THE FUTURE OF EXPLOSIVES DETECTION?

Following the Christmas day Detroit bomber, there has been considerable discussion and some heated debate in the media about the need for full body scanners. But do they work? Are they safe? Would these systems prevent a future incident and are all the systems the same?

Full body scanners identify explosives concealed under clothing. Each unit does this in one of two ways: Imaging based units rely upon the visual interpretation of conventional images (these utilise backscatter x-ray and millimetre waves). The alternative terahertzbased systems identify a ´terahertz fingerprint´ and so do not rely upon imaging the human anatomy or operator interpretation. Each approach has its advantages and disadvantages.

Full body scanners: backscatter x-ray and millimetre waves
The major advantage of backscatter x-ray, such as systems provided by Rapiscan and AS&E, is the high image quality. This technology is developed and proven and consequently is capable of being deployed now. This technology however suffers from a number of drawbacks that have limited its deployment. Despite the low dose of x-ray used and
safety arguments articulated by experts, concerns over safety still need to be overcome if passengers are to accept regular irradiation. Deployment of the technology has also been limited by persistent questions over invasions of privacy and related civil liberty issues. The resultant images are often referred to by its critics as a ´virtual strip search´. Companies active in the field have attempted various solutions to dispense with direct anatomical images, but concerns still remain. Before the alleged attempted Christmas bombing, body-scanning technologies such as backscatter were criticised by the American Civil Liberties Union and other groups; the US House of Representative voted 310-118 to prohibit the use of such whole body imaging for primary security screening. The situation is now, however, clearly changing. Finally, the ability of backscatter to detect the presence of certain forms of explosives has also been questioned - the technique relies on the ability of x-ray to pick up density variations, and if these
are insufficient the contrast may not be picked up by the operator. Operator training and alertness are therefore key to its effectiveness.

Millimetre waves address some of the limitations currently cited against x-ray, namely that it is safe and that the images are less sharp (and hence have a lower impact on privacy). These units can be passive. (ie they image the mm waves naturally emitted by the human body) or active (ie they have a mm wave source which is directed at the subject). A number of companies – L3, Brijot and Smiths, have systems deployed or in trial. Consequently systems are available to deploy now. Despite these advantages, millimetre waves also suffer from some of the drawbacks of backscatter x-ray, namely concerns over privacy and their sensitivity to explosives. The latter drawback is more severe than with x-rays, because the images are more blurred and require more operator training and alertness. The technique critically relies on image contrast between the body, clothing and explosive/weapon. Recent examples have indicated that there are limitations on this contrast due to the need for the concealed objects to be at a different temperatures and/or have a different emissivities. Doubts remain that millimetre waves could have detected the explosives concealed in the attempted Christmas bombing. Some of this lack of contrast is addressed by employing the active millimetre wave systems. (ie they image the mm waves naturally emitted by the human body) or active (ie they have a mm wave source which is directed at the subject). A number of companies – L3, Brijot and Smiths, have systems deployed or in trial. Consequently systems are available to deploy now. Despite these advantages, millimetre waves also suffer from some of the drawbacks of backscatter x-ray, namely concerns over privacy and their sensitivity to explosives. The latter drawback is more severe than with x-rays, because the images are more blurred and require more operator training and alertness. The technique critically relies on image contrast between the body, clothing and explosive/weapon. Recent examples have indicated that there are limitations on this contrast due to the need for the concealed objects to be at a different temperatures and/or have a different emissivities. Doubts remain that millimetre waves could have detected the explosives concealed in the attempted Christmas bombing. Some of this lack of contrast is addressed by employing the active millimetre wave systems.

Terahertz fingerprinting of concealed threats
Terahertz light has the potential to overcome many of the limitations noted above. The technology can safely, non invasively and rapidly detect explosives and other anomalies through different types of clothing and other concealment/confusion materials, without the need
to perform any imaging of the human anatomy. It has the capacity to detect and identify hidden explosives in an automated fashion without operator interpretation. The technology works by passing Terahertz light, that lies between radio and light waves in the electromagnetic
spectrum, through the explosive material.

Explosive materials absorb this light strongly at certain terahertz frequencies in this range but not at others, and the resultant ´terahertz fingerprint´ can be used to identify an explosive, and to distinguish it from clothing or other inert materials; libraries of material fingerprints are held in software and compared to that from clothing and other items on a passenger in an automated fashion. For example PETN, the explosive used in the alleged Christmas bombing attempt, has a unique Terahertz fingerprint, distinct from clothing and other common materials. Since clothing is transparent at terahertz frequencies, the terahertz light can pass through several layers, including common garments and shoes.

Terahertz thus has major advantages over other body scanner technologies; unlike x-rays, terahertz is non-ionizing and safe; Terahertz fingerprinting typically uses one one-millionth of the power naturally emitted by the human body in this frequency range, and extensive European Union-sponsored programs (such as the three year Terahertz Bridge project) have verified the safety. The lack of conventional images and the ability to use software to automatically recognise terahertz fingerprints also eliminates the need for operator interpretation and address the significant privacy concerns that have plagued both millimetre wave and x-ray technologies.

Terahertz scanning is also fast. terahertz fingerprints can be
collected in under one tenth of a second from a point on the body. The combination of this speed and automation means the technology has the potential to provide throughputs akin to metal detectors. The technique is also less likely to be subject sensitivity issues that could affect other technologies; trained operators interpreting complex images are not required, and the fingerprinting is based on chemical analysis of the target, not subtle changes in density.

The chief drawback of terahertz light in body scanning is its relative immaturity compared to the other techniques. Field deployable systems are not yet available and will require an additional 12-18 months of development, as well as the appropriate level of investment from governmental and/or commercial organisations. The above attributes have been demonstrated in conjunction with the US and UK governments by TeraView Ltd in the UK, a pioneer in the uses of terahertz light across a range of industries.

TeraView has demonstrated the ability of terahertz light to detect a range of explosives with the UK Home Office and TSA in the US. They have developed the world´s first stand off explosives detection system for laboratory evaluation, as well as a portable hand-held wand prototype capable of detecting and identifying hidden explosives. Whilst prototype systems exist and have been verified by US and UK government agencies, these capabilities have yet to translate into field deployable systems. The challenges are however considerably derisked by progress to date, but until field trials commence, they have not been fully verified.

All body scanning techniques have advantages and disadvantages. Millimetre and backscatter x-ray are being deployed now and afford a new level of passenger screening. However they are unlikely to fully address
the concerns over privacy, safety, sensitivity and throughput. Terahertz fingerprinting has the potential to overcome most of these limitations and afford the simplicity of operation of today´s metal detectors.

However further investment and field testing is required. In reality it is likely that the future will see a combination of these technologies used. And all will also have to evolve to meet the on-going threat of countermeasures employed by increasingly sophisticated terrorists.

Please note: The units pictured in this article are prototypes. TeraView is currently looking for partners in design and manufacturing to go to full scale production with the units. For more information visit: www.teraview.com (opens a new window)

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