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Existing and Potential Standoff Explosives Detection Techniques 7 Unexploited Potential Bases of Detection As part of the charge of this committee, committee members undertook the development of novel concepts for explosives detection. “Novel” here refers to the fact that the idea appears not to have been exploited for explosives detection, rather than implying that the idea is new. Each of these ideas is presented in a separate section below, with the purpose of identifying future research directions. DYNAMICAL BEHAVIOR OF AN EXPLOSIVE VAPOR PLUME Hypothesis and Concept An understanding of the dynamical behavior of the vapor plume emanating from an explosive will assist the development of devices for detecting explosives at a distance via their molecular properties, namely, their spectroscopic properties.1 This understanding should answer questions such as: What is the shape of the plume? What is its behavior over time? How is it affected when one introduces airborne dust particles, surfaces of different composition, and air currents? A way to study the plume behavior is to image it. Two possible “imaging” approaches to this problem are the ultraviolet (UV) imaging of the explosive and schlieren photography. For the former, nitrogen-based explosives have a very strong absorption band in the short-wave UV due to 1 Additional detail on the explosive vapor plume is given in Chapter 4.
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Existing and Potential Standoff Explosives Detection Techniques the presence of aromatic rings.2 Therefore, explosive vapors, if they are dense enough, should look dark when viewed with a UV image intensifier. A UV reflective background would be necessary to enhance the contrast. Schlieren photography, on the other hand, is based on vapor density differences, the image being formed by refraction and scattering from areas of varying refractive index. The evaporation of acetone from a container or the heat waves emanating from a human body are clearly “seen.” The question is whether or not the current schlieren technology is sensitive enough to detect the vapors coming off an explosive. Novelty The imaging of an explosive vapor plume has not been done. The closest we have is the theoretical and semitheoretical work of Dr. Thomas A. Griffy of the University of Texas at Austin.3 Although this work considered the diffusion of explosive vapors emanating from a block of explosive, the more useful results are what happen at the end. Griffy showed that most of the vapor being emitted by a nitrogen-based explosive ends up adhering to surfaces; there is hardly any that is airborne. His results help explain why swabbing surfaces is much more effective in gathering explosive residue (most of which is from contamination) than sampling the air. Relevance to the Committee’s Mission The mission of the committee is to generate ideas for the standoff detection of explosives. If standoff detection technology is used that probes the spectroscopic properties of the explosive, it would be helpful to know how much explosive vapor is airborne around an explosive mass and how much is adhered to nearby surfaces. Furthermore, if most of it is adhered, it would be useful to know how readily it can be dislodged from a distance. Clearly, understanding the “physics of the plume” is critical for designing and using spectroscopy-based detection technology. Spectroscopy-based standoff detection techniques that would rely on the plume behavior include remote sensing and remote imaging optical techniques. Among these are those that use LIDAR ([laser] light detection 2 Yinon, J.; Zitrin, S. The Analysis of Explosives, In Ultraviolet and Visible Spectroscopy. Pergamon Press: 1981, Chapter 10, pp 141-153. 3 Griffy, T.A. A model of explosive vapor concentration II. In Advances in Analysis and Detection of Explosives, Proceedings of the Fourth International Symposium on Analysis and Detection of Explosives, September 7-10, 1992, Jerusalem, Israel, J. Yinon, Ed.; Kluwer Academic Publishers: Dordrecht, Netherlands, 1992, pp 503-511.
