The changing face of airport security
Keith Rogers and Paul Evans* discuss the technologies being used to keep people safe at airports.
Ensuring safe air travel is a high priority for operators and governments, as the travelling public is aware. The increasing sophistication of terrorist threats is driving the need for improved technologies, designed to detect the myriad of potential risks. This comes at a time when passenger numbers are increasing, which in turn creates tension for airport and airline operators as demands for higher throughput and requirements for greater detection accuracy often conflict.
Specific terrorist events often produce responses in terms of technological innovation. For example, one reaction to 9/11 was the introduction of widespread computer tomographic (CT) imaging for hold luggage. The reaction to the London transatlantic bomb plot of 2006 and the ‘printer cartridge bomb plot’ of 2010 included a limit on the amount of liquids, aerosols and gels (LAGs) in carry-on luggage, which led, among other innovations, to the development of spectroscopic methods for routine liquid identification.
Prior to the 1988 Lockerbie bombing, threats arose primarily from concealed weapons and technologies were developed to detect the shape of relatively high-density objects such as guns and ammunition. The use of X-ray imaging technologies became the de facto standard and such screening systems are ubiquitous today, albeit with enhancements.
Single-view X-ray screening has been replaced by dual, orthogonal-view systems, for improved object visualisation. This X-ray absorption-based imaging continues to evolve with multi-view systems providing enhanced depth resolution and fast CT creating 3D reconstructions of bag contents. An example of a state-of-the-art CT system is Rapiscan’s RTTTM, which has a relatively high scanning speed of up to 1ms and a stationary gantry.
Despite the disadvantages of CT – including relatively high cost, maintenance and large footprint – such systems with enhanced capability are likely to find increasing use. In contrast to identification on the basis of morphological features, threats from concealed explosive materials have presented new challenges, particularly for material analysts. Developing technology to address the identification of lower atomic number [density] substances in various formats, at relatively high speed within the operational constraints imposed has been a challenge. The problem is magnified as materials within suitcases are generally inaccessible for direct sampling.
Materials identification methods used in aviation security are often thought of as either bulk or trace techniques. Trace detection requires a physical sampling strategy followed by examination using characterisation probes such as mass spectrometry, chemiluminescence, ion mobility spectrometry, immunoassay and bio sensors. These methods detect trace amounts [<1mg] of explosive from baggage surfaces or vapours, but are slow and require operator training. Only a small proportion of luggage may be examined by such methods due to the throughput demands of airport operators.
Bulk detection systems attempt to non-invasively detect explosives while screening large quantities of baggage. Non-X-ray-based screening systems include neutron techniques, nuclear quadrupole resonance and terahertz time domain spectroscopy, although these are not currently in widespread use.
Unfortunately, the X-ray absorption characteristics determined from a single measurement do not generally provide sufficient materials discrimination to separate all threats, for example explosives, from benign materials. However, the differential variation of X-ray interaction cross-sections with material properties and photon energy may be used as a material discrimination tool.
Therefore, dual energy imaging is capable of coarse material discrimination – the false colour images are viewed as luggage is scanned and material space is separated into three categories – organic, inorganic and mixed. The firearm on the X-ray image is easily recognisable but the light organics, in orange, cannot be uniquely identified. The dual-energy approach is used by most X-ray screening systems employed within airports. It can be applied to multi-view and CT modalities, with the principal advantage of such CT being the ability to identify organic materials in sheet form.
A need to keep up with the threats
As detection technology advances, the terrorist threat becomes more sophisticated and the need to develop screening tools with greater materials discrimination fidelity becomes essential. This is particularly the case for homemade explosives where the compositional heterogeneity results in increased false alarm rates from X-ray absorption based methods.
Governments and industries have therefore been looking for new approaches to achieve more accurate material discrimination without compromising the high throughput demands of airports.
Whenever X-ray photons pass through materials, several physical interaction processes occur, including scatter. Materials scientists are very familiar with the coherent scatter process, as it forms the basis of X-ray diffraction (XRD) and crystallographic methods.
Numerous threats from concealed explosives and other illicit materials, such as narcotics, appear in a polycrystalline form and are routinely examined by laboratory diffractometers to establish their identity and study their crystallographic structures.
The conversion of X-ray diffraction from the laboratory into the aviation security domain is an attractive proposition, as the analysis is orthogonal to any absorption-based method and is highly specific for the material phase. However, scatter signature intensities are several orders of magnitude less than those exploited within image formation and therefore, in general, the technique demands significantly longer data acquisition times to achieve practical S/N values. Although this problem can be offset by higher-power X-ray sources and more sensitive detectors, it is amplified by a reduction in relative coherent scatter cross-sections at the photon energies required to penetrate suitcases.
Several attempts have been made to incorporate X-ray diffraction into aviation security screening equipment, but there is only one notable commercial system incorporating diffraction, the XRD 3500TM, developed by Morpho [Smiths Detection]. In this system, X-ray diffraction is usually used as a secondary screening technique after CT identification of suspicious materials within an inspection volume, i.e. the diffraction component acts as an alarm resolver. It uses high-power X-ray sources to increase the amount of diffracted flux available for analysis and high aspect ratio collimators to locate the source of the diffraction signal.
Despite this, practical solutions for the deployment of XRD within mass transit screening systems have remained elusive. An alternative approach in development used an effect know as focal construct geometry (FCG), which was first reported in 2007. Here, in contrast to high-power X-ray sources and photon starving collimation, the incident beam topology is modified so that objects are illuminated with hollow cones of photons. This produces high-intensity maxima along a principal axis that occurs when Bragg’s Law is satisfied, analogous to the intensity maxima observed within a conventional diffractogram.
With FCG, the signal is formed from converging cones and corresponds to compact, diffracted ray geometry, unlike conventional systems where diverging Debye cones are intersected by a detector. It is possible to collect the diffraction signatures using monochromatic incident radiation and translating a point detector along the principal axis or through a polychromatic source and fixing an energy resolving point detector at some distance from the object. The scattering patterns produced in this way resemble a conventional pencil beam arrangement but, although there is a central intensity maximum, the direct beam does not intersect the detector.
FCG has an important advantage over other approaches in that it produces an increased relative intensity that results in acquisition times that can meet the demands of current airport screening. In high-energy dispersive mode (~150kV), FCG can also penetrate typical carry-on luggage. This approach was recently used in a digital device scanner commercialised by Halo X-ray Technologies, shown in Figure 4 [above]. Another advantage concerns the imaging modality that comes from the use of hollow conical beams of X-rays – both absorption and diffraction caustics can be used in a tomosynthetic approach to directly image an object.
This is similar to conventional computer tomography scanning, as the object appears to have been rotated about an axis normal to the detection surface in an oblique beam formed from a composite of annular projections. This is an interesting and counter-intuitive result, as the conical shell incident beam is produced using a point source and only linear motion is employed in the image collection process.
Overall, the identification of materials within aviation security screening has become a demanding technological problem. Threats change almost monthly and all present a new challenge. An advantage with a diffraction-based method is its inherent future proofing – as new explosive compositions emerge, the measurement systems require little or no modification to still achieve high-accuracy identification. Risk-based intelligence, combined with advanced imaging and materials identification, will provide a high degree of reassurance for travellers and result in an improved passenger experience.
*Keith Rogers is Deputy Director at the Cranfield Forensic Institute, Cranfield University, UK.
Paul Evans is Head of The Imaging Science Group at the College of Science and Technology, Nottingham Trent University, UK.