EXECUTIVE SUMMARY

The threat posed to commercial aviation by small, concealed explosive devices is particularly severe since they are difficult to detect using current techniques and can readily cause tremendous destruction and loss of life. Protecting air travelers from terrorist actions is an essential mission of the Federal Aviation Administration. The FAA plays a critical role in defining the terrorist threat, stimulating the development of explosive detection devices and systems, and in regulating their use. It can issue rules, set minimum performance standards, and set up a system for compliance that will allow private industry and open market competition to play an active role in research, development, manufacturing, and self monitoring. Successful examples of this strategy are the Federal Drug Administration's approach for regulating medical devices, and the FAA's approach for regulating the certification and operation of commercial aircraft. The regulatory framework created by the FAA will influence in a major way the direction of many crucial aspects of the development of explosive detection systems. This would include the resources the FAA will require in order to execute its regulatory duties, the level of investments that private industry would be willing to risk, the pace of performance and cost improvements of deployed systems, and the degree to which air carriers, airport operators, and vendors can cooperate in the common objective of improving U.S. commercial aircraft security.

The committee recommends that the FAA define the regulatory strategy to be used for explosive detection systems. The committee further recommends that the FAA encourage the participation of air carriers, airport operators, and equipment suppliers in the rule-making process.

The United States government has several means to respond to the terrorist threat, which include: deterring the terrorist action; strengthening aircraft against explosions; and detecting and removing an explosive device before it is brought on-board an airplane.1

Deterrence assumes that a terrorist would rationally weigh the perceived risks and benefits of the proposed action. From the perspective of a terrorist, risks would include the assessment of the certainty of apprehension and the severity of the punishment. While deterrence may reduce terrorist incidents, it alone is not sufficient to prevent them entirely. Aircraft hardening will increase structural integrity against a given amount of

1  

NMAB-463, Reducing the Risks of Explosives on Commercial Aircraft, pages 9–14.



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Detection of Explosives for Commercial Aviation Security EXECUTIVE SUMMARY The threat posed to commercial aviation by small, concealed explosive devices is particularly severe since they are difficult to detect using current techniques and can readily cause tremendous destruction and loss of life. Protecting air travelers from terrorist actions is an essential mission of the Federal Aviation Administration. The FAA plays a critical role in defining the terrorist threat, stimulating the development of explosive detection devices and systems, and in regulating their use. It can issue rules, set minimum performance standards, and set up a system for compliance that will allow private industry and open market competition to play an active role in research, development, manufacturing, and self monitoring. Successful examples of this strategy are the Federal Drug Administration's approach for regulating medical devices, and the FAA's approach for regulating the certification and operation of commercial aircraft. The regulatory framework created by the FAA will influence in a major way the direction of many crucial aspects of the development of explosive detection systems. This would include the resources the FAA will require in order to execute its regulatory duties, the level of investments that private industry would be willing to risk, the pace of performance and cost improvements of deployed systems, and the degree to which air carriers, airport operators, and vendors can cooperate in the common objective of improving U.S. commercial aircraft security. The committee recommends that the FAA define the regulatory strategy to be used for explosive detection systems. The committee further recommends that the FAA encourage the participation of air carriers, airport operators, and equipment suppliers in the rule-making process. The United States government has several means to respond to the terrorist threat, which include: deterring the terrorist action; strengthening aircraft against explosions; and detecting and removing an explosive device before it is brought on-board an airplane.1 Deterrence assumes that a terrorist would rationally weigh the perceived risks and benefits of the proposed action. From the perspective of a terrorist, risks would include the assessment of the certainty of apprehension and the severity of the punishment. While deterrence may reduce terrorist incidents, it alone is not sufficient to prevent them entirely. Aircraft hardening will increase structural integrity against a given amount of 1   NMAB-463, Reducing the Risks of Explosives on Commercial Aircraft, pages 9–14.