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Existing and Potential Standoff Explosives Detection Techniques and ranging], DIAL (differential absorption LIDAR), and DIRL (differential reflectance LIDAR). These involve laser illumination usually in the infrared (IR) region, but there are also multi- and hyperspectral imaging and sensing techniques that operate in this and other spectral regions. All of these optical techniques operate on the principle of radiation (from an illuminating source) being absorbed by the target material and detecting the attenuated backscattered radiation. At present, there exists a dual spectral explosive imaging system designed to detect adhered particles of a particular nitrogen-containing explosive. This system uses solar illumination for viewing. Existing Technology Related to This Idea At present, the only two technologies known to us that address imaging the dynamic behavior of the plume are the two mentioned above, namely, UV imaging and schlieren photography.4 Advantages For imaging the explosive vapor plume, we would ideally like to see a technique that reveals the presence of any explosive material within its view. One common way to emphasize a target within the viewing field of an imager is to colorize it, giving it strong visual contrast. A major advantage of exploring the imaging idea is the ability to view in real time the dynamics (time behavior) of the sublimation-diffusion process. Disadvantages Based on the work of Griffy, we expect to see less airborne explosiveladen particles and more adhered explosive material on surfaces. By adhered explosive materials is meant adsorbed vapors, adsorbed explosiveladen particles, and adhered explosive particles or residue. Therefore, there may be no advantage to studying the dynamics of the vapor plume if the explosive is in a closed environment where almost all of the emitted explosive vapor is adhered to surfaces. 4 Settles, G.S. Schlieren and Shadowgraphs Techniques: Visualization Phenomena in Transparent Media, Springer-Verlag: 2001, 390 pp.
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Existing and Potential Standoff Explosives Detection Techniques Research Needed to Demonstrate Feasibility To gain an understanding of the vapor plume, research that focuses on imaging these plumes is necessary. Experiments would be needed that reveal the dynamic behavior of a vapor plume, from its origin at the surface of the explosive to its final destination on surfaces, under idealized laboratory conditions. Additional research should be extended to outdoor conditions, including the behavior of the plume from a moving target such as a person on foot or a moving vehicle. Furthermore, most “real-world” cases are not at the elevated temperatures that might be necessary in these experiments for imaging the plume. Thus, attempts should be made to extrapolate the findings to anticipated environmental conditions. Additional research on the vapor plume would model vapor generation, transport, adsorption to airborne particles, natural convection, and a moving versus stationary source; experimental testing of the model through measurements of vapor concentration versus time and spatial position would also be needed. In order to perform standoff measurement of plume characteristics, schlieren photography and other techniques to detect chemical information about a plume should be utilized to detect chemical signatures of explosives. DETECTION OF A SUICIDE BOMBER’S LOCAL ATMOSPHERE—ANOMALIES IN THE ATMOSPHERIC ION BACKGROUND Hypothesis and Concept Would the tendency for high explosives containing electronegative nitro or peroxide groups to undergo electron attachment cause depletion in negative ions around a person carrying concealed explosives as he or she walks through the background ion field? It is well known that the atmosphere contains background positive and negative ions.5 The ambient atmospheric ion concentration is 250 positive ions and 200 negative ions per cubic centimeter. A net current of 2000 A flows from the atmosphere to the Earth’s surface. One highly speculative approach would be to image the ion charge aura in a surveillance area and see whether anomalies could be correlated with the presence of explosives. Although the approach is unlikely to be highly specific, it may be useful as part of a systems design using orthogonal sensors. Several issues would have to be explored. 5 Wahlin, L. Atmospheric Electrostatics; John Wiley & Sons: New York, 1989, 130 pp.
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Existing and Potential Standoff Explosives Detection Techniques Novelty To the committee’s knowledge, such a detection approach has not yet been explored. There are several possibilities for measurement. The ion density of air can be measured and tracked with inexpensive ($60) electronics based on the Gerdien Cylinder.6 However, this would require sampling at many points over an area under surveillance. Another possibility would be to measure the variation in the 4 × 10−12 A/m2 current that flows between the atmosphere and Earth’s surface because of the excess positive ions in the atmosphere and compensating negative charge at the Earth’s surface. This would be a very challenging measurement, where new research ideas could be useful. Ideally, one would prefer to image the electric field distribution in a surveillance area. The average “fair-weather field” strength at the Earth’s surface is 100 V/m. Relevance to the Committee’s Mission This approach might provide a method of broad-area surveillance for concealed explosives, which would be nonhazardous to subjects being examined. Existing Technology Related to This Idea Measurable perturbation of the background ion concentration has been claimed in human breath.7 It may be possible to amplify the atmospheric ion background for improved detection sensitivity at an entry portal through the use of van de Graff generators or some other method. This would provide a higher density of ions, if the rate of charge attachment is a limiting factor. Advantages This technology represents a nondestructive and noninvasive method for broad-area surveillance to detect concealed explosives. Challenges Background variations with altitude, solar wind strength, and electrical storms will be significant; however, these effects would be expected to 6 Carlson, S. Counting atmospheric ions, Scientific American, September 1999, 96-97. 7 Suchanowski, A.; Wiszniewski, A. Changes in ion concentration in the air during the breathing process of a human being. Polish Journal of Environmental Studies 1999, 8(4), 259-263.
