Engineering Research Paper Summary Project - Section A 2012

Millimetre Wave and Terahertz Technology for detection of Concealed Threats

GAURAV PANDEY (33)

There has been intense interest in the use of millimetre wave and terahertz technology for the detection of concealed weapons, explosives and other threats. Radiation at these frequencies is safe, penetrates barriers and has short enough wavelengths to allow discrimination between objects. In addition, many solids including explosives have characteristic spectroscopic signatures at terahertz wavelengths which can be used to identify them.

                                                              The increased threats of criminal or terrorist action in recent years have led to development of many techniques for detection of concealed weapons, contraband, explosives or other objects.  Systems based on electromagnetic radiation between 30- 300GHz in the millimetre-wave and 300GHz- 3THz in the terahertz region have particular advantages that:
1.) Radiation penetrates many common barrier materials enabling concealed objects to be seen

2.) Wavelengths are short enough to give adequate spatial resolution for imaging or localisation of threat objects and radiation at these frequencies is non-ionising and, at modest intensities, safe to use on people.

                        It is harder to work at terahertz frequencies, the lack of practical sources and detectors for many years led to the region becoming known as the ‘terahertz gap’. However,   the     higher

frequency means that systems can be physically smaller for the same image resolution.  Also, many materials, including common explosives, exhibit characteristic terahertz spectral features which can be used to identify them. Concealed-threat detection applications include screening of people ,screening for people such as stowaways and mail screening. These focus on detection of metallic and non-metallic weapons and explosives in aviation security and protection of sensitive facilities; detection of contraband by customs authorities and stolen items in loss-prevention applications; as well as stand-off suicide bomber detection and detection of weapons carried by potential intruders or assailants. Mail screening applications include detection of drugs-of-abuse .Detection of hidden objects depends on the transmission of radiation through barrier materials as well as through the atmosphere.  Inhomogeneous materials also scatter incident radiation to a greater or lesser extent. At millimetre wave frequencies, non-conducting solids and liquids behave as dielectrics and reflect between 1% and 25% of the incident radiation.  Absorption coefficients are typically a few dB/mm at 100GHz and rise with frequency. Conducting liquids such as water have a reflectivity of 40% at 100GHz falling quickly to 20% at around 500GHz and then levelling off, and are very strong absorbers such that penetration into water or the human body is only a millimetre or so.

                                                               At terahertz wavelengths materials absorb more strongly and refractive indices tend to be lower leading to smaller reflectivity. The increased absorption is due both to resonances in the materials and scattering by the microstructure of many substances.  Absorption coefficients vary very widely. Some materials, such as plastics, remain virtually transparent .Others, including glass, pottery and porcelain are strong absorbers (~35dB/mm at 1THz).

                                                             Practical detection systems will usually need to operate in reflection rather than transmission due to the high absorption of the explosives themselves and absorption by the body. In reflection geometry, the spectral features due to the resonances are still visible, but are much less strong. Most barrier materials such as different types of cloth, paper, cardboard, plastics are semi-transparent to terahertz with an absorption which rises smoothly with frequency.   Other substances may have features in the THz range, but we have not observed significant confusion with explosives. A variety of methods have been used to produce two and three dimensional imaging systems.  A single detector may be mechanically scanned across a scene using mirrors. Scanning time can be reduced by using a line array of detectors or a full two dimensional array, at the cost of providing many detectors. . Terahertz imaging systems, with few exceptions, are currently limited to a single detector and these currently take several minutes to capture an image. To be useful, spectral features must be distinct from those of barrier and harmless, potential confusion materials. Most barrier materials such as different types of cloth, paper, cardboard, plastics are semi-transparent to terahertz with an absorption which rises smoothly with frequency.   Other substances may have features in the THz range, but we have not observed significant confusion with explosives.

                                                       Terahertz imaging systems, with few exceptions, are currently limited to a single detector and these currently take several minutes to capture an image. Developments of millimetre-wave systems over  a number of years have led to commercial mm wave systems, mainly operating  at 30GHz or 94 GHz, designed for a range of checkpoint and stand-off people screening applications and these are now beginning to become more widely used in the field. Higher frequencies enable more compact systems and these are also starting to appear.  Before terahertz systems can be produced for operational use, further development is required, both in source and detector technology and in system architectures. Nonetheless terahertz continues to show promise as a technique for people screening due to its potential for materials specific detection, an area where few other candidate technologies exist.