May 29, 2003 A novel technique that uses far-infrared (terahertz) radiation to rapidly identify bulk or airborne materials inside sealed paper or plastic containers has been demonstrated by scientists at the National Institute of Standards and Technology (NIST) and SPARTA Inc., of Rosslyn, Va. Described at a recent technical conference,* the technology has potential applications in homeland security such as detection of explosives in the mail or other non-metallic portable containers. The method involves directing a far-infrared light source at a sample in a closed container, detecting the light transmitted through the materials, and then analyzing the light that was absorbed by the sample while making adjustments for the light absorbed by the container. Far-infrared radiation, which falls between visible light and radio waves on the electromagnetic spectrum, is partially transmitted through many materials. The pattern of light frequencies or spectra absorbed by a material depends specifically on the vibrations of the material's atoms and its crystalline structure.
This method can readily identify compounds made of molecules containing three to hundreds of atoms, the size of many threat materials. The two instruments employed, one using a pulsed laser and the other a glowing filament, are tabletop-sized and work at room temperature. Two years of experiments have demonstrated that the technique detects aerosols (such as those that might contain anthrax spores), pharmaceutical powders, most gases, several explosives and other common materials. The researchers have compiled a database of spectral characteristics for more than 100 materials and developed an automated software tool for rapidly identifying bulk materials based on their absorption spectra.
Further research aims to increase the sensitivity and throughput speed of the technology.
Campbell, M.B. and Heilweil, E.J., "Non-invasive detection of weapons of mass destruction using THz radiation," in Proceedings of SPIE Vol. 5070 Terahertz for Military and Security Applications, edited by R. Jennifer Hwu, Dwight Woolard, (SPIE, Bellingham, WA, 2003) in press.
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