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Existing and Potential Standoff Explosives Detection Techniques be homogeneous over a given area being monitored. Metal objects may also provide an impediment. Research Needed to Demonstrate Feasibility Measurement sensitivity and detection specificity are expected to be the most serious issues in testing this new approach. Detailed knowledge of the chemical distribution of component ions present in the atmosphere, and their role in any variations detected, should be defined. Although these experiments would be more technically challenging, they would provide understanding of the origin of any observed effects that might aid in rational design of the method. DETECTION BY DETONATION Hypothesis and Concept The goal of this approach is to detect the presence of an explosive by remotely detonating it at a safe standoff distance. Detonation takes place by adding sufficient energy to the explosive that the exothermic reaction initiates. Alternatively, energy is added to the explosive or allied circuitry by one of several possible means: radio frequency (RF) excitation, acoustic shock, focused millimeter wavelength radiation, and perhaps others. For example, RF radiation will excite a resonance in some metallic structure in a bomb (wires, detonator, etc.). One can find dimensional resonances by sweeping the RF frequency at each point of focus during some (relatively short) dwell time. Novelty Coupling chemistry and remote energy is common (consider microwave ovens), but applying this general idea to the initiation of an explosive is novel.8 Detonation by coupling the electronics or wires of a detonator in a bomb using remote energy is directly novel. Relevance to the Committee’s Mission Detection is accomplished using relatively simple physical devices. Issues of background noise and other interferent confusions are entirely 8 Won, I. J.; Keiswetter, D. Electromagnetic Induction Spectroscopy. In Detection and Remediation Technologies for Mines and Minelike Targets III, A. C. Dubey, J. F. Harvey, and T. Broach, Eds. Proceedings of SPIE 1998, 3392, 14-21.
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Existing and Potential Standoff Explosives Detection Techniques finessed. Sensitivity of the system is high for those explosives that couple to the delivered energy. Sensitivity barriers can be overcome in other explosives (those that do not couple) with greater power. Existing Technology Related to This Idea Some consideration has been given to this technology during research for advanced general luggage scanning (now being performed by X-ray and computer tomography [CT]). Stealth technology modeling (absorption of RF) could provide design guidance for these systems. Microwave oven technology and microwave curing of composites may provide insights. RF induction is a common process. Advantages The approach has a dual effect: the explosive is detected and destroyed, and the human bombers are deterred, because they do not reach their intended target. From an information point of view, other schemes work to return substantial information to the detector and identifier. This approach returns only one bit: explosion yes or no. This system has merit for military checkpoint scenarios. Standoff detonation is quite possible with both millimeter wavelength radiation and RF radiation. Both millimeter wavelength and RF radiation (typically at frequencies of 1 GHZ or less) can be transported with transmission lines and focused with phased array antennas. Challenges Any of the methods in this approach transfer energy to the explosive or possibly to the detonation hardware. A reasonable impedance match between explosive, or detonation hardware, and incoming energy is necessary to minimize the extent of the energy source. Various explosives admit to different means of energetic excitation, so this system may be somewhat selective both in terms of explosives detonated and in terms of energy source specifics. For example, shock-resistant explosives would essentially not be detonated by acoustic energy. Several of the methods are not appropriate for human exposure. For example, relatively high-energy RF exposure is excessively painful. Essentially any method of energy delivery would require focusing to accomplish standoff detection.
