Presentations to the Committee
The Committee on the Review of Existing and Potential Standoff Explosives Detection Techniques met four times over the course of its study. At these meetings, the committee was briefed by experts on various aspects of standoff explosives detection. Included in this appendix are brief summaries of each of the presentations. These summaries are the work of the committee and NRC staff.
STANDOFF EXPLOSIVES DETECTION STUDY
Lisa Porter, Defense Advanced Research Projects Agency (DARPA)
DARPA has requested that the National Research Council (NRC) undertake the present study in order to address the issues and problems related to standoff explosives detection. Standoff explosives detection is the ability to detect explosives at a distance. Two main scenarios can be envisioned for the application of standoff explosives detection. The first is broad-area surveillance, particularly at events with large crowds such as the Super Bowl. A second scenario is the ability to detect a suicide bomber before the bomber is able to reach his or her target and detonate the explosive. Low-probability, high-consequence situations such as these are the main priority for DARPA in its request for this study. Detection of land mines is not a primary consideration for the NRC study.
DARPA requests that the committee identify new approaches and new ways of thinking about the problem of standoff explosives detection. When considering standoff explosives detection, sensitivity and specific-
ity must be taken together. Requirements for standoff explosives detection are a high probability of detection combined with a low probability of false alarms. In order to meet this goal, it is necessary to consider the background in the field when explosives detection is being performed and both its impact on the explosives signal and its potential to contribute to false alarms.
Linking together different detectors for a systems approach to standoff explosives detection is desirable, but doing so must invoke a good systems approach. Effective sensor fusion will depend upon the orthogonality of the sensors. True orthogonality is defined in terms of the false alarms that trigger the sensors, and not necessarily the signal that is being detected.
The committee is tasked with both reviewing what detection systems now exist and trying to determine what could exist. In undertaking this task, the committee may want to consider physical or chemical aspects of explosives that have not yet been exploited. The committee should consider techniques for exploiting these characteristics. The committee should also consider whether there are systems-level approaches that should be exploited.
In addition to these considerations, the committee should take into account the speed and applicability of any novel standoff detection system. The committee should be willing to suggest “out of-the-box” approaches to this problem—including ones that combine chemical, physical, and visual means of detection.
STANDOFF EXPLOSIVES DETECTION
Lyle Malotky, Science Advisor
Transportation Security Administration (TSA),
U.S. Department of Homeland Security
The mission of the TSA is focused more on civilian protection than on protection of military personnel or assets. With this mission, the TSA seeks modes of detection that ensure the safety of the population being screened and can perform the detection task in a timely manner, so that neither commerce nor traffic is significantly impacted.
When looking at the issue of standoff explosives detection, a number of factors must be taken into account. A variety of bombing scenarios can be envisioned, and detection must be geared toward one or more of these. Scenarios include suicide bombers, car or truck bombs, boat bombs, abandoned packages, and booby traps, to name a few. In addition to bombing scenarios, the variety of explosives to be used and to be detected must also be examined. Ammonium nitrate, dynamite, military explosives,
black and smokeless powder, acetone and hydrogen peroxide, and acid, to name a few, might all potentially be used in an explosive device.
In addition to the explosives and the scenarios in which they might be used, other challenges confront those who work to detect explosives. In addition to the aforementioned variety of explosives, the quantity of explosives used plays a large factor in determining whether a particular mode of explosives detection will be adequate. Another significant challenge is presented by materials that can produce false alarms, such as nitroglycerin used for medical purposes.
When considering novel approaches to explosives detection, all chemical and physical aspects of the explosive device should be examined for possible exploitation. Full use of the electromagnetic spectrum may provide novel means by which explosives can be detected. For example, use of radio-frequency, neutron analysis, or X-ray backscatter may offer opportunities for finding concealed explosives. Electro-optical properties, including millimeter wave imaging, radar, terahertz imaging or spectroscopy, the dielectric constant of explosives, or Raman spectroscopy are just some of the other options available for this task.
The elemental composition of commonly used explosives may be a means by which explosives can be detected in a package or in luggage. The high nitrogen and oxygen content in many explosives can provide a characteristic signal for these compounds. Characteristics such as the low vapor pressure of many explosives result in vapor clouds that can be detected using infrared (IR), ultraviolet (UV), or double photon techniques. In addition, vapor sensors can also be sent to the bomb—for example, through the use of nanoparticles, indigenous or dispersed plants or bacteria, trained insects or animals, or potentially nano-robots.
Thermal and mechanical properties of bombs may also be used for detection. IR imaging of a suicide bomber, sonic imaging, anomalous movement of cars or people, or detection of the components of a bomb such as the battery, switch, detonator, or container also may be utilized for standoff explosives detection.
Federal Bureau of Investigation Explosives Unit
Any standoff explosives detection system must be adaptable and flexible. Terrorists and their methods will change and adapt when confronted with new barriers to execution of their attacks. From the standpoint of detection, the use of homemade mixtures to produce explosives and the development of compounds such as acetone peroxides that are easy to
produce and difficult to detect—although they are also extremely difficult to handle safely—present challenges to those seeking to stop terrorist attacks.