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Detection of Explosives for Commercial Aviation Security explosive.2 Hardening, however, adds weight and cost, may require an appreciable time to implement, and may be difficult to accomplish for commuter-sized aircraft. Deterrence, aircraft hardening, and explosive detection are complementary. Deterrence reduces the number of attempts to penetrate the security system, while a good detection capability acts as a deterrent. Hardening an aircraft increases the minimum quantity of explosives required to cause catastrophic damage. This makes the detection task easier since an explosive detection system would be able to search for larger devices which have a greater likelihood of detection. The detection of small quantities of explosives is a difficult challenge, but one that is technically feasible. The detection problem is complicated by the following considerations: The terrorist has literally dozens of choices of explosives; highly energetic "plastic" explosives are widely available, and thus small, powerful bombs can be inexpensively constructed. Nitrogen-based (nitramine) explosives have three qualities that make them very attractive to a terrorist. They have high energy yields per unit weight, they have small critical diameters (i.e. the small diameter that can sustain a detonation), and they require little or no confinement (i.e. heavy metal walls). However, other types of explosives and devices also pose significant threats. Small amounts of explosives have small signatures regardless of the instrumental method used for detection. For a given instrument as the detection threshold is lowered, the probability of detection increases, but the probability of false alarms also increases. The threat is infrequent among an enormous volume of bags; during the course of a year, only a few devices might be placed in over one billion pieces of baggage.3 A wide variety of items is packed into luggage, presenting a broad spectrum of random background signals to an explosive detection instrument. Advanced explosive detection technologies offer considerable promise, but have not yet been demonstrated to be effective, much less cost-effective in an airport environment. An explosive detection system would not be acceptable if it substantially slowed down airport operations, or otherwise adversely affected the flow of passengers through the airport. 2   S. Ashley, "Safety in the Sky: Designing Bomb Retardant Baggage Containers," Mechanical Engineering, January 1992, pp. 81–86. 3   Report of the President's Commission on Aviation Security and Terrorism, 1990, page 48.

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Detection of Explosives for Commercial Aviation Security Fully automated detection equipment can minimize the human role, but cannot eliminate it since judgment will still be required to clear all alarms. A sophisticated terrorist can adjust his strategy more quickly than can the opposing security system. The FAA fulfills an essential function by fostering the continued development of explosive detection devices and their integration into systems. The FAA is funding research and development in relevant technology areas with the expectation that private industry will commercialize the results. The FAA must establish the standards that explosive detection equipment must meet, as well as the procedures to verify and certify the performance of this equipment. The key issues for the FAA regarding explosive detection technology are: What can the different detection methods do in principle? What can they actually do in practice? How can the different methods be best employed to counter the terrorist threat? This report of the Committee on Commercial Aviation Security addresses the above issues from a detection technology perspective. It discusses and assesses system considerations, testing protocols and performance criteria, and recent explosive detection technology developments. Key conclusions of the committee are: There does not appear at present to be any single detection technology that can, by itself, provide a high probability of detection coupled with a low false alarm rate that will reduce the threat of terrorism at an acceptable cost to airport operations. Individual detection devices can be integrated into a system that takes advantage of the strengths of each method. A large range of performance and cost outcomes is possible for a given configuration of detection procedures and devices, depending on the organization of the search strategy, the system architecture and the operational parameters of the individual devices. The selection of instrumental methods integrated in an EDS could be based, in part, on the consideration that the vulnerability of one detection technology (EDD) to a potential countermeasure could be compensated by another EDD. In order to protect deployed EDS equipment against countermeasure attack, the FAA should work with the airline industry and the EDS equipment suppliers to secure an agreement that the configuration of particular EDS equipment at a particular location will not be made available to the general public. If this cannot be done voluntarily, then appropriate enabling legislation should be sought by the FAA.

