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Assessment of Millimeter-Wave and Terahertz Technology for Detection and Identification of Concealed Explosives and Weapons Executive Summary The security of the U.S. commercial aviation system has been a growing concern since the 1970s when the hijacking of aircraft became a serious problem. Since the early days of screening airline passengers, the aviation security system has grown increasingly complex and comprehensive. Current protocols include the screening of passengers and their carry-on baggage for threat items that could be used to damage the aircraft or threaten the crew and passengers and the screening of passengers’ checked baggage for items that could damage the aircraft. While many threats exist in the nation’s transportation infrastructure, the Committee on Assessment of Security Technologies for Transportation focused this effort on the aviation security portion of the U.S. national infrastructure because, with the air transportation environment’s more controlled passenger access and its experience with passenger screening, the committee believes that the air transportation environment can serve as a ready testbed for assessing screening
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Assessment of Millimeter-Wave and Terahertz Technology for Detection and Identification of Concealed Explosives and Weapons technologies that might be extended or modified for use in securing other transportation modes. Prior to the terrorist bomb that brought down Pan Am Flight 103 over Lockerbie, Scotland, in December 1988, the focus of airline passenger security checkpoints was the detection and interdiction of metallic weapons, either carried on the person or concealed within carry-on items. Since the introduction in 1994 of certified explosive detection systems for checked luggage, a technology based on computed x-ray tomography, explosive threat detection has significantly improved. While potential attacks on all modes of transportation are of concern, the committee believes that the U.S. air transportation system continues to have a high priority for counterterrorism resources, both because of its economic importance and because of the intensified public perception of risk following the September 11, 2001, attacks. The Transportation Security Administration (TSA) provided the National Research Council (NRC) with the following statement of task for this study: This study will explore opportunities for technology to address national needs for transportation security. While the primary role of the committee is to respond to the government’s request for assessments in particular applications, the committee may offer advice on specific matters as required. The committee will: (1) identify potential applications for technology in transportation security with a focus on likely threats; (2) evaluate technology approaches to threat detection, effect mitigation, and consequence management; and (3) assess the need for research, development, and deployment to enable implementation of new security technologies. These tasks will be done in the context of current, near-term, and long-term requirements. The committee will perform the following specific tasks: Identify potential applications for technology in transportation security with a focus on likely threats derived from threat analyses that drive security system requirements. Review security system developments structured to meet the changing threat environment. Assess government and commercial industry plans designed to address these threats. Evaluate technology approaches to threat detection, effect mitigation, and consequence management. Delineate the benefits of the insertion of new technologies into existing security systems. Evaluate the trade-offs between effectiveness and cost, including the cost of changing the security system architectures. Assess the need for research, development, and deployment to enable implementation of new security technologies. Review and assess the potential benefit of existing and advanced detection technologies, including scanning technologies, sensing technologies, and the use of computer modeling and databases. Review and assess emerging approaches to effect mitigation and consequence management. An overarching goal of this committee has been to provide timely reports that meet the technology-evaluation priorities of the Transportation Security Administration
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Assessment of Millimeter-Wave and Terahertz Technology for Detection and Identification of Concealed Explosives and Weapons for defeating terrorist threats. The committee judged that this could best be done by issuing a series of short reports, of which this report is the third. This report focuses on maturing millimeter-wave and terahertz imaging and spectroscopy technologies that may offer promise in meeting aviation security requirements. The committee considered the spectrum often mistakenly referred to as the terahertz region to encompass the radio frequencies from 10 GHz to 10,000 GHz, the latter being the top of the true terahertz region (10,000 GHz or 30 micron wavelength). When referring to this entire spectrum, the committee has chosen the nomenclature “millimeter wavelength/terahertz.” To assess the potential for millimeter-wavelength/terahertz technologies to detect weapons and explosives in an airport screening environment, the committee examined four aspects of the problem: The currently available phenomenology associated with the atmosphere, concealing materials, and materials to be detected; The maturity of electronic and electro-optic components; The suite of millimeter-wavelength/terahertz scanning systems currently undergoing development; and A potential implementation strategy for the Transportation Security Administration (TSA). BACKGROUND The sense of urgency about addressing emerging terrorist threats and the availability of funds to develop potential technologies to address these threats have combined to elicit a plethora of proposals for funding. However, the committee believes that there has been significant overselling of the potential of these technologies to address screening requirements. Proposals that are not well founded on the principles of physics or that are driven by those who lack a sound understanding of the technology and its strengths and limitations appear to exaggerate the potential benefits of millimeter-wavelength/terahertz technology as being more widely applicable to security screening than it is. The electromagnetic spectrum from submillimeter wave through terahertz can be used both to create an image of an object by measuring the intensity of reflected or emitted energy and to gather information on the chemical makeup of an object by measuring the absorption of electromagnetic energy. There are two general classes of millimeter-wavelength/terahertz imaging techniques examined in this report: passive and active. Passive imaging detection techniques rely on collecting naturally occurring radiation and using the contrast between apparently “warmer” and “colder” objects, which usually results from contrasts between the emissivities of different materials. For example, millimeter-wavelength/terahertz technologies are being examined for their ability to detect metal guns concealed underneath clothing by detecting the contrast between the warmer human body and the apparently cooler metal weapon.
