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Aspects of Explosives Detection
ElsevierCopyright © 2009 Elsevier B.V.
All right reserved.
Chapter OneThe Detection Problem
M. Marshall and J.C. Oxley
1. Explosive Detection Technology – The Impetus 1 2. The Problem 3 3. Detection Technologies 4 References 10
1. Explosive Detection Technology – The Impetus
There has always been a need to detect the presence of threats. The classical threats from smuggled weapons and poisons remain, but new threats from explosives as well as from chemical and biological agents must also be considered. Threat must be defined rather broadly, to include both immediate threats, for example, a bomb on an airplane, and longer term threats, for example, smuggled drugs. To prevent explosions requires the detection of bombs, bomb makers, and bomb placers.
The functional components of a bomb are a control system, detonator, booster, and a main charge. Such threats can often be recognized from their shape. These can be viewed as bulk detection issues, historically addressed by imaging techniques such as sight or touch. Other threats may take no particular physical form and can only be recognized by their chemical composition. These are often trace detection issues, historically detected by the sense of taste or smell.
In modern times, many techniques have been investigated for the detection of explosives and illicit chemicals. The main impetus has been for military applications. For example, a great deal of work was carried out in the period 1970–1990 to develop rapid methods and instruments for battlefield detection of chemical warfare agents. There was the development of colored test papers (e.g., the M256 detection kit introduced into US Army service in 1978), the UK deployment of nerve agent-immobilized enzyme alarm and detector, and the production of early models of ion mobility spectrometers (IMSs) for identification of a range of chemical warfare agents (e.g., the Chemical Agent Monitor adopted by the UK Armed Forces in the late 1980s). Similarly, instruments for breath alcohol were developed to aid law enforcement officers in the fight against drunken driving, and a number of simple field tests and kits for the rapid screening of suspect illicit drugs were produced.
Throughout the latter part of the twentieth century the UK experienced a ferocious campaign of Irish terrorism, Israel was the target of frequent Palestinian bombings, and Spain suffered from attacks by Basque terrorist group ETA (an abbreviation in the Basque language: "Euskadi Ta Askatasuna", which translates into English as "Basque Homeland and Freedom"). In Eastern Europe, Chechen terrorists conducted a number of horrific attacks in Russia, whereas in Asia numerous terrorist bombings took place in Sri Lanka. At the same time, attacks against US interests continued around the world (Table 1).
Although both the UK and US military had supported some developmental work on explosive detection, the event that launched new efforts in explosive detection was the downing of Pan Am Flight 103 over Lockerbie on 21 December 1988. Shortly afterward the US Congress passed the Aviation Security Improvement Act (Public Law 101–604), directing the Federal Aviation Administration (FAA) to set standards for acceptable detection (covering not only the type and amount of explosive, but also sample throughput) and "certify" instruments that met those standards. Other countries also mounted similar programs, notably the UK and Israel.
The investigation into the sabotage of Pan Am Flight 103, which left 269 dead, indicated that the explosive used was Semtex H, a plasticized mixture of hexahydro-1,3,5-trinitro-s-triazine and pentaerythritol tetranitrate, and that the amount used was half the quantity that the fledgling technique of Thermal Neutron Analysis (TNA) was designed to detect. Although the placement of the explosive device was fortuitous (from the terrorists' point of view) and the suitcase had not been screened by TNA, this event killed the TNA prototype program.
Progress toward setting certification standards was slow so that in 1991 the Office of Technical Assessment was quite critical of the FAA's efforts [4, 5]. One of the FAA's responses was to create the first of many National Research Council committees to review and advise. By 17 July 1996, when TWA 800 crashed taking off from New York, one system, the InVision CTX 5000, had obtained certification but appeared nowhere near deployment. The crash of TWA 800 and the accompanying suspicion that it was an act of terrorism changed the paradigm. Congress mandated deployment of non-certified detection systems and, more importantly, supplied the funding to support their purchase. The influx of money provided by Congress spurred the industry, which had previously only dabbled in explosive detection, to spend serious money on research. Furthermore, the terrorist attacks on US soil on 11 September 2001 made the threat sufficiently real to the American public that stringent security measures, once thought to be intolerable, could be put in place. The ensuing Afghanistan and Iraq wars provided a massive increase in government funding for explosive detection and defeat. Further impetus was provided to research in the UK by the attacks against public transport in London in July 2005 and threats in the summer of 2006.
2. The Problem
In considering detection issues, we need to find ways of narrowing down the scope so as to focus on questions that can be addressed and solved in practice. This involves making some assumptions about what a terrorist or other bomb maker might do and why they might do it.
The problem of threat detection can be considered on several different axes, as follows:
1. the malefactors;
2. the location – airports, public buildings, vulnerable high-value facilities;
3. the target – airplanes in luggage, in cargo, on people; and
4. the threat – weapons, drugs, explosives, bombs.
