Spectral Signatures for Identifying Explosives With Wideband Millimeter-Wave Illumination

Millimeter-wave imaging systems used in airports, government buildings, and other facilities for personnel screening use advanced imaging technology (AIT) to detect explosives and weapons concealed under clothing. Additional information in the imaging data can be applied to identify the composition of the detected objects. The method described here demonstrates that material data in the form of the dielectric constant can be derived from the variation of reflectivity in millimeter waves over a range of frequencies from 18 to 40 GHz. By fitting the reflectivity to an optical model, the thickness and dielectric constant, including attenuation, can be computed. The method is applied to samples of inert substances and a military sheet explosive to show that detected anomalies can be distinguished as distinct materials through their dielectric constants. For absorptive materials, a frequency band of lower frequencies, 2-18 GHz, can be applied to detect the dielectric, as is demonstrated in the case of a commercial explosive. Used with AIT, the technique can facilitate the evaluation of threats at personnel checkpoints.