Existing and Potential Standoff Explosives Detection Techniques (2004)

Recent terrorist attacks have led to an elevated concern with regard to national and international security and have prompted security measures to be increased. Following the 1988 bombing of Pan Am Flight 103 over Lockerbie, Scotland, airline security procedures, such as luggage and passenger screening, were assessed by a number of organizations, including the Federal Aviation Administration, the Office of Technology Assessment, and the National Research Council (NRC). These groups also looked into new bomb detection methodologies. The terrorist attacks of September 11, 2001, and the attempted shoe bombing of American Airlines Flight 63 in December 2001, led to reexamination of the issues related to airline security, but once more the increased scrutiny focused on the screening of luggage and passengers utilizing close-proximity explosives detection.

These security measures, however, were not designed for scenarios in which individuals appear in an open environment and a security decision must be made at a distance from the suspected explosive. For scenarios such as these, standoff explosives detection is required, where physical separation puts individuals and vital assets outside the zone of severe damage should an explosive device detonate. The difficulty of the standoff explosive detection task is exacerbated by several factors, including dynamic backgrounds that can interfere with the signal from the explosive, the potential for high false alarms, and the need to ascertain a threat quickly so that action can be taken.

To assist the Defense Advanced Research Projects Agency (DARPA) in its efforts to develop more effective, flexible explosive and bomb detection systems, the NRC has agreed to examine the scientific techniques

currently used as the basis for explosives detection and to determine whether other techniques might provide promising research avenues with possible pathways to new detection protocols. This report addresses the following tasks:

• Describe the characteristics of explosives, bombs, and their components that are or might be used to provide a signature for exploitation in detection technology.

• Consider scientific techniques for exploiting these characteristics to detect explosives and explosive devices. Particular consideration must be given to discriminating possible signals from the background and interferents that can be anticipated in real applications.

• Discuss the potential for integrating such techniques into detection systems that would have sufficient sensitivity without an unacceptable false-positive rate. In proposing possible detection protocols, give consideration to trade-offs between desirable system characteristics, including relative ease of implementation.

• Propose areas for research that might be expected to yield significant advances in practical explosives and bomb detection technology in the near, mid, and long term.

Successful standoff explosives technology involves detection of a weak signal in a noisy environment. This background is also often dynamic, so that exemplary performance in controlled laboratory settings may be quite poor performance in the field. The speed with which the detection is performed is a crucial factor when a potential threat is rapidly approaching. Finally, all explosives detection methods both generate alarms in the absence of threat, and do not alarm in the presence of a true threat.

Detection of explosives involves receiving a signal, processing the signal, assessing the results, and ultimately deciding whether explosives are present or not. To assess the performance of a given detection methodology, concepts such as sensitivity (a measure of when a detector alarms if the substance of interest is present) and the receiver operating characteristic (ROC) curves are considered. ROC curves, which plot the probability of detection against the probability of false alarm (and thus combine sensitivity and specificity performance), are of particular interest because they provide a means of comparing two competing detection techniques.

Although these performance measurements are important, it is equally important to note that for a very low probability of explosives presences in the field, even tests with very high sensitivity and specificity can have unacceptably low proportions of observed alarms yielding true threats. It is important to note here the subtle but important difference between the rate of laboratory “false alarms” (the probability of an alarm given that no explosives are present) and the rate at which observed alarms in the field turns out to be false (the probability that no explosives are present given than an alarm has sounded). The latter is of particular concern for implementation because users may cease to react to alarms if this rate is exceedingly high. The proportion of alarms that turn out to be false associated with a particular detector is a function of the detector’s sensitivity, specificity, and the underlying probability of the true presence of explosives.

When assessing a detection system built on multiple detection technologies, a measure called system effectiveness (SE) is used to characterize the overall system performance in the presence of environmental, threat, and other potential detection confusers. SE is a measure of the degree to which a detection system can be expected to achieve a set of specific mission requirements; it can be expressed as a function of availability, dependability, and capability.

In considering any situation involving standoff detection of explosives, one must have some general understanding of the scenarios of concern (e.g., suicide bombing, concealed bombs in roadways, bombings in a stadium) and the parameters that describe the explosive device and the surrounding environment. Two scenarios were given primary consideration by the committee. The first—suicide bombings—is of concern because there is little time or opportunity to detect the bomb before detonation. The second—wide-area surveillance, or monitoring a large area for the presence of explosives—is of interest in order to prevent the illicit introduction of high explosives into an area being monitored.

These scenarios can be further refined by defining and identifying threat parameters involved in any particular scenario, including the following:

• Threat parameters related to the local environment. Particularly important is the identification of the intended target. Other parameters include meteorological conditions, as well as the possible presence of trace explosives and any other chemicals in the area where the detection will take place.

• Threat parameters that describe the device. These include the type of explosive, its mass, and the device’s construction.

• Threat parameters that characterize the bomber. These can include both psychological and physiological aspects of the bomber or bomb maker.

