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We report the use of a terahertz pulsed imaging technique for three-dimensional chemical mapping. Terahertz radiation reflected from a sample was measured pixel-by-pixel in time domain using a terahertz pulsed imaging system developed at TeraView Ltd, UK. The recorded terahertz waveforms were then transformed into frequency domain using time-partitioned Fourier transform. Structural maps of samples were obtained by analyzing the terahertz time-domain data whilst chemical maps were obtained from terahertz spectral data sets. For a sample comprising chemical A at the surface of a polyethylene pellet and chemical B buried inside the pellet, we have separated the component spatial patterns of the two chemicals using their spectral fingerprints. The reconstructed three-dimensional chemical maps not only locate the chemicals in the object, but also identify each chemical. We also demonstrate the capabilities of terahertz pulsed imaging for non-destructive analysis of coating thickness and quality, and for detecting and identifying explosive materials such as RDX.

Recent events have led to dramatic changes to the methods em ployed in security screening. For example, following the failed shoe bombing, it is now common for shoes to be removed and X-rayed at airport checkpoints. There is therefore an increasing focus on new technologies that can be applied to security screening, either to simplify or speed up the checking process, or to provide additional functionality. Terahertz (THz) technology is a promising, emerging candidate. In previous publications we have shown how our THz pulsed imaging systems can be used to image threat items, and have demonstrated that explosive materials have characteristic THz spectra. 1 We have also demonstrated that non-metallic weaponry can be imaged when concealed beneath clothing. In this work we examine more closely the properties of barrier and potential confusion materials. We demonstrate that barrier materials have smooth spectra with relatively low attenuation. We further demonstrate that the terahertz spectra of several common chemicals and medicines are distinct from those of threat materials. Keywords : security screening, terahertz, applications, explos ive detection, people screening, imaging, spectroscopy 1. INTRODUCTION 1.1 Critical issues in security screening Recent events have demonstrated the need for ever more effective security screening, and for systems capable of detecting contraband. A wide variety of techniques are already available, for the detection of a variety of threats, such as weapons or explosives, or illicit items, ranging from drugs to illegal immigrants. Current methods of bag screening in the United States and elsewhere typically use X-ray inspection techniques with some use of further image analysis, manual search and chemical trace detection. Passenger screening relies heavily on archway and handheld metal detectors that are deployed throughout most airports. However, these techniques are not foolproof, and beneficial enhancements could be made in detecting the following : € Weapons containing a small amount of metal; € Ceramic weapons; € Explosive materials; € Chemical and biological threats. Emerging technologies, such as X-ray backscatter and millimeter wave imaging might be employed to address some current deficiencies. However, the routine use of ionising X-rays raises health concerns, whilst millimeter wave approaches have a limited spatial resolution. It is also unclear whether either technique can provide spectroscopic substance identification. Thus, X-ray and millimeter wave portals are likely to prompt frequent further searching whenever an image indicates a suspect item or area. False alarms can greatly increase the cost and reduce the throughput of screening systems; throughput is crucial in high-volume situations such as in major airports. A further factor is the high dependence of current techniques on human