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Existing and Potential Standoff Explosives Detection Techniques Research Needed to Demonstrate Feasibility Coupling of energy source and classes of explosives of interest is one research focus. Energy source wavelength and physics, focusing, directing, intensity, and cost of generation will all be of interest. Focused millimeter wavelength electromagnetic radiation and focused radio-frequency energy are of particular interest. Low observable research in the millimeter wave regime could serve as a basis for this work. Similarly, radiation effects laboratories have considerable experience in remotely energizing structures and circuits. Scenarios for using this technology are limited to certain kinds of military checkpoint situations. Understanding interaction between energy sources and intensities, and the human body, will be important to expanding the use of this technology. Remotely causing current flow, or sparking, in wires associated with electrical explosive detonators may require a measure of research study as well as engineering. Hyperspectral radiation may be used for greater sensitivity in initiating detonation if the explosive is simultaneously responsive to several frequencies. Hyperspectral radiation may also expand the range of explosives affected by exposure, because different explosive classes are sensitive to different energy wavelengths. DETECTION BY SELF-REPORTING SENSORS Hypothesis and Concept The goal of this approach is to detect the presence of explosives using remote standoff mine technologies such as neutron activation analysis. The idea is to distribute small sensors that are silent until they reach a critical threshold of detection. Each device can have multiple sensors. Once a threshold is crossed, the device activates and sends a signal that locates it in a physical space. If the sensors are spread out, the path to the explosive can be tracked at a rate dependent on the rate at which the detectors can determine the presence of explosive material. The key element of this idea is to use vast numbers of detectors that need not be probed but report back only when a certain threshold is reached. Novelty This approach couples detection technologies with the need for area surveillance. The standoff mine technology requires energy sources that last for long periods of time and are activated by motion. This concept
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Existing and Potential Standoff Explosives Detection Techniques would couple microfluidic detection technologies with reporting systems that are location sensitive. It does not require continuous interrogation of sensors but provides opportunities for large numbers of sensors to be distributed overa large area. Relevance to the Committee’s Mission Detection is accomplished using proximal probes (i.e., probes that must be close to explosive), but this provides opportunity for wide-area detection. Existing Technology Related to This Idea Microfludic technologies are being developed based on chemically specific methods. The limitation of these technologies lies in the need to get close to explosive. The concept being suggested here is to marry this with communication systems where small probes can be distributed widely, have a long working life, and report in when threshold levels are crossed. Advantages This approach has the advantage of allowing detection systems to be distributed easily (dropping or gluing down at random or strategic locations for a big event, e.g., a Super Bowl Game). The device will be needed only for a short period of time, and when not needed the listening device is simply turned off. This system has applications is wide-area surveillance. Challenges This method relies on the sensitivity and speed of measurements of microdetectors. If the microdetectors cannot measure the trace of the explosive, the distribution of sensors will not be useful. Making the detectors and responding units requires miniaturization of sensors. Research Needed to Demonstrate Feasibility Research and demonstration of reliable detection sensors with the required sensitivity and accuracy for use in these devices is required. Research is also needed to couple microfluidic detectors with radio-frequency transmission systems that have location sensitivity.