A number of detection technologies presently exist or are close to implementation, but many of these pose difficulties for use in standoff detection. Ion mobility spectroscopy (IMS) provides detection in the nanoto picogram range, and recycling of the detector for multiple scans can be achieved in minutes. However, the IMS detector must be quite close to the explosive for effective detection. Likewise, Raman spectroscopy must be close to the source. Gas chromatography-mass spectroscopy (GC-MS) and portable Fourier transform infrared (FTIR) can achieve standoff detection using a laser beam and a telescope, but issues of background interference and the form of the explosive (e.g., FTIR requires a solid sample) must be considered when using these technologies. Dogs present the most versatile means for explosives detection, but they cannot provide truly standoff detection and they require frequent breaks when performing their task. Trained insects may potentially be used for explosives detection, but there are problems to be overcome in training and specificity of detection.
Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF)
ATF has seen a steady decline in the use of commercial high explosives in bombings since the 1970s. Currently, the most prevalent explosives used in criminal bombings in the United States are commercially manufactured low explosives such as black and smokeless powders. Many of the more recent high-profile bombings seen both in the United States and abroad utilize improvised explosives. Homemade explosives such as black powder, triacetonetriperoxide (TATP), hexamethylene-triperoxidediamine (HMTD), chlorates, and perchlorates mixed with sugar or other fuels have come to be used instead of commercial explosives.
While many bombings are acts of vandalism with very few fatalities, the growing utilization of suicide bombings overseas demonstrates the damage that can be inflicted by a single person. In Israel, suicide bombers mostly used TATP as the main charge, and in many cases the bomb is fronted with shrapnel on a belt, copper foil for extra shrapnel, a battery switch, and anticoagulants.
ATF has developed a program to use dogs for explosives detection. This program has trained more than 400 dogs for use in explosives detection, both domestically and in other countries.
MILLIMETER WAVE TECHNOLOGY
National Institute of Justice, Office of Science and Technology (OST)
Passive millimeter wave technology has been developed for use in standoff detection. This technology is based on measurement of emissivity and is used mainly to detect metals, thus providing a means of detecting at least some explosive devices. Presently, this technology has effective standoff capability at approximately 10 m.
Millimeter wave machines are presently large (approximately 650 pounds) and expensive (on the order of $100,000). For this technology to be used regularly on the state and local level, portable devices must be developed with cost on the order of $10,000 to $30,000. Resolution of objects using this technology must be in the millimeter to centimeter range, and standoff capability must be at least 10 m.
Millimeter wave technology is based not on a chemical signature, but rather on millimeter wave thermal emission by all objects. The differences in emission between objects make imaging possible. The difference in temperature between an object and the environment also impacts the ability to resolve objects using this technology. Since millimeter wave technology is used to measure changes, it provides a complement to other technologies in its ability to image an area to see if a change has occurred, for example, to determine if a package has been left.
REPRESENTATION OF ODOR INFORMATION IN THE OLFACTORY SYSTEM: FROM BIOLOGY TO AN ARTIFICIAL NOSE
Through the use of olfactory coding and the development of an “artificial nose,” real-world problems can be solved using neurobiological principles. Biomimetic approaches to sensing allow researchers to learn from nature. For example, in biological organisms, any one sensor in a group of a thousand may not be particularly effective, but the cooperative nature of this mass of sensors provides the organism with the ability to detect with great sensitivity and selectivity.
Analysis of specifically how sensing is performed in living systems provides still further insights. Detection of a specific odor does not involve detection of a specific compound, but rather groups of as many as 500 different molecules. For an organism to effectively sense and identify an odor, a number of capabilities must work in concert. Sensors must be
regenerative so that bound molecules can be dislodged. Multiple sensing as opposed to single specific binding must occur. With multiple sensing, only 32 receptors are required to discriminate among 1000 agents. Using only single specific binding, 1000 receptors would be required to detect 1000 agents. For multiple sensing to be effective, the brain must undertake the sorting of the multitude of information coming from the receptors.
Insights such as these have led to the development of an artificial sensing device. A 16-channel fluorimeter, consisting of a 16-sensor array composed of polymeric sensors, has been developed.
Bureau of Customs and Border Protection
There are approximately 300 ports of entry for cargo into the United States. Seventeen million cargo containers cross U.S. borders every year. The Bureau of Customs and Border Protection has responsibility for screening and examining these cargo containers and is able to screen approximately 5 to 10% of all containers. For detection of illegitimate explosives, a variety of detection methods are used. IMS technology in the form of hand-held ion track vapor tracers is used to screen packages. For examination of trucks and rail cars, gamma-ray imaging and X-ray systems are used, the goal being to search for changes in density that might indicate illicit cargo. Present detection technologies pose problems—particularly in the form of false positives when using vapor tracers even with legitimate cargo such as fertilizer and new leather, for example.