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Detection of Explosives for Commercial Aviation Security The system architecture or search strategy will vary considerably for each airline and airport situation due to individual terminal designs, utilization rates, and other local factors. Within the level of R&D resources available, the technologies chosen by the FAA for advanced development appear to be appropriate. The FAA R&D program should not be the sole source of funding for further development of these technologies. A balanced investment strategy that integrates the FAA plan with the program plans of other government agencies and industry would be the most desirable approach. The testing of candidate devices should differ significantly from testing of systems ready for deployment. Candidate devices and systems not ready for deployment should undergo parametric testing to verify operational performance at specified levels of statistical confidence. Systems ready for deployment should undergo certification testing in which the system will be judged as pass/fail against a performance specification. Accurate, unbiased data on operational parameters for individual detection systems (e.g. probability of detection, false alarm rate, throughput, cost, size, and weight) are essential for rational decisions on the system architecture. Unbiased testing is an essential element of the FAA's Security Technology Program. Additional features of a comprehensive certification program would include a process that reviews and encourages cost-effective upgrades to fielded equipment, and provides an atmosphere in which industry will be stimulated to invest in the development of new equipment. A general test protocol, which abstracts the most important testing features, insures that all tests consider the same critical factors in a consistent way. A test director, together with a team of experts, could use the general protocol to prepare a detailed test specification tailored for a particular piece of explosive detection equipment without biasing the test. The testing of explosive detection devices, in the context of the underlying system architecture, under realistic airport operating conditions, against a FAA-required performance standard must be the keystone of the FAA's certification program. The certification program must include provisions to assure that each explosive detection device in the field will perform over a period of time at least as well as the one that passed the certification test. A comprehensive proactive, needs-driven research program can provide options to counter the terrorist threat as it evolves. SYSTEM CONSIDERATIONS System considerations include discussions of explosive detection system (EDS) architectures and design issues, and critical issues bearing on the

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Detection of Explosives for Commercial Aviation Security development of computerized simulation tools which could assist in the design and analysis of an operational EDS. The primary systems engineering challenge is to combine detection devices and procedures in such a way as to achieve high detection probabilities and throughput rates with an acceptable false alarm rate and cost (initial investment and operating costs). In order for the FAA to address the systems engineering challenge, the committee recommends that a single organizational entity be responsible for simulation tool development and analysis of explosive detection system architectures. Computer-based simulation tools and data bases should be developed and maintained under the auspices of the FAA; these tools should incorporate many practical constraints and complex factors arising from airport operating environments which would affect the design and use of explosive detection systems. These tools should be made available to the airline industry, airport operators and designers, and equipment vendors. TESTING PROTOCOLS AND PERFORMANCE CRITERIA Effective, unbiased testing of explosive detection equipment is an essential element of the FAA's Security Technology program. The development of standard test protocols and performance criteria, and the FAA's role in testing are key ingredients of airport security regulatory process. Because of the importance of testing, it is essential that the organization charged with conducting such tests be independent of and insulated from potential pressures internal and external to the FAA. Furthermore, this activity must be conducted by an organization with established expertise in testing matters using specified protocols. An important distinction is made regarding the categorization of the equipment being tested. An Explosive Detection Device (EDD), which employs one particular instrumental method to detect the presence of explosive material, would be tested to verify that its performance matches the data provided by the manufacturer. On the other hand, an Explosive Detection System (EDS), which could be composed on one or more integrated EDDs, would be certified by the FAA as meeting the operational standards.4 A comprehensive EDS certification program which includes but is not limited to certification testing, is strongly recommended by the committee. GENERAL TEST PROTOCOL FOR BULK DETECTION A general test protocol containing the framework of significant testing considerations required to design a test plan, conduct the tests, and analyze the test data and evaluate bulk detection equipment was developed in 4   Parametric testing could be performed on EDS equipment to fully characterize the operational characteristics prior to the formal certification process.

