7
Conclusions and Recommendations

At the current stage of development, constructing an airport-compatible prototype of a PFNTS-based explosives detection device, such as the MDNR, would be ill advised. Current PFNTS designs appear to be at a substantial disadvantage compared to x-ray CT-based technologies that have already been certified. Based on the information currently available, an installed PFNTS-based device would cost considerably more than currently available FAA-certified EDSs. Even if the PFNTS per-unit costs could be brought down to a level comparable to those of CT-based systems, the PFNTS system would still be at a disadvantage because it requires much more space to install and is considerably heavier, which would make it much more difficult to install in existing airports; in fact, the entire baggage transfer system would have to be redesigned and reconstructed.

The promise of PFNTS—and other element-specific technologies—is their potential to detect a wider variety of threats and lower threat masses than are currently included in the FAA's EDS certification standards. Thus, a critical remaining step in the development of PFNTS technology is characterizing the range of potential threats and threat masses it could reliably detect. This kind of research would not be practical with a prototype system designed to meet current EDS performance certification standards and packaged in a compact configuration for integration into an airport environment.

Another attribute of PFNTS is its capability of responding to changing explosive threats to airline security. It seems reasonable to expect that eventually the certification standards will be tightened to require detection of smaller explosive quantities and a wider range of explosive types with less defined densities. As terrorists become more sophisticated in the placement of explosive devices, the threat quantities of interest could become smaller. The original categories for EDS certification testing reflect the explosives of choice by terrorists at the time the certification standard was developed. The promise of PFNTS is not that it can detect explosives at the current certification standards but that it has the "potential" to detect smaller threat quantities and a wider range of explosives. Therefore, research should be focused on characterizing the potential performance of PFNTS rather than on optimizing the airport-installed performance of a current point design. This type of research could best be done in a research setting rather than in an airport environment with a prototype designed to meet current certification standards.

Questions Posed by the FAA

At the panel's first meeting, the FAA asked that four questions be addressed during the course of this study. The panel and the NRC staff determined that these questions fit within the Statement of Task.

Question 1. Given the choice, would airlines select the PFNTS instead of equipment based on currently available x-ray CT for checked baggage inspection?

Answer. No, the airlines would not choose a PFNTS-based explosives-detection device for three reasons. First, PFNTS has not demonstrated an ability to meet the FAA's certification requirements for detecting Class A explosives. Second, the area detector configuration has not demonstrated an ability to meet the FAA's false alarm requirements. Third, because of integration difficulties that would be caused by the size, weight, and safety precautions attendant upon the deployment of a PFNTS-based device, the airlines would choose currently available x-ray CT-based EDSs.

Question 2. If so, is their preference for this technology strong enough to justify the remaining costs to develop this technology (estimated to be $20 million to $30 million)?

Answer. Not applicable.