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Assessment of Millimeter-Wave and Terahertz Technology for Detection and Identification of Concealed Explosives and Weapons Active imaging systems illuminate the detection space with a beam of millimeter-wavelength/terahertz power, either by illumination of the entire space or as a focused beam scanned over the object, with detectors specifically sensitive to the illuminating frequencies. Although millimeter-wavelength/terahertz energy passes through typical clothing materials, this non-ionizing energy penetrates the human body to only about skin depth. Therefore, the potential health effects of this radiation are significantly lower than those from the competitive imaging technology using ionizing x-rays, although the general population may not fully understand this. There are debates about the relative quality of imagery from a passive versus an active imaging system. The active system has an advantage, however, in that it can illuminate people and objects with the amount of power sufficient to penetrate materials, whereas a passive system must rely on the natural radiation, which is much lower intensity. Some portal-type1 active systems have already demonstrated the capability of providing sufficient information to locate and identify concealed items on people. The terahertz region is now the subject of aggressive university research driven by the availability of short-pulse generators that can produce a wide spectrum of frequencies through this region. These short-pulse generators are being used as sources for collecting broadband spectral features of solid materials as well as in performing slow imaging experiments. As frequencies increase, spectral features of solid materials of interest become more apparent, but the ability to penetrate materials, a desirable feature for the identification of concealed objects, is reduced. The hope for transportation security is that millimeter-wavelength/terahertz energy may provide detection and identification capability for explosive materials concealed underneath a person’s clothing or in nonmetallic baggage. The challenge to detecting and classifying the spectra of explosive materials is that they are not as clearly defined as the spectra from gaseous materials and may be difficult to discern through the atmosphere or through other benign materials that may have their own spectral features. So while there appear to be some unique spectral features of explosive materials in the millimeter-wavelength/terahertz spectrum, conducting screening without corroboration with other sensor modalities could prove to be difficult in other than very controlled situations. In addition to having the capability of imaging concealed objects, a millimeter-wavelength/terahertz imager also has the capability of revealing some anatomical features of the individuals being screened. In the United States, displaying detailed anatomical features of a person is considered a violation of that individual’s privacy. The issue of whether the acquisition of an image with anatomical detail, even though the image is never publicly displayed, needs to be addressed rigorously by experts in the legal, human factors, and psychology areas. CONCLUSIONS As a result of this study, the committee concluded the following: 1 A system through which a passenger passes.
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Assessment of Millimeter-Wave and Terahertz Technology for Detection and Identification of Concealed Explosives and Weapons The technology base for millimeter-wavelength/terahertz security screening is expanding rapidly internationally, yet there is insufficient technology available to develop a system capable of identifying concealed explosives. Millimeter-wavelength/terahertz technology has potential for contributing to overall aviation security, but its limitations need to be recognized. It will be most effective when used in conjunction with sensor technologies that provide detection capabilities in additional frequency regions. Millimeter-wavelength/terahertz technology in portal applications has been demonstrated for detecting and identifying objects concealed on people. Millimeter-wavelength/terahertz image quality raises personal privacy issues that need to be addressed. Millimeter-wavelength/terahertz technology and x-rays provide images of similar quality. However, millimeter-wavelength/terahertz energy has the safety benefit of being non-ionizing radiation, while x-rays are ionizing radiation. Millimeter-wavelength/terahertz energy cannot penetrate metal objects. Universities, national laboratories, and the commercial sector (both national and international businesses) continue to increase investment in millimeter-wavelength/terahertz technologies for security, medical, nondestructive inspection, and manufacturing quality-control applications. A decision by the TSA to invest in an imaging portal depends on the potential threat posed by passengers carrying either weapons or explosives or other material. The cost of a system, the probability of detection, the false-alarm rate, and the throughput versus that of a competing x-ray system would impact the management decision. RECOMMENDATIONS Building on the conclusions presented above, the committee makes the following recommendations to the Transportation Security Administration regarding the application of millimeter-wavelength/terahertz technology to security screening. To perform an accurate assessment of the applicability of millimeter-wavelength/terahertz-based technology to explosive detection, the TSA will need to do the following: (1) decide on the range of materials to be detected, (2) assess the state of knowledge of what chemical structures and/or features of the scope of materials lend themselves to detection by millimeter-wavelength/terahertz-based spectroscopy, (3) assess the presence of these features in other common materials (such as clothing) within the range of uncertainty for such features, and (4) assess the contribution of additives to explosives to the millimeter-wavelength/terahertz signature. The TSA should examine how millimeter-wavelength/terahertz technology can be employed with other technologies to enhance the detection of weapons and explosives.
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Assessment of Millimeter-Wave and Terahertz Technology for Detection and Identification of Concealed Explosives and Weapons The TSA should commence developmental and operational testing of millimeter-wave-based portals to assess their effectiveness and suitability. As with x-ray-based passenger imaging, the TSA needs to address issues associated with personal privacy raised by millimeter-wavelength/terahertz imaging. The TSA should actively pursue joint projects through agreements such as cooperative research and development agreements with industry, academia, the Department of Defense, and the national laboratories to benefit from their investments in millimeter-wavelength/terahertz technology and applications. The TSA should follow a two-pronged investment strategy: Focus on millimeter-wave imaging as a candidate system for evaluation and deployment in the near term, and Invest in research and development and track national technology developments in the terahertz region.
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