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Detection of Explosives for Commercial Aviation Security consultation with the committee.5 It is a guide in designing detailed verification and certification test data analysis plans. This protocol clearly delineates the differences between verification testing and certification testing, elaborates on the composition of the standard bag set, requires documentation of the rationale for deviations from the protocol, and includes provisions for updating as additional test experience is gained. STATUS OF VAPOR AND PARTICLE DETECTION TEST PROTOCOL The committee was asked to develop a generic test protocol for vapor detector instruments. By their very nature, the results from vapor detectors are inferential; i.e when a vapor is detected the presence of the bulk explosive (threat) associated with the test object must be inferred. From vapor detection alone, nothing can be said about the amount of explosive present. For instance, vapor in sufficient quantity to cause current detectors to alarm can come from crumbs of explosive material. Conversely, amounts of explosive ten times the lethal threat quantity, if properly encapsulated, would not necessarily cause these detectors to alarm because there would be no vapor to detect. Aside from the special cases of complete containment or the availability of particulate material, the amount of vapor available for sensing from a given quantity of a concealed explosive is not known. At this time, there is no certified vapor generator available to produce a known small quantity of an explosive's vapor, and there is no reference instrument available to check these generators or the background contamination at low levels. Until the role of vapor detection in the explosive detection system can be specified and the level of detection necessary to fulfill successfully this role is quantified, a generic test protocol cannot be prepared. The committee did provide informal input to a group at Idaho National Engineering Laboratory (INEL) preparing a preliminary detailed test specification for developmental testing of candidate vapor detection devices. This specification is still evolving as the various test procedures are reduced to practice. The committee recommends that, if vapor detection devices are to be certified, a test facility must be maintained that has standard vapor generators and a reference instrument. The facility will have to be free from contamination and capable of remaining in that condition after the tests. RECENT TECHNOLOGY DEVELOPMENTS The committee reviewed and updated the status of specific technologies and devices currently under development for explosive detection. Many of these technical approaches were the result of investments by the FAA's research and development (R&D) program or by industry encouraged by the FAA's 5   In this context, bulk detection refers to the sensing of some physical or chemical property of the solid phase of an explosive.

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Detection of Explosives for Commercial Aviation Security active interest in soliciting their ideas. The committee did not find any significant approach that had not been explored by the FAA through direct funding, by monitoring, or by a combination of funding and monitoring. Broad investment strategy priorities are suggested below for various technologies. Table ES-1 is a short summary of each detection technology. The assignment of priorities was guided by the following considerations: High priority projects directly support devices and systems which could be deployed within the next several years, or support efforts with a longer lead time that could have a high future payoff. Medium priority projects, if successful, could result in an improvement of the next generation of EDS, or could have a payoff of intermediate value. Low priority projects were considered to be technically high risk or to have a low payoff, which should be pursued only as funds allow. For the purpose of evaluating the stage of development for the various instrumental technologies, the following life cycle phases were used: Concept: the concept is clearly described, proof-of-principle calculations performed, and drawings of laboratory experimental set-up completed. Demonstration of Principle: laboratory apparatus assembled, signal-tonoise measurements completed, and detection of pure standards and interferrants tested. Engineering Prototype: detection module integrated with other devices and sub-systems, testing of key operational parameters completed, and physical size and facility requirements of a fully capable device defined. Deployable Device: specifications, operational tests, manufacturing and assembly methods, configuration and software finalized; pricing established; available for integration into a qualified explosive detection system. Following is a summary of the instrumental technologies, which have been updated from the previous report:1 1. Thermal Neutron Activation (TNA) TNA provides an important detection capability against larger quantities of explosives. Although the probability of detection decreases and probability of a false alarm increases for smaller quantities of explosives against the background of nitrogen containing materials in passenger luggage,