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Existing and Potential Standoff Explosives Detection Techniques STANDOFF COMPTON BACKSCATTER X-RAY IMAGING Hypothesis and Concept Using low-energy x-rays, the target is illuminated and Compton backscatter photons are collected that are subsequently emitted from the target.9 Photomultipliers detect light flashes in plastic that result from the backscatter photons. Standoff capability results from locating the detector near the target (e.g., in the ground) or from using a relatively large detector near the source X-ray emitter. The image is assembled by scanning the X-rays over the target and detecting in synchronization the backscattered photons. Backscattered photons are produced relatively efficiently by substances of low atomic number. Novelty The novelty of this approach is in using a large detector or standoff placement of the detector very near a checkpoint while keeping emitter and electronics at a distance. Furthermore, low-energy X-rays are quite effective for this application. In fact, higher-energy X-rays will produce increasingly less response. Relevance to the Committee’s Mission This is an imaging technology that favors explosive-like low effective atomic number materials. The result is an image of the interior of the target with reasonable resolution and highlighted explosives. This would be particularly significant for the military checkpoint challenge. Existing Technology Related to This Idea A number of companies make one-side Compton backscatter scanning systems (the emitter and detector can be placed near one another, in contrast to transmission X-ray approaches). Modeling of the Compton backscatter effect, leading to design of an effective system, can be undertaken using standard codes such as MCDP. Lawrence Livermore National Laboratory has a group advertising this capability, for example. 9 Additional information on X-ray backscatter is presented in Chapter 5.
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Existing and Potential Standoff Explosives Detection Techniques Advantages Backscatter systems produce an image from X-rays that are scattered back from the screened object toward the source (and not only transmitted as in the standard machines described above). Because low-Z (low Zeff, the effective atomic number) materials are more efficient at scattering X-rays, explosive-like materials are contrasted more—they stand out clearly in the backscatter image, while they are often barely visible in a transmitted image (low contrast). Metals, of course, are highly visible in the transmitted image. The low-energy X-rays mean that target exposure to higher-energy photons is minimized. As to shielding, this class of backscatter interaction is possible to about 1/2 inch of aluminum and about 6 to 14 inches of carbon epoxy composite material. In dual systems (transmission and backscatter), a quantitative measure of the backscattered X-rays, together with absorption measurement, provides information that can help separate the effects of density and Zeff, in order to identify high-density, low-Zeff materials (the signature of explosives). Challenges This approach is fundamentally limited by the number of backscatter photons detected. Backscatter photons emit essentially in all directions from the target. Detecting enough of the photons to assess the target’s structure requires a relatively large sheet of plastic in terms of solid angle or a modest one close to the target. The system will not produce a particularly high-resolution image of the target. Resolution would be increased by more photons, but not in a linear manner. Also, X-rays penetrate deeply into the target, so backscatter emission can happen anywhere in depth. The image is just a kind of integration in depth. Research Needed to Demonstrate Feasibility If modeling of Compton backscatter shows promise for standoff, such a device should be built and assessed. Modeling of the system to guide its design should accompany the implementation planning stages. Both small portable systems and standoff systems, especially those that are checkpoint based or portal or tunnel based, should show promise.
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Existing and Potential Standoff Explosives Detection Techniques DISTRIBUTED BIOLOGICAL SENSORS Hypothesis and Concept Could bees, moths, or butterflies be trained on bombers’ biomarkers such as those outlined in Chapter 6? Could rat packs trained on explosives or drugs be fitted with a wireless global positioning system (GPS), a bioluminescent reporter gene, and a microphotocell and released in sewers to locate outflow from bomb factories or drug labs? Novelty The novelty of this approach is the use of bees and rats for preemptive explosives detection. Bioluminescent genes have been demonstrated with microluminometers. Relevance to the Committee’s Mission Preemptive location of bombers and bomb makers for arrest or surveillance would be possible with this technology. Drug labs might also be located with this approach. Existing Technology Related to This Idea Bees and rats have been prepared for explosives detection. Advantages This approach offers a broad sampling method for wide-area screening. There is the potential to find the problem at its source. Challenges The return of butterflies, moths, and rats and the relatively short lifetime of butterflies and moths are challenges given the investment in training. Also, rats are disease vectors. Research Needed to Demonstrate Feasibility Field trials would be needed to test this approach. In principle, the
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Existing and Potential Standoff Explosives Detection Techniques technology exists, so a rat could be modified with a bioluminescent reporter and sensor (photocell). RECOMMENDATION Recommendation: Feasibility studies should be developed on the ideas suggested in Chapter 7 to assess their potential in sensors suitable for standoff detection.
Representative terms from entire chapter: