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--> Appendix A Explosives-Detection Technologies X-Ray Technology Many of the explosives-detection technologies that are based on x-ray techniques measure the x-ray attenuation of the materials that make up the baggage. Because x-rays interact primarily with electrons, the x-ray attenuation coefficient is strongly correlated with the electron density of the material under investigation. The x-ray attenuation of a particular material will also depend on the energy of the x-rays being used. The mechanisms primarily responsible for x-ray attenuation in materials at the energy ranges typically used by explosives-detection equipment are Compton scattering and the photoelectric effect. The photoelectric effect results in x-ray absorption, whereas Compton scattering merely scatters x-rays, potentially altering the path and energy of the scattered photons. The significance of the photoelectric effect is greater for materials composed of elements with a high atomic number (z), such as metals or other inorganic materials. However, this significance drops off rapidly with increasing x-ray energy. For organic materials (low z), Compton scattering is the dominant x-ray attenuation process, and it varies less with x-ray energy. Comparing attenuation measurements at different x-ray energies will therefore allow for distinguishing materials from one another. For example, inorganic materials can be identified by rapidly changing x-ray attenuation with changing x-ray energy, whereas organic materials will display a more subtle change. Multienergy x-ray-based detection equipment have been developed and are suitable for distinguishing organic and inorganic materials and for semiquantitative density measurements. Combining the measurement of transmission and backscatter x-rays will further improve the detection of light (low z) elements as they are found in explosives; however, it will not uniquely identify explosives. The x-ray-based systems described above will provide electron density and therefore mass density information but very limited spatial or geometric information. The x-ray radiograph or projection image is a collection of x-ray attenuation line integrals in two dimensions, but it will not be able to resolve the third dimension along the incident x-ray direction. Computed tomography (CT) adds the capability to visually display the physical appearance of the materials in question from all three dimensions. The ability to reconstruct two-dimensional cross-sectional images (tomographs) and then full three-dimensional volumes can greatly improve the ability to determine explosive threats by identifying certain shapes or patterns, such as wires, batteries, or detonators, as well as measure the volume of the material in question. The additional geometrical information supplements the material x-ray attenuation information and results in a more specific discrimination of explosive materials. Dual-energy CT is capable of providing geometrical information as well as information pertaining to both the physical density and the effective atomic number of a material. The effective atomic number is not enough to uniquely characterize a material but provides discrimination capability over and above physical density alone. Other x-ray-based methods utilizing high-energy photons have been discussed in the literature (Hussein, 1992; Gozani, 1988). They require x-ray energies between 10 and 30 MeV. Due to low-reaction cross sections, they also require high x-ray flux rates produced by powerful accelerators that may not be suitable for airport use. Those high-energy techniques are based on photon interactions with the nuclear properties of nitrogen, carbon, and oxygen. They provide definitive detection of the amount of material with some spatial information, but due to the small-reaction cross sections it is difficult to distinguish between the three elements. Neutron Technology Four basic techniques utilizing neutrons are being proposed for explosives-detection equipment (Hussein, 1992; Gozani, 1988; Barfoot et al., 1981; Overley, 1987; Hussein
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--> et al., 1990): (1) fast neutron activation, (2) thermal neutron activation, (3) fast neutron transmission, and (4) neutron elastic scattering. The first two activation methods use the nuclear reaction of the neutron with the nuclei of nitrogen, carbon, and oxygen, which produces gamma rays at specific energies. By detecting and identifying the energy of these gamma rays, one can then determine the amount of those elements in the probe material. Using single-photon imaging techniques similar to medical applications, one can then generate a two-dimensional image of the luggage. Neutron sources can be reactors (fast neutron activation), radioactive isotopes (thermal neutron activation), or neutron tubes. In all cases, other materials in the luggage, particularly metallic items, can be activated, producing relatively long-lived isotopes (on the order of a few seconds) resulting in low-intensity radiation. Fast neutron activation can be used to determine carbon, nitrogen, and hydrogen densities. However, fast neutron nitrogen lines are generally weak, and thermal neutrons can provide a better determination of nitrogen. The thermal neutron activation method is a nitrogen-only method and is not able to provide adequate spatial information, potentially leading to a high rate of false positives. The neutron transmission and scattering methods make use of the specific material- and energy-dependent absorption and scattering cross sections of neutrons interacting with the nuclei of different elements. They can be used to determine hydrogen, carbon, nitrogen, and oxygen content in an object. Both methods allow for limited generation of tomographic images and use accelerators as the neutron source. Due to the nature of the neutron interaction with the probe nuclei, radioactive activation of some luggage content may result. Trace Detection Technology Trace detection technologies are based on direct chemical identification of particles of explosive material or vapors given off by explosive material. These techniques require three distinct steps to be effective: (1) sample collection, (2) sample analysis, and (3) comparison of results with standard spectra. Sample collection is accomplished by using high-volume air flow to gather vapors or dislodge particles from surfaces or by making physical contact with the subject. Sample analysis techniques employ a variety of chemical separation and detection methods including gas chromatography, chemical luminescence, and ion mobility spectrometry. These methods estimate the molecular weight, electron affinity, and various other chemical properties of the vapor or particulate matter collected (NRC, 1993, 1996). Vapor detection can also be accomplished by using dogs. Although the sensitivity of dogs to explosive vapors has not been quantified, it is generally believed that dogs are more sensitive than the best electromechanical trace detection devices available (OTA, 1992). The major advantage of vapor detection devices is their noninvasive nature and applicability to screening both passengers and baggage. However, vapor detection techniques suffer from potentially high nuisance TABLE A-1 Mass Density and Composition of Common Explosive Materials and Selected Nonthreat Items Material Mass Density (Kg/m3) N Density (Kg/m3) H Density (Kg/m3) C Density (Kg/m3) O Density (Kg/m3) Ammonium nitrate 1,700 595 85 0 1,020 Nitroglycerine 1,600 296 35 254 1,015 PETN 1,800 319 45 342 1,094 TNT 1,700 315 37 629 719 RDX 1,830 693 49 297 791 HMX 1,900 719 51 308 822 Black powder 1,800 190 0 281 610 C4 (RDX-based) 1,640 620 44 266 710 Smokeless powder 1,660 325 39 402 984 Natural rubber 1,300 141 111 726 322 ABS plastics 1,050 0 81 962 0 Silk/Wool fibers 1,440 316 90 271 723 Silk/Wool cloths 200 37 13 97 53 Ground nuts 1,400 252 98 728 315 Nylon 6 1,140 161 111 142 0 Nylon 11 1,140 142 111 726 161 Nitrate rubber 1,000 679 121 57 264 Melamine 1,800 1,200 86 514 0 Orlon 1,180 312 67 801 0 Source: Hussein, 1992.
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--> Figure A-1 Hydrogen and nitrogen content of various explosive and nonexplosive materials. Source: Hussein, 1992. alarm1 rates (NRC, 1993). Current trace detection instruments are designed to detect particulate amounts of material, which reduces the potential for nuisance alarms considerably. Mass Density and Composition of Various Explosive and Nonthreat Materials As discussed in Chapter 3, bulk explosives-detection techniques often exploit the high nitrogen and oxygen densities found in explosives. These elements are also present in nonthreat materials such as plastics, clothing, and narcotics. The relative densities of hydrogen, nitrogen, oxygen, and other elements (e.g., carbon) can be used to discriminate explosive from nonexplosive materials. As can be seen in Table A-1 and Figure A-1, however, each density window in which there is a cluster of explosive materials also includes some nonexplosives. Conceptual Explosives-Detection System System Description: An explosives-detection system (EDS) whose principle of operation is predicated on the transmission of light through an object as shown in Figure A-2. The system illuminates one side (input) of the object with visible light. The interaction of light with the object results in a reduced amount of light on the other side of the object (output). The amount of light transmitted through the object is measured with the photodiode, which converts the signal into a physical parameter related to the amount of transmitted light (i.e., electrical current). For this conceptual system, a threat would be represented by an object with very low transmission properties. So detection of a threat relies on a comparison of the value of electrical current observed for the object under illumination with a threshold value of electrical current. The subsystems of this system are presented in Table A-2. The infrastructure for this conceptual explosives-detection system consists of the mechanical stand and electrical interconnections. Figure A-2 View of a conceptual EDS based on visible light transmission. 1 A nuisance alarm is caused when a trace detection instrument correctly identifies the presence of trace amounts of explosive material, but there is not an explosive device present. This may be caused by gun owners having trace amounts of black powder on their hands or miners having trace amounts of dynamite on their hands.
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--> TABLE A-2 Tabular Description of Each Operational Subsystem of a Conceptual EDS Based on Visible Light Transmission Subsystem Associated components Sampling Flashlight —lightbulb, batteries, lenses, and "on/off" switch Photodiode Analyzing Photodiode, ammeter Classifying Electrical current comparator —decision based on level of electrical current "Threat" versus "ok" indicators User Interface "On/off' light switch "Threat" versus "ok" indicators Ammeter Note: Individual components may be associated with more than one subsystem. Example of Explosives-Detection System System Description: An EDS whose principle of operation is x-ray CT, as shown in Figures A-3 and A-4. The subsystems of this system are presented in Table A-3. The system generates an image of the baggage as a map of the x-ray attenuation coefficient in each volume element. The attenuation coefficient depends on the density and the elemental composition of the objects within the baggage. The system then determines the size and shape of any objects whose attenuation coefficient matches that of known explosive materials. Detection of a threat object in the baggage is made when the physical parameters (shape, atomic number, and density) match or exceed predetermined threshold values. The infrastructure of this EDS is the mechanical and electrical framework, including the conveyor system, electrical and mechanical interconnections, and radiation shielding. Figure A-3 Front view of an EDS based on x-ray CT. Figure A-4 Side view of an EDS based on x-ray CT. TABLE A-3 Tabular Description of Each Critical Module of an EDS Based on X-Ray CT Subsystem Associated Components Sampling X-ray source —x-ray tube, x-ray housing, x-ray collimator, and x-ray filtration X-ray control —x-ray tube current, x-ray tube potential, and exposure time X-ray detector —detector, electronics Analyzing Reconstruction algorithms —digital filtration, backpropagation Pattern recognition software Classifying Data analysis and manipulation Comparator (human or computer) —decision function and threat notification User Interface X-ray status indicators —x-ray "on," thermal heating load, and x-ray tube operating conditions System status indicators —mechanical and electrical Threat notification —audible and visible indicators —retention of image data for operator interpretation References Barfoot, K.M., C.W. Cheng, J.D. MacArthur, B.C. Robertson, S.K. Saha, and K.M. Wilson. 1981. Bulk elemental analysis by the inelastic scattering of neutrons produced with a Van de Graff accelerator. IEEE Transactions on Nuclear Science 28-S(2): 1644-1646. Gozani, T. 1988. Nuclear detection technology, from plutonium through coal to explosives. American Nuclear Society Transactions 56(3): 38. Hussein, E.M.A. 1992. Detection of explosive materials using nuclear radiation: a critical review. Pp. 130-137 in X-Ray Detector Physics and Application, R.B. Hoover, ed. Proceedings of The International Society for Optical Engineering, vol. 1736. Bellingham, Wash.: SPIE. Hussein, E.M.A, P.M. Lord, and D.L. Bot. 1990. An empirical fast-neutron technique for detection of explosive-like materials. Nuclear Instruments and Methods in Physics Research A299(1-3): 453-457. NRC (National Research Council). 1993. Detection of Explosives for Commercial Aviation Security. National Materials Advisory Board. Washington, D.C.: National Academy Press. NRC. 1996. Airline Passenger Security Screening: New Technologies and Implementation Issues. National Materials Advisory Board. Washington, D.C.: National Academy Press. OTA (Office of Technology Assessment, U.S. Congress). 1992. Technology Against Terrorism: Structuring Security. Washington, D.C.: Office of Technology Assessment. Ovefiey, J.C. 1987. Element-sensitive computed tomography with fast neutrons. Nuclear Instruments and Methods in Physics Research B24/25: 1058-1062.
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