In describing explosives detection systems, two or more explosives technologies are considered completely orthogonal if the detection methods detect independent characteristics of the explosives device. Three potential significant advantages of a system of orthogonal detection technologies are:

1. a higher probability of detecting explosives over a range of potential threats,

2. increased difficulty in defeating the detection system, and

3. greater effectiveness in detecting explosives than any single technology.

A positive indication of an explosives threat from a sensor does not mean that an explosive is present. Standoff explosives detection must take into account more than the single sensor indication, because a system that depends on a single signal yields excessive false alarms. The intent of a system of orthogonal detectors is detection of an explosive when one is present and extremely few indications of an explosive when one is not present. In order to achieve this goal, careful system design is needed to resolve ambiguous sensor information. In addition, a system allows one to consider additional aspects such as mass of explosive, available sampling time, available response actions and times, and the role of human judgement in assessing effectiveness.

To design an effective standoff explosives detection system—explosives detection where physical separation puts individuals and valuable assets outside the zone of severe damage from the potential detonation of an explosive device—the following issues must be considered:

• Multiple sensors of different types increase the number of possible indications that can be searched for in the environment.

• Both specificity and sensitivity can continue to increase with additional sensor types, as long as there are indications that each sensor type can find an explosive if an explosive is present during its interaction with the environment.

• The result coming from a standoff explosives detection system is not static, nor is it desirable that it be static.

• Novel threats will be recognized only incidentally via intersection with threat parameters currently considered by the system.

• Choice of sensor types and system design must be integral with the nature of the threats.

Threat parameters can be used to identify the performance challenges that must be addressed when developing standoff explosives detection. These threat parameters include, but are not limited to, the following:

• Means of delivering the device to the point of detonation

• Location and timing of detonation

• Composition of the explosive

• Mass of the explosive

• Other components of the explosive device

• Dispersed materials

Additional considerations that may impact standoff explosives detection include ambient environmental conditions and the influence of humans present in the event.

In order to properly evaluate a system comprised of multiple technologies, system effectiveness must be utilized. Within these orthogonal detection systems, both false positives (false alarms) and false negatives (misses) will occur. While system effectiveness will be a function of the sensitivity and specificity of the system, the system in turn will comprise the sensitivity and specificity of each component, as well as how the system is put together.

The architecture of a system of explosive devices could consist of multiple arrays of different types of technologies. Distributed arrays can be fixed in one location with multiple sensors over a geographical area. While there are advantages in wide area coverage and standoff potential, there are significant technical challenges, including

• Communication between sensors

• Sensor sampling

• Data transfer

• Fusion of information

• Sensor fault detection

• Time to sample

• Detection decision making

• Deployment issues.

A system for standoff detection cannot be static. Intended capability and ongoing performance must change to respond—at least—to new

threats, new background conditions, changes in existing threats, and threats actively attempting to defeat the system.

The diversity of potential explosive formulations makes detection of explosives based on their chemical characteristics a challenge. However, this diversity suggests that a consideration of the elemental composition of explosives might lead to new or improved detection approaches. If elemental formulations are considered, then few common chemicals would be mistaken for explosives.

All self-contained explosives must contain both oxidizing and reducing agents. This leads to a high preponderance of the more electronegative elements nitrogen and oxygen, and helps make explosives readily detectable by ion mobility spectrometry. As new explosives containing other electronegative elements are utilized, detection based on these atoms may be possible.

Several different atomic and molecular properties might be exploited in explosives detection. Nuclear properties identified from gamma-ray emissions provide a unique signature for some elements. Core electron ionization and subsequent characteristic X-ray emission gives rise to another broad class of methods that might be used for atom identification. Molecular spectroscopic techniques can be used to uniquely identify explosive molecules in the vapor phase, but the low vapor pressure of many explosives limits their use. LIDAR (light detection and ranging) techniques show promise for standoff applications; however, selectivity is problematic when detecting complex explosives mixtures with broad spectroscopic properties. Sensor array detectors (e.g., resistive, fluorescent) or “electronic noses” offer the possibility of specificity with relatively inexpensive instrumentation. Key issues that have to be addressed for these arrays are sample collection and concentration.

Explosives detection techniques usually focus on either bulk explosives or traces of explosives. Detection of bulk explosives is carried out either by imaging characteristics of the explosive device or by detection of the explosive itself. Trace detection utilizes either emitted vapors from the explosive or explosive particles deposited on surfaces. Many explosives detection techniques are limited either by fundamental physical limits or by the circumstances of a particular scenario, for example, background interference.

• X-rays. This technology has good potential for imaging at standoff distances of 10 to 15 m. X-ray backscattering images reveal outlines of explosive devices. The imaging distance can be extended by developing new X-ray sources; X-ray optics (lenses and mirrors); and compact, inexpensive remote detection apparatus. An alternative approach may be coded aperture imagers since they are able to achieve high sensitivities with practical devices.

• Infrared. Preliminary experiments show that concealed explosives can be detected beneath clothing in an indoor setting using infrared techniques; these are less viable outdoors. To improve these techniques, research is needed on spectroscopic properties of human skin, clothing, and other relevant materials.