LAND MINE DETECTION AND IDENTIFICATION USING TERAHERTZ SPECTROSCOPY AND IMAGING
Johns Hopkins University Advanced Physics Laboratory
Terahertz (THz) spectroscopy and imaging presents several advantages. High-resolution imaging can be attained, and terahertz radiation can penetrate dielectric materials. When THz imaging is used in the scattering mode, excellent resolution can be attained when imaging. For example, 1-mm resolution at a 10-m distance from an object has been achieved. Work on in this field has also pointed to the use of THz radiation for spectroscopy.
Although THz technology presents many advantages, there are still
obstacles to be overcome. Water absorption presently limits the effective range of THz instruments for use in standoff imaging to approximately 10 m on a rainy day and 100 m on a clear day. In attempting to develop portable THz systems, issues of power sources, signal discrimination and background interference, scattering by particles (both in the soil and as aerosols), and the aforementioned problem with atmospheric water must all be addressed and overcome.
A number of labs are working to use THz radiation in real-time explosives-specific chemical sensors. So-called passive THz spectroscopy for imaging is potentially feasible, but the technology for such a device is not yet available.
NOVEL METHODS FOR EXPLOSIVES DETECTION—SUICIDE BOMBER DETECTION
Because it is difficult to detect explosives long range, research has focused on the detection of the hardware of a bomb, not the explosive itself. With this approach, an assumption is made that at least part of an explosive device contains metal.
When developing standoff detection devices, scenarios must be taken into account. For example, it is assumed that a potential bomber approaching a checkpoint advances at 1 m/s. Therefore, if a standoff detector has an effective range of 30 m and an individual is identified approaching a checkpoint from 30 m away, there is a 10-second window during which identification of any bomb and appropriate action must be made before the bomber is close enough to inflict major damage and/or casualties.
To perform detection under these tight time constraints, a number of approaches have been developed. For a potential suicide bomber, nonlinear radar, which detects metal-metal friction through harmonic returns, is a possible means of detection. Terahertz imaging at 0.1- to 1.0-mm wavelengths can provide both visual identification of an explosive device and chemical imaging through the use of THz spectroscopy.
Distributed sensors to detect metals and/or chemicals can also be used for standoff detection. Tracking can be done through optical means, and feedback provided through wireless communications. Once an effective sensor has been developed, miniaturization should be undertaken to increase its applicability.
An orthogonal systems approach to standoff detection provides advantages such as increases in standoff range and increased spatial resolution.
A MILLIMETER WAVE IMAGING ARRAY FOR CONCEALED WEAPONS DETECTION
National Institute of Standards and Technology (NIST)
The THz program at NIST is focused on concealed weapons detection. The goal is to develop a low-cost, widely deployed system with standoff capability of at least 8 m. THz radiation is particularly well suited for such detection because clothing is transparent at less than 500 GHz, while metals are perfect reflectors.
Terahertz detection is possible using both active and passive methods. While passive detection outdoors is effective because the difference in temperature between the human body and the sky (background) is approximately 100 degrees, providing sufficient contrast, indoor contrast is much lower so that coherent detectors are needed to boost low-power contrast indoors.
For active THz detection, high-powered sources are required. Active sources are relatively inexpensive, on the order of $5000, but the lack of these sources is a primary reason why THz spectroscopy has not been explored in more depth.
Lithographic antennas are being developed, the goal of which is to adapt the pixel idea to THz imaging by having several antennas in an array tuned to a specific wavelength. When combined with multiple sources, imaging of metals is possible.
In order for THz imaging and spectroscopy to be widely used for detection, a combination of high-powered sources and low-cost detectors must be developed.
STANDOFF EXPLOSIVES DETECTION
Technical Support Working Group (TSWG)
For standoff detection to be effective at protecting personnel and valuable assets, standoff must be defined as a minimum of 30 to 50 feet from an individual suicide bomber, and 500 to 1000 feet for a vehicle bomb. TSWG approaches this task (and others) by conducting interagency research with oversight provided by the U.S. State Department.
To this end, TSWG has undertaken a number of research programs to address the challenge of standoff detection. For example, detection of ammonium nitrate powder on a vehicle was successful at close range but could not be made to work at standoff distances. This and other projects point out the difficulties involved in standoff detection because of issues
such as background interference and interference occurring as a result of temperature variations over the course of a day, for example.
Other practical concerns impact the ability to perform effective standoff detection. Nuisance alarms, caused by actual detection of explosives that are not of interest, such as spent munitions, must be taken into account when determining whether an actual threat exists. While the technology for explosives imaging may be more feasible than that for explosives spectroscopy, the fact that imaging requires human interpretation whereas spectroscopy can be automated must be considered.
In any explosives detection scenario, choice of the proper tool for the job is essential. To that end, a variety of technologies are in place for explosive detection in a number of widely different locations and facilities. Imaging by low-dose X-ray backscatter is now used at entry points within the United States and is under consideration for additional uses as well. Infrared imagers are being assessed, and thermal emission technology is being revisited. Tests using this technology indicate that it may be useful in identifying suicide bombers. Passive millimeter wave imaging is a project in development, the goal of which is to provide a man-portable imaging system. In addition, nonimaging millimeter wave technology is being developed, whose goal is to look at anomalies in reflectance.