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7 Conclusions and Recommendations At the current stage of development, constructing an airport-compatible prototype of a PFNTS-based explosives detection device, such as the MDNR, would be ill advised. Current PFNTS designs appear to be at a substantial disadvantage compared to x-ray CT-based technologies that have already been certified. Based on the information currently available, an installed PFNTS-based device would cost considerably more than currently available FAA-certified EDSs. Even if the PFNTS per-unit costs could be brought down to a level comparable to those of CT-based systems, the PFNTS system would still be at a disadvantage because it requires much more space to install and is considerably heavier, which would make it much more difficult to install in existing airports; in fact, the entire baggage transfer system would have to be redesigned and reconstructed. The promise of PFNTS—and other element-specific technologies—is their potential to detect a wider variety of threats and lower threat masses than are currently included in the FAA's EDS certification standards. Thus, a critical remaining step in the development of PFNTS technology is characterizing the range of potential threats and threat masses it could reliably detect. This kind of research would not be practical with a prototype system designed to meet current EDS performance certification standards and packaged in a compact configuration for integration into an airport environment. Another attribute of PFNTS is its capability of responding to changing explosive threats to airline security. It seems reasonable to expect that eventually the certification standards will be tightened to require detection of smaller explosive quantities and a wider range of explosive types with less defined densities. As terrorists become more sophisticated in the placement of explosive devices, the threat quantities of interest could become smaller. The original categories for EDS certification testing reflect the explosives of choice by terrorists at the time the certification standard was developed. The promise of PFNTS is not that it can detect explosives at the current certification standards but that it has the "potential" to detect smaller threat quantities and a wider range of explosives. Therefore, research should be focused on characterizing the potential performance of PFNTS rather than on optimizing the airport-installed performance of a current point design. This type of research could best be done in a research setting rather than in an airport environment with a prototype designed to meet current certification standards. Questions Posed by the FAA At the panel's first meeting, the FAA asked that four questions be addressed during the course of this study. The panel and the NRC staff determined that these questions fit within the Statement of Task. Question 1. Given the choice, would airlines select the PFNTS instead of equipment based on currently available x-ray CT for checked baggage inspection? Answer. No, the airlines would not choose a PFNTS-based explosives-detection device for three reasons. First, PFNTS has not demonstrated an ability to meet the FAA's certification requirements for detecting Class A explosives. Second, the area detector configuration has not demonstrated an ability to meet the FAA's false alarm requirements. Third, because of integration difficulties that would be caused by the size, weight, and safety precautions attendant upon the deployment of a PFNTS-based device, the airlines would choose currently available x-ray CT-based EDSs. Question 2. If so, is their preference for this technology strong enough to justify the remaining costs to develop this technology (estimated to be $20 million to $30 million)? Answer. Not applicable.

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Question 3. Does PFNTS have any realistic potential for application to full cargo container inspections? Answer. No. Based on Tensor's analysis of PFNTS for cargo screening of LD-3 containers containing single items (a reasonable projection given the sparseness of existing test data), PFNTS could only interrogate 57 percent of air cargo shipped in LD-3 containers. The other 43 percent contains large amounts of hydrogenous and, therefore, highly neutron-attenuating material, rendering PFNTS ineffective for screening. Further complicating the use of PFNTS for cargo screening is that containerized cargo is not always uniform in composition. The likelihood of false alarms from nonhomogeneous containerized cargo assembled by cargo consolidators has not been determined. Finally, the capability of PFNTS to detect explosives concealed in thick containers (e.g., the LD-3) has not been experimentally verified. Current estimates on neutron attenuation (based on crude exponential algorithms) are sufficiently accurate to raise concerns about highly attenuating hydrogenous cargo. These estimates do not treat beam divergence from neutron scattering in the cargo and are, therefore, not sufficient to validate the PFNTS detection for low-attenuating scenarios. Question 4. What experiments, if any, should be pursued in the near future to further define this potential? Answer. Because PFNTS does not appear to be either practical or currently desirable for airport deployment, the panel does not recommend that experiments addressing the airport integration of PFNTS be pursued at this time. However, experimental verification of Tensor's simulation of PFNTS performance for cargo screening might be useful. Prototype A critical limitation of PFNTS-based explosives-detection technologies is their inability to detect Class A explosives at the Pd level required for FAA certification. Other limitations are the high Pfa demonstrated for area detectors and their unproven capacity for resolving alarms. Unless and until these limitations are overcome, there is no reason for the FAA to address the technical or operational issues associated with integrating the technology into an airport setting. Even for the current certification testing requirements for Class B explosives (as opposed to Class A explosives), PFNTS-based technologies would not be selected by airports for primary screening of carry-on baggage or checked baggage because of the difficulties of integrating these technologies into existing baggage lines and the operational issues associated with ensuring the safe operation of radiation-producing accelerators that meet regulatory requirements. To date, no one has demonstrated that any PFNTS-based technology, including the MDNR design proposed by Tensor Technology, could be feasibly integrated into existing airport operations. The 7.9 m x 12.6 m (26 ft x 42 ft) rectangular space requirement described in the MDNR design could not be provided easily at most airports. Furthermore, few if any existing airport terminal structures could accommodate the 657-tonne (723-ton) weight without significant alterations. The size and weight of PFNTS-based explosives-detection devices are largely dictated by shielding requirements and by the necessary distance between the accelerator and the detector array for time-of-flight energy resolution. The panel believes it is unlikely that a PFNTS-based device could be designed with a much smaller footprint than the MDNR proposed by Tensor Technology. Therefore, x-ray CT-based systems are a better choice for detecting current explosive threat levels. If the PFNTS system were configured for the smallest possible size, a complex baggage belt transfer system would be required to move baggage through the unit and to orient it properly in front of the detector array. In actual operation, this complexity is likely to lead to time-consuming baggage jams that would have to be cleared manually. The time required to shut down the system and enable an operator to safely enter the shielded environment and clear a jam would substantially reduce the throughput of the system. Even though the panel believes a PFNTS-based device could be operated safely, the public, and perhaps even airline employees, might believe the system posed health risks. Because of the perceived risks and the physical size and weight of PFNTS-based devices, operations personnel could decide to locate the device in a remote facility, either a standalone facility or a separate building outside of the terminal complex, which would inhibit passenger involvement in resolving false alarms. Therefore, a PFNTS system would have to demonstrate a much lower Pfa than the current certification standard to offset the difficulty of resolving alarms. The potential of PFNTS for screening small loose cargo packages has not been sufficiently explored using existing research accelerators, and the existing characterization of air cargo (type, size, weight, delivery constraints, method of delivery to the airport) is not sufficient to develop a valid testing protocol. Furthermore, although the FAA may have intelligence data that support determining the type and mass of an explosive threat, the agency has not endorsed a specific threat definition that could be used as a basis for evaluating the potential of any explosives-detection technology for cargo inspection. Based on cargo characterization data and analytic estimates of neutron attenuation, the panel concluded that PFNTS does not have a realistic potential for screening the full spectrum of cargo containers and pallets. Recommendation. The FAA should not fund the development of a prototype of a multidimensional nuclear radiometer-based explosives-detection device. Recommendation. Based on the current explosive threat levels and the state of the art of pulsed fast neutron transmis-