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Detection of Explosives for Commercial Aviation Security TABLE ES.1. Summary of Explosive Detection Devices (EDD) EDD Technique Principle Of Operation Characteristic Detected Advantages Thermal Neutron Activation (TNA) Low energy neutrons captured by nitrogen atoms, resulting deexcitation produces characteristic gamma rays. Nitrogen content. Many highly energetic explosives have high nitrogen content. Operational experience with some pre-production equipment. Can be automated; regions of high nitrogen content identified. Elastic Neutron Scattering Monoenergetic neutron source used to scan objects; elastically back-scattered neutrons are detected. Carbon, nitrogen, oxygen content calculated from neutron energy loss. Measures quantities of light elements. Suspect location identified. Pulsed Fast Neutron Activation (PFNA) Fast pulses of neutron beams used to excite characteristic gamma rays. Provides carbon, nitrogen, and oxygen compositional information. Measures quantities of light elements present. Determines position and depth of suspect material. Photon Activation A powerful electron linear accelerator produces bremsstahlung x-rays, which in turn produce a radioactive isotope of nitrogen when encountering nitrogen atoms. The resultant nitrogen isotope has a 10 minute half life, and decays by emitting a positron. Nitrogen content. Builds on experience with existing medical PET devices. Excellent spatial resolution possible. Nuclear Resonant Absorption (NRA) Proton beam bombards a target to produce high energy gamma rays which preferentially excite nitrogen atoms. Nitrogen content. Able to penetrate shielding around an explosive. High sensitivity combined with good spatial resolution for detecting nitrogenous material. Fast Neutron Associated Particle (FNAP) High energy neutrons produced from a deuterium-tritium reaction are ejected in known directions relative to an ejected associated alpha particles. These fast neutrons activate nuclei by inelastic scattering which results in emission of characteristic gamma rays. Correlation of the direction of the gamma rays with the alpha particles yields directionality. Relative amounts of carbon, nitrogen, and oxygen. Provides elemental compositions and locations of explosives. Uses highly penetrating neutrons. Dual Energy X-Ray Alternating x-ray beams of high and low energy levels produce two different images due to differences between the photoelectric and Compton attenuation coefficients of the different elements. By comparing the images, the areas with light elements can be identified. Average atomic number, density, and shape. Extension of a medical device. Provides narrow resolution in average atomic number which allows identification of materials. Backscatter Analysis X-Ray Compares normal x-ray transmission image with a Compton backscatter image. By comparing the images, the areas with light elements can be identified. Average atomic number, density, and shape. Uses off-the-shelf technology. Extension of x-ray technology already familiar to airport security. Can be readily available. Extremely Low-Dose X-Ray Same as backscatter analysis x-ray but at a lower x-ray intensity level. Average atomic number, density, and shape. Can be used on people. Extension of available medical devices. Can be readily available. Coherent X-Ray Scattering X-ray diffraction. Crystal structure. Depends on unique crystal structure of the explosive. Dual Energy X-Ray Computed Tomography (CT) A CT image is a map of the x-ray attenuation coefficient in each voxel. Attenuation depends on density and composition. Differences between photoelectric and Compton attenuation coefficients at two different energy levels are used to solve for density and composition. Shape, atomic number, and density. Similar to medical CAT scanner. Produces true cross-section slices. Suspect areas can be imaged in greater detail. Can be readily available. Vapor/Particle Detection Devices Devices employ a variety of methods, including: Gas Chromatography; Chemical Luminescence; Mass Spectrometry. Volatility, molecular weight, and electron affinity. Non-invasive method. Can be used on people. Experience in other applications. Commercially available equipment. Dogs Not known for certain. May sense chemical vapor from the explosive itself or some other odor associated with the explosive; or may sense particulate components from the explosive. Volatility and possibly other characteristics. Non-invasive. Very mobile. Can cover wide area rapidly. Can be used on people.

