The aim of the project is to develop two THz imaging systems:

A: Stand-off imaging system: Detection of threats and explosives carried by a distant person (up to approx. 20 m).
B: Close-by detection system: Detection of explosives and bioagents hidden in close-by objects such as letters.

Fig. 1: Scheme of a T-ray stand-off imager

Stand-off imaging system

The stand-off imaging system, as sketched in Fig. 1, the sensor will be configured similarly to metal detectors used in airport security systems. The reflected radiation from the THz sources is collected and analyzed to determine the presence of target compounds in personnel, clothing and shoes. It should also be noted that the approximate distance of 25 m, at which the stand-off system should detect explosives, is set by a typical lethal radius around a suicide bomber. Consequently, this scanning distance allows preventing harmful action from a save distance. However, this distance makes it difficult to achieve sufficient contrast with passive THz imaging techniques. Therefore an active THz imaging system with a source for illumination is advantageous. This allows for high spatial resolution, which is fundamentally limited by wavelength of at which the system operates and a high sensitivity. Compared to passive remote imaging it is possibly more sensitive because the reflected power from the illumination source is larger than the thermal emission.

Fig. 2: T-ray scanner for close-by detection of explosives, phathogens and chemicals hidden in objects, e.g. mail.

Close-by imaging system

Fig. 2 depicts the principle close-by detection system. It is based on pulsed THz radiation opto-electronically generated by femtosecond lasers. Pulsed THz imaging systems have a number of potential advantages for close-by inspection of packaged objects, because of their high sensitivity and spectral resolution, which will allow an identification of the object or substance. Because many common packaging materials are relatively transparent in THz frequency range, a spectrum of the packages and their content can be obtained. Pulsed T-ray imaging has already been utilized in laboratory experiments to allow the identification of different powdered materials concealed inside envelopes. Using the THz spectral information different powders may be uniquely identified. Different thicknesses of the powder layers can also be imaged to investigate the influence of scattering on the measured THz pulses.

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