the roughness). For comparison, the substrate diffuse reflectivity (aluminum) is also shown. Characteristic TNT differential reflection spectra on various substrates Explosives are generally attached to a substrate, such as a suitcase, or a paper box. Pure substrates have been scanned with the DR and then a small amount of TNT has been attached to their surface. None of the substrate displayed the characteristic differential reflectogram known for TNT, whereas all samples consisting of TNT on those substrates showed the reflectogram known from Figure 2. Among the substrates used were felt, velvet, cotton, nylon, eyeglass wipe, canvas, Elmer's glue (dry), Kim wipe, Ziploc bag, business card, dielectric ceramic, black garbage bag, synthetic leather, natural leather, aluminum foil, and particle board. CONCLUSIONS Differential reflectometry has been shown to identify traces of explosive substances on surfaces having sizes as small as a few hundreds of nano-grams. The technique allows differentiating the spectra of various explosives which are identified by characteristic shoulders in differential reflectograms in the blue and UV spectral region. Harmless substances do not possess these characteristic shoulders. This standoff technique is fast (few seconds response time), safe (no laser beam or x rays), and does not need human judgment for identification of explosives. REFERENCES 1. R. E. Hummel, A. M. Fuller, C. Schoellhorn, and P. H. Holloway, Appl. Phys. Lett. 88, 231903 (2006). 2. R.E. Hummel, A.M. Fuller, C. Schoellhorn, and P.H. Holloway, in Trace Chemical Sensing of Explosives, edited by R. L. Woodfin, John Wiley N.Y. (2007) Chapter 15, page 301. 3. A. M. Fuller, R. E. Hummel, C. Schoellhorn, and P. H. Holloway, Proceedings SPIE Optics East 6378 (2006). 4. C. Schoellhorn, A. M. Fuller, J. Gratier, and R. E. Hummel, Appl. Optics 46, 6232, (2007). 5. C. Schoellhorn, A. M. Fuller, J. Gratier, and R. E. Hummel, SPIE Defense and Security proceedings 6554 (2007). 6. A. Fuller-Tedeschi and R. E. Hummel, J. Appl. Phys. 107, 114902 (2010).