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Detection of Explosives for Commercial Aviation Security TABLE ES.1. Summary of Explosive Detection Devices (EDD) Limitations Critical Issue(s) Development Stage Committee Recommendation Only detects nitrogen; large size; sensitivity degrades as explosive quantity decreases; uses radioactive source. Cannot be used on people. Sensitivity against background and interferents. Pre-production prototypes deployed at several international terminals. High priority for continuing to refine the devices already deployed until further efforts are assessed to have low marginal benefit. Shielding required; background scattering noise reduces sensitivity. Cannot be used on people. Requires accelerator to produce 1 MeV to 5 MeV proton beam. Must demonstrate time-of-flight measurement capability. Laboratory demonstration of principle. Low priority for the development of an engineering prototype. Shielding required. Large size. Durability unknown. Cannot be used on people. Engineering issues related to the accelerator (must produce approximately 7.5 MeV neutron beam), target, scanning system, and detectors. Engineering prototype. Medium priority for the testing of the critical elements of anengineering prototype. Only detects nitrogen. Susceptible to background interference from other elements. Would destroy any radiation-sensitive material in baggage (e.g. film). Cannot be used on people. Engineering issues related to the accelerator (13.5 MeV). Laboratory demonstration of principle. Low priority for the development of an engineering prototype. Only detects nitrogen. Durability unknown. Cannot be used on people. Development of high current accelerator. Long life multi-layered carbon target. Data acquisition and analysis system. Laboratory demonstration of principle. Low priority for the development of an engineering prototype. High priority for development of accelerator technology. Shielding required. Severe background problem. Requires nuclear accelerator. Requires multiplexed alpha particle detection system. Cannot be used on people. Handling of radioactive tritium target. Reliability of accelerator. Design and operation of detector system. Processing time required for the computerized algorithms. Laboratory demonstration of principle. Low priority for supporting the development of a prototype. Looks for average atomic number, not a specific atomic number. Cannot be used on people. Complex computer analysis required. Engineering prototype. High priority for the testing of existing engineering prototypes. May have a high false alarm rate due to difficulty of performing computer analysis. Cannot be used on people. Effectiveness and speed of computer analysis. Engineering prototype. Medium priority for testing of existing engineering prototypes. Image may be confusing due to diversity of articles carried by travelers. Public acceptance of even a low radiaton dose. False alarm rate. Engineering prototype. High priority testing of existing engineering prototypes. Cannot be used on people. Scan speed. Verification of unique explosive crystallinity ''signatures.'' Concept. High priority for demonstration of principle. Slow speed may require much greater computer power. No experience to date in scanning actual passenger bags. Cannot be used on people. Speed of the analysis algorithm. Engineering prototype. High priority for performance testing of improved prototypes. Very low vapor pressure of plastic explosives results in femtograms of available vapor. Sample collection step is critical. Does not give a direct measure of the amount of explosive present. Demonstrated effectiveness in detecting high performance explosives in luggage. May be more effective as particle detectors since much more sample materials is available for analysis. Varies. Some units are commercially available. High priority for determining quantity of vapor associated with explosives in baggage and carried on persons; high priority for the development of particulate detection standards; and high priority for well-controlled testing of deployable devices with particulate detection standards. Reliability over a period of time. Performance difficult to quantify. Determining absolute detection threshold for explosive vapors, and what key factors influence a dog's detection performance. Operational. High priority for performance testing of dogs.

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Detection of Explosives for Commercial Aviation Security TNA still provides a significant detection capability for smaller quantities of explosives in selected items with an inherently low nitrogen background, such as electronic devices and smaller items of carry-on baggage. TNA is likely to be an important element in a larger architecture of detectors. The committee recommends continued support for the refinement of this deployable device at a high priority level until an alternate explosive detection device of greatly improved performance becomes available, or until further improvements result in only small marginal gains. 2. Automated Neutron Source Accelerator Although existing TNA devices utilize a radioactive 252Cf isotope as a source of thermal neutrons, the source might be made more compact, require less radiation shielding and be less expensive if a small, completely automated particle accelerator were developed for the neutron source. In fact, all of the proposed nuclear-based explosive detection devices, with the exception of TNA, are dependent on the development of specialized particle accelerators. Therefore, development of this type of accelerator is assigned a high priority by the committee. 3. Elastic Neutron Scattering This technology in principle can detect specific elemental components of an explosive by measuring the energy loss of elastically scattered neutrons. However, no tests have yet been made using realistic baggage while imposing the shielding conditions necessary at an airport. Much more research and development is needed before a judgment can be made regarding the practicality of this technology as an explosive detection device. The committee gives the development of an engineering prototype device utilizing this technology low priority. 4. Pulsed Fast Neutron Activation In principle, PFNA is the most promising of the nuclear technologies, since it can be used to identify the different atoms that make up an explosive—e.g. carbon, nitrogen and oxygen—and identify the explosive by its composition, quantity, and location. The pulsed fast neutrons make possible fast correlation analysis that then allows for three-dimensional construction of the relative location of the explosive materials within the suitcase on a single pass. This technology is currently under development for use as a device for scanning entire containers for explosives before allowing their entry into the tunnel under the English Channel. However, the technology suffers from potentially high cost and complexity. The committee recommends medium priority for the testing of an engineering prototype. 5. Photon Activation This approach activates nuclei with high-energy photons produced by a powerful electron accelerator. The resulting radioactive nitrogen and other nuclei then decay by positron emission, allowing for both detection and location by positron tomography methods. Significant safety and passenger acceptance issues involving the high level of gamma irradiation, as well as