• Terahertz. Imaging in the terahertz region of the electromagnetic spectrum allows for detection of explosives hidden beneath clothing without exposing people to the danger of ionizing radiation. However, a fundamental limit on image resolution is encountered for wavelengths longer than 300 microns. Therefore, the shortest wavelength possible should be chosen to resolve items in the terahertz region. Absorption of terahertz radiation by atmospheric water absorption is also a limitation.

• Microwaves (mm waves). Even though the resolution of images using microwaves is fundamentally limited at standoff distances, explosive devices that use large amounts of metal will give anomalously large reflection that can be detected.

• Neutrons, gamma rays, magnetic resonance and magnetic fields. Although the use of neutron and gamma-ray explosives detection suffers from a combination of potential health hazards and limitations in sensitivity for standoff detection, explosives detection based on these technologies can potentially be used to screen large cargo containers at points of entry. Magnetic resonance techniques require close proximity and/or a large amount of bulk explosives, making them ill suited to standoff detection. Extremely sensitive magnetometers have been developed to detect metal objects; however, clutter from other metal objects is a significant limitation to their use for standoff explosive device detection.

• Optical absorption. Explosive molecules may be identified by using their ultraviolet (UV), electronic, and vibrational resonances (absorptions). The need for large samples and the use of relatively fragile laboratory instrumentation remove these techniques from the standoff category unless lasers are used (see below).

• Optical fluorescence. This technique, for use in detecting granular materials, has standoff potential. Lack of very high sensitivity and problems of environmental quenching must be overcome.

• LIDAR, DIAL, and DIRL. LIDAR, differential absorption LIDAR (DIAL), and differential reflectance LIDAR (DIRL) may suffer from sensitivity limits in the 10- to 30-m range due to the very low molecular concentrations of explosive. However, nonlinear optical techniques can be used to increase signal-to-noise ratios since these techniques have the potential for increased signal-to-noise relative to linear techniques.

• Array biosensors using captured antibodies. These are not likely to be useful for remote explosives applications unless the size and cost of these sensors are dramatically reduced. Enzymes can detect explosives at the parts-per-trillion level but require concentrators and long analysis times.

• Biomimetic sensors. An important avenue for future research is the development of robotic “insects” with onboard sensors or samplers. One would hope to develop low-cost, nonintrusive devices that could be controlled remotely in any weather.

Hyperspectral imaging from widely disparate regions of the electromagnetic spectrum and the combination of explosive device imaging with

identification of the material composition of the explosive in the device are two general approaches that should be applied in the development of orthogonal explosives detection systems.

Biologically based systems for explosives detection are quite numerous, but their sensitivity, robustness, and efficacy for standoff monitoring remain undefined. Biological or biosensor approaches for explosives detection combine the specificity of molecular recognition of biomolecules with electronics for signal transduction. In many cases, modern molecular biology has provided the tools to isolate and modify the genes for receptor proteins to make biosensors. Efforts have been made to identify genes that are induced or activated by explosives. The utilization of such information could permit the engineering of plants and animals with luminescence or fluorescence reporter genes for passive monitoring of explosives.

Based on existing data, it appears that a variety of standoff spectroscopic or acoustic surveillance techniques could be used to detect physiological changes in bombers or bomb makers. These changes include body heat signatures, color changes of skin and tissues, and irregularities in heart beat and rate. Approaches such as these could overcome one of the main limitations to the use of biosensors for standoff detection, providing real-time feedback to the detector operator.

As part of its charge, the committee considered a number of novel concepts for explosives detection. These concepts are described in detail in Chapter 7. A brief overview is presented here:

• Dynamic behavior of an explosive vapor plume. An understanding of this dynamic behavior will assist in the development of standoff explosives detection based on the explosive’s spectroscopic properties.

• Detection of a suicide bomber’s local atmosphere. Electronegative at-

oms present in high explosives may cause electron attachment and a subsequent depletion in negative ions around a person carrying concealed explosives as he or she walks through a background ion field. This depletion may be detectable.

• Detection by detonation. Remote detonation could be accomplished by mechanical or acoustic shock, high-intensity electromagnetic pulse, microwave radiation, or radio-frequency (RF) induction heating. This technique could be used only in situations where it was possible to disarm the bomber and explosive device without harming innocent bystanders.

• Detection by self-reporting sensors. The presence of explosives would be accomplished using standoff mine technologies, such as neutron activation analysis. Small sensors would be silent until a critical threshold of detection is reached.

• Standoff Compton backscatter X-ray imaging. Using low-energy X-rays, a target is illuminated and backscatter photons that have been emitted from the target are collected. Photomultipliers could be used to detect light flashes in plastic that result from these photons.

• Distributed biological sensors. Bees, moths, butterflies or other insects would be trained on biomarkers in the bomber such as those described in Chapter 6. Other options include fitting rats trained to detect explosives with a wireless global positioning system (GPS), a bioluminescent reporter gene, and a microphotocell. The goal would be for the rat to find clandestine bomb production facilities in crowded urban areas.