We can reasonably divide such malefactors into the following four groups: (1) state-sponsored actors; (2) non-state-sponsored actors; (3) criminals; and (4) mentally disturbed or immature persons. Each of these groups has individual characteristics that impinge on our strategy for bomb detection. However, each also requires the same basic four requirements, namely, motivation, knowledge, capability, and access.
Criminals and mentally disturbed or immature persons are both likely to be limited by the availability of materials and knowledge. In addition, criminals are quite likely to be more susceptible than the other groups to deterrence by visible and effective security measures. Thus, the first two groups – state-sponsored actors and non-state-sponsored terrorists – are the main threats on which explosives detection needs to focus. Unfortunately, this conclusion implies the need for detection of military, commercial, and improvised explosives and does not greatly help in narrowing down the issues.
The geographical location, such as an airport or building, whether the threat is on (or in) people or objects, the target mobility, and whether the target is moveable or fixed are all factors that impose constraints on the detection techniques employed and influence the operational deployment. For example, high-energy X-rays might be used to screen baggage containers but most certainly could not be used to screen thoroughbred racehorses. In many instances, the location and nature of the target will be the predominant factor in choosing a detection strategy. This is a multidimensional problem. The issue needs to be viewed as a whole; different approaches are needed depending on the scenario, and what works in one arena may be operationally impractical in another. For example, the installation of security screening and explosives detection systems at airports has had significant physical impacts in terms of the space required for equipment and also upon the flow of passengers through facilities. Quite detailed studies of traffic flows need to be conducted to ensure that security procedures do not impair the overall function of a facility. Such considerations are of course much easier to resolve in new buildings when the requirements can be built into the design, as opposed to existing buildings where modifications can be costly and difficult.
Related to the issue of threat detection is that of threat resolution. We can distinguish the following three types of positive detection: (1) false alarms where an innocent substance is incorrectly identified as a threat; (2) innocent detections where a threat substance is correctly identified but is not a threat, for example, traces of explosive on members of the security forces or other persons who legitimately work with explosives; and (3) genuine threat detections. This implies a need for an understanding of the environment, that is, what background levels of target species may be present in the public environment from legitimate activities, and what potentially interfering species may be present. Background surveys to answer these questions would assist in the difficult decision as to where to set alarm levels for instruments. And, of course, operators need a plan and a system for resolving alarms when they occur.
A general issue is the need to design detectors against a specific set of threat scenarios or target materials. It is important not to be driven by the technology but to address the operational requirement by whatever means is most effective. An explosives trace detector is unlikely to be the right solution if the threat is from smuggled knives.
Human factors need to be properly considered in the design and application of any detection system. Studies have shown that explosive detection systems generally perform less effectively in realistic field trials than in laboratory tests and that one of the biggest causes of this shortfall is failure to properly consider the operator/ system interface.
3. Detection Technologies
Apart from explosives there is a great deal of interest among law enforcement agencies in both the detection of caches of illegal drugs and in determining whether a person has taken an illegal drug. There is also a medical requirement for the diagnosis of unconscious patients admitted to hospital emergency rooms where treatment depends on diagnosis and delay may be fatal. Typically, this latter requirement is met by laboratory analysis rather than field portable detectors. In the drug field there are potentially many thousands of possible drugs of abuse, whereas in the explosives field there are also theoretically very many potential threat materials. It is one thing to design an instrument to detect a single compound with great sensitivity, selectivity, and speed, but quite a different proposition to achieve the same performance against a range of compounds, particularly if they have rather disparate characteristics. And, of course, the example of roadside breath testing for alcohol demonstrates that the technical challenges can be substantial even in the single-compound scenario.
If we consider equipment for drug detection, outside the hospital scenario, it is likely to be required to be portable so that it can be used at crime scenes, to be sufficiently robust, safe, and easy to operate so that it can be deployed with individual police officers, to have adequate sensitivity and selectivity so that false positives and false negatives are avoided, to provide rapid results and to operate in a way that does not infringe subjects' civil rights (see Chapter 12). Given that any results are likely to be used in court proceedings, the methodology must be subject to thorough scientific peer review and validation. The techniques used must be open and susceptible to ready explanation. Finally, the technology must be affordable so that it can be deployed on an operationally useful scale. Many of these same requirements also apply to portable explosives detectors for use by either law enforcement officers or military forces engaged in anti-terrorist operations.
Imaging techniques such as radiography are quite good for recognizing bombs either visually or by computer-aided image recognition, but as they are not particularly sensitive, they will only detect suspect items of a certain minimum size. And, of course, the imaging equipment does have to look at the right thing, which may also be disguised to avoid recognition.
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