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Detection of Explosives for Commercial Aviation Security cost and complexity of the tomography detector system, make this technology seem less promising than some of the alternative nuclear methods. Low priority for development of an engineering prototype is suggested by the Committee. 6. Nuclear Resonant Absorption This technology utilizes a high energy gamma ray that is strongly absorbed (resonant) by nitrogen and nothing else. A very high current accelerator is required along with specially prepared targets and possibly detectors as well. Three different organizations are working on this technology; however, the engineering problems are formidable. Although it would be useful to expend effort on accelerator development, development of an engineering prototype is accorded low priority by the committee. 7. Fast Neutron Associated Particle (FNAP) This method utilizes the nuclear reaction of two hydrogen isotopes, deuterium and tritium, that results in a 14 MeV neutron and an associated alpha particle. The 14 MeV neutron and alpha particle move in opposite directions, so that detection of the alpha particle's direction with respect to the target or nuclear reaction point determines the direction of the neutron. The alpha particle detection also provides a start signal for a neutron time-of-flight measurement for any detected deexcitation gamma rays, which can then be used for elemental identification and three dimensional imaging. Although this method has potential as an explosive detection device, the engineering problems are formidable and a working prototype has yet to be demonstrated. A low priority is recommended by the committee for the support of prototype development. 8. Dual Energy X-Ray Systems This promising technology uses alternating beams of high and low energy-x-rays to produce two different images. This technique relies on the differences between the photoelectric and Compton attenuation coefficients of elements for the two different x-ray energies. Heavy elements are more effective at absorbing high-energy x-rays while light elements are more effective at scattering low energy x-ray beams. The differences in signal between the two x-ray beams can be used, after suitable analysis, to compute the average atomic number of an object. The results to date are impressive. Near-term implementation at reasonable cost appears feasible. The committee recommends that testing of existing engineering prototype devices for deployment be given high priority. 9. Backscatter X-Ray This x-ray technique makes use of two images: the normal transmission image which shows areas of photoelectric absorption by the high atomic number elements, and backscatter image due to compton scattering. Since compton

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Detection of Explosives for Commercial Aviation Security scattering is relatively independent of atomic number unlike photoelectric absorption, the comparison of the backscatter image to the transmitted image, allows the average atomic number of various areas to be computed. The ability of this technique to discriminate explosives from other "low-Z" materials such as paper has not been demonstrated. However, the method, if successful, would be relatively inexpensive. The committee suggests that testing of the existing engineering prototypes for feasibility be given medium priority. 10. Extremely Low-Dose X-Ray Devices for Searching Passengers The performance of these devices, which use x-ray backscattering, is very impressive. The radiation risks are reported to be negligible. However, passenger acceptance will be a major issue, as will false alarms. Since almost all objects carried on a person will be readily detected, discrimination between threatening and non-threatening objects will be a problem. At present, these devices provide the only demonstrated capability for detecting explosives carried on a person. The committee recommends that testing of existing engineering prototype units for deployment be given high priority. 11. Coherent X-Ray Scattering Coherent x-ray scattering is similar to conventional x-ray diffraction in that Bragg's Law is used to compute crystal structure and lattice spacings. The powder diffraction pattern can then be compared to known x-ray powder diffraction patterns of explosives of interest. The capability to identify specific explosive compounds from their powder diffraction pattern is impressive. This technology appears extremely promising and has significant long-range potential. Demonstration of principle should be given high priority. 12. Dual Energy X-Ray Computed Tomography (CT) A computed tomography image is a map of the x-ray attenuation coefficient in each volume element of an object. Typically, the object is imaged one cross-section at a time, and a three-dimensional image reconstructed from each slice. The x-ray attenuation coefficient depends on both the density and composition of an object. Dual energy imaging (as noted above) provides enough additional information to determine the individual contributions of density and composition to the x-ray attenuation in each volume element. Dual energy CT has the potential to determine the shape, atomic number, and density of objects in luggage. It holds significant promise for a detailed analysis of suspect pieces identified by other screening methods. A high priority is suggested for testing engineering prototypes.