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sion spectroscopy (PFNTS) technology, the FAA should not deploy PFNTS technologies or devices for primary screening of carry-on baggage, checked baggage, or cargo. Recommendation. At this stage, the FAA should not fund the development of an airport test facility. Research After reviewing the laboratory-demonstrated detection performance of PFNTS (Chapter 3) and comparing PFNTS-based devices to x-ray CT-based EDSs (Chapter 6), the panel concluded that deploying a PFNTS-based system would not be practical at this time. Indeed, laboratory testing has not demonstrated PFNTS to be technically desirable compared to the FAA-certified InVision CTX-5000 SP, CTX-5500 DS, and L3 Communications 3DX-6000, which are much easier to integrate into the airport environment. However, because the threat to aviation security is dynamic, the requirements for explosives-detection system certification may very well change over time; and, at some point, new explosives-detection approaches will probably be necessary. Among the technologies currently in development, PFNTS has shown the most promise of meeting more stringent certification testing requirements because of its element-specific detection technology. In other words, because PFNTS can determine multiple element-specific metrics of the materials it characterizes, it has the potential to detect lower threat amounts with a lower Pfa and to adapt to new threats. Therefore, although the deployment of a PFNTS prototype (e.g., MDNR) is not desirable at this time, valuable research could be conducted on the application of PFNTS for explosives detection. Priorities Existing neutron sources cannot readily provide the beam characteristics (high-current, low-radiation background) for significantly improving the quality of the test data. If innovative approaches to the use of currently available research accelerators cannot resolve this deficiency, research on the PFNTS technology for explosives detection may have reached a logical stopping point. If so, the experimental data (including baggage scan data from blind tests and complete documentation on the application of the detection algorithm to these scans), details of the detection algorithm, and suggested paths for refining the detection algorithm should be archived to preserve the knowledge gained from FAA-funded research. The technology could then be reevaluated if new security threats evolve for which current CT-based systems are inadequate and for which element-specific approaches might have the potential to meet aviation security requirements. If funding is available for further research on PFNTS technology for explosives detection, the greatest benefit would be derived by addressing the current shortfalls of the technology rather than by developing and assembling a prototype MDNR unit that could be integrated into an airport. Because this panel has not studied and is not acquainted with the whole spectrum of research requests for explosives-detection technologies submitted to the FAA, the panel neither supports nor opposes the allocation of funds for further research on PFNTS. The recommendations in the following paragraph are based on the assumption that funds are available but should not be interpreted as endorsing such allocations. The research recommendations in this report are directed at improving the detection performance of PFNTS-based explosives-detection technologies and do not address the practical limitations of deploying PFNTS-based devices. Because of the large mass of the cyclotron magnets and the extensive radiation shielding required for PFNTS, the size and mass of a PFNTS-based explosives-detection device can probably not be significantly reduced. These practical limitations should be considered in evaluating the potential of PFNTS for explosives detection in airports. Recommendation. The research priorities for pulsed fast neutron transmission spectroscopy should be directly related to the current shortfalls of the technology. Three major problem areas that should be addressed are listed below in order of importance: 1.   detection of Class A explosives 2.   reduction of false alarm rates 3.   development of alarm resolution procedures The first priority for research on PFNTS is the development of a methodology for detecting Class A explosives, the one category of explosives for which PFNTS would not pass certification testing. Recommendation. The pulsed fast neutron transmission spectroscopy detection procedure should be refined to detect Class A explosives reliably. These refinements will certainly involve using smaller target pixel sizes and may require multiple views or tomographic representations. Reducing the false alarm rate of PFNTS will be critical for it to become a viable explosives-detection technology. Based on laboratory test data, the false alarm rate for a two-dimensional open detector array is uncertain. A strict interpretation of the existing laboratory blind test data, however, suggests that PFNTS would not pass the false alarm rate requirement for an FAA-certified EDS. Recommendation. The FAA should validate the potential for a pulsed fast neutron transmission spectroscopy-based explosives-detection device with low false alarm rate and a high probability of detection. This will require resolving the sensitivity of the probability of false alarm to the detection

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algorithm (neural net versus B-matrix), evaluating the use of an area rather than a linear detector array, and assessing the effect of higher background neutron/gamma radiation on the detection algorithm. Unless PFNTS is developed to the point that potential users have enough confidence in the system to shut down a terminal and call in a bomb squad for every unresolved alarm, alarm resolution will remain a critical issue. Because the image produced by transmitted neutrons is not sufficient for resolving false alarms, PFNTS will probably have to be paired with a high-resolution x-ray-based imaging technology for the purpose of alarm resolution. Recommendation. The FAA should develop and validate a procedure for resolving false alarms for the pulsed fast neutron transmission spectroscopy technology that meets or exceeds the capabilities of deployed certified explosives-detection systems. This may involve pairing a pulsed fast neutron transmission spectroscopy-based device and an advanced x-ray-based high-resolution imaging system. Several other issues should also be evaluated. The performance bounds for the PFNTS technology should be explored to determine the lowest amount of explosive that can be detected reliably while maintaining a low Pfa. In addition, the flexibility of the PFNTS approach should be validated experimentally by testing with a range of new threat materials. The applicability of PFNTS for screening cargo or small containers within cargo should be assessed. The research priorities to address these issues are listed below: Test the ability of the PFNTS algorithm to detect lower threat quantities and characterize the receiver-operating-characteristic curve for various threat quantities of existing classes of explosives. Characterize the detection performance (Pd and Pfa) of PFNTS for small boxes in containerized and palletized air cargo. Quantify the influence of deuteron current stability, deuteron energy, and room size on detection performance. Quantify the performance of PFNTS technology for bags that have produced false alarms on existing certified EDSs, and characterize the performance for bags that contain explosives that were not detected by existing certified EDSs. Validate the flexibility and adaptability of the PFNTS detection algorithm to incorporate changes that address new threat agents, and characterize the time required to develop the expanded detection algorithm, highlight algorithm implementation problems if the new algorithm were implemented in older systems, and characterize any change in the Pfa associated with the increased range of threat agents. Develop and demonstrate an automated hardware system that could orient general commercial airline bags so the PFTNS detection algorithm would be effective against Class A explosives (e.g., multiple views or standing rectangular bags on a side for the shortest cross section to facilitate neutron transmission). Facilities and Equipment Consistent with the recommendations in the second interim report of the CCAS (1997), this panel believes that if the FAA continues to pursue research and development on PFNTS, current neutron sources, detector technology, and radiation modeling capabilities should be used. However, it appears that no existing research accelerator facility can meet the requirements for demonstrating the potential of PFNTS for explosives detection. These requirements include a 1-ns pulsed deuteron beam, high beam current (~50 µ-amps), a large experimental area consistent with a small bag pixel area and a long neutron time of flight path, and a low scattered neutron and gamma background that would provide PFNTS data with a high signal-to-noise ratio. If a research facility is acquired, it should not entail the development of a new accelerator; a commercially available accelerator should be acquired instead. If a new accelerator is acquired, it should be configured to support a broad range of research activities. Rather than developing an airport-integrated system, further development of PFNTS for explosives detection should be focused on characterizing the potential of PFNTS technology and the sensitivity of the detection process to deuteron beam current stability, the selection of the deuteron energy, and the influence of the room size on the detector signal-to-noise ratio. The utility of an accelerator facility would be greatly enhanced if its design features could accommodate the research requirements of other postulated neutron-based applications. Because the projected PFNTS research will probably be completed in the next few years, and given the expense of developing and constructing a new accelerator-based neutron test facility, the FAA should look for partners (e.g., Defense Advanced Research Projects Agency, U.S. Customs Agency, National Institutes of Health, or the National Science Foundation) to ensure the long-term usefulness of the facility. Locating the facility in a university environment might be the best way to promote broad cooperation and multidisciplinary research and ensure its long-term utility. In this way the facility would contribute to the education of students and promote low-cost innovative experiments that might lead to new uses of neutrons for explosives detection, as well as other applications. The panel does not support or oppose the allocation of funds for a research facility. All recommendations that require government funding are offered on the assumption that funds are available and should not be interpreted as recommendations in support of such allocations.

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Recommendation. If the FAA acquires an accelerator to meet the testing requirements of PFNTS, it should be configured to support a broad range of research activities. Recommendation. To ensure the availability of an accelerator research facility to a diverse range of researchers, the facility should be located in a university environment and not at an airport or private industrial site. Recommendation. To ensure the flexibility of an accelerator research facility to address near-term and far-term research, it should have the following attributes: the capability of generating monoenergetic neutrons from the 2H(2H,n)3He (DD) reaction in addition to the broad-energy "white" neutron spectrum from the 9Be(d,n)10B reaction required by PFNTS a large experiment room with configurable shielding walls to permit changing the size of the room and varying the scattered neutron background shielding in the experimental room and an approved experimental envelope consistent with the operation of a small 150-keV 2H(3H,n) 4He (DT) sealed-tube source in conjunction with other explosives-detection methods compatibility with other missions, such as the production of short half-life radioisotopes that can be used for research on the production and use of medical radioisotopes, which may entail accelerating protons as well as deuterons staff and faculty who can function as a core group for research activities and a radiation metrology laboratory to support characterization of the neutron field