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Detection of Explosives for Commercial Aviation Security 13. Vapor Detection Devices The major virtue of vapor detection devices is their non-invasive nature and applicability to screening passengers. But, vapor detection suffers serious problems arising from the lack of understanding of the mechanisms involved in the evolution of the vapor, and potentially high false alarm rate. Therefore, at the present time, the committee concludes that vapor detection devices geared to sensing extremely small concentrations (femtogram level) of explosive vapor are unsuited for use in airport terminals as a primary explosive detection method. The committee recommends the FAA focus a research activity toward determining quantitatively the amount of vapor available for detection in baggage and passenger scenarios. However, current vapor detectors, if suitably configured, can readily detect particulate plastic explosives. The amount of material available for detection from particles can be more than a million times greater than that available from vapors. Particulate detection, rather than vapor detection, represents a technology that could be successfully deployed very quickly. Testing standards and airport calibration and testing would be relatively straightforward for particulate detection and the devices would not be challenged to work at their absolute limit of detection. However, no particulate standards are currently available. The FAA should assign a high priority to the development of particulate detection standards. 14. Dogs Dogs potentially provide a viable capability, although they have limitations in search time capability and performance retention over long time periods. Controlled tests of dog performance equivalent to those used with explosive vapor devices will be fundamental to developing a rationale for their use. The committee recommends that such testing be given high priority. 15. Bar Coding Positive matching of each piece of baggage to a passenger can be an effective terrorist deterrent. Unaccompanied bags would be treated as suspicious and would not be brought on-board an aircraft without a thorough examination. The application of linear bar code technology offers significant improvement over existing manual-intensive approaches. Currently several different bar code technologies are being applied by air carriers. The committee recommends that international standardization of bar coding symbology be given a high priority to allow bag tracking among all carriers world-wide. 16. Pattern Recognition This emerging technology area offers the potential to automate the interpretation of data from various detection instruments. Implementation of pattern recognition algorithms can help automate the interpretation of results from the instruments, reducing the dependence on human judgment in recognizing explosive materials in scanned luggage. Various technical approaches are

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Detection of Explosives for Commercial Aviation Security possible, including different implementations of neural networks. The committee recommends a medium R&D priority to determine the most effective pattern recognition approaches. SUMMARY OF TECHNOLOGY RECOMMENDATIONS The following efforts are recommended with a high priority: Continued testing of Thermal Neutron Activation units at airports, and refinement of the method until a determination is made that little additional benefit is being gained. Development of an Automated Neutron Source Accelerator. Performance testing of a Dual Energy X-ray prototype. Demonstration of principle of Coherent X-ray Scattering for explosive material detection. Performance testing of Dual Energy X-ray Computed Tomography engineering prototypes. Performance testing of Extremely Low Dose X-ray prototypes. Statistically-Valid Testing of Deployable Vapor Detection equipment using a test protocol. Development of particulate detection standards, and testing of deployed devices using the standards. Performance testing of Dogs. International standardization of Bar Coding for checked luggage. The following efforts are recommended with a medium priority: Performance testing of Pulsed Fast Neutron Activation prototypes. Performance testing of Backscatter X-ray prototypes. Demonstration of proof of concept of Pattern Recognition approaches. The following efforts are recommended with a low priority: Development of an Elastic Neutron Scattering engineering prototype. Development of a Photon Activation engineering prototype. Development of a Nuclear Resonant Absorption engineering prototype. Development of a Fast Neutron Associated Particle engineering prototype.