A field-portable instrument has been developed for the purpose of classifying and identifying chemical agents within munitions by measurement of the linear attenuation property of the agent. For most chemical agents of interest, the gamma-ray linear attenuation coefficient is sufficient to separate the agents by class (nerve, blister, and blood). In addition, many chemical agents of a particular class are separable by the attenuation coefficient. Complications in gamma ray transmission measurements arise due to the packaging of the chemical agents in thick-walled steel containers of various configurations, corrosion, and changes in the state of the material from liquid to solid or gas. Identification of chemical agents within a container and without imaging is contingent upon sampling a region of the container that has a homogeneous distribution of the agent. The instrument allows for several degrees of freedom to accommodate multiple data acquisition protocols, including tomographic imaging. A variety of algorithms have been investigated including single-ray transmission to complete 2D-computed tomography using a collimated isotopic source. Recent results indicate that gamma ray measurements can provide identification of chemical agents in reasonable time frames. This paper will describe the system, data acquisition and processing, and present results from laboratory and field studies.
Portable systems for x-ray imaging of objects up to 20-cm in diameter have been developed for field inspection of industrial objects. These systems can be configured with either a linear diode array (45-cm long, 1024-elements, 12- bits/element) or a large-area amorphous-silicon (a-Si) detector (30 X 40-cm2, 2304 X 3200-elements, 12- bits/element). Each detector utilizes gadolinium oxysulfide as the scintillation element. X-rays are emitted from an 80 to 300-kVp constant-potential source with a spot size of approximately 1.6-mm. The object can be rotated and the source and detector translated vertically for collection of 'spiral' fanbeam or 'helical' conebeam computed-tomography (CT) data. For low-density objects, the reconstructed spatial resolution of CT data collected with either detector is about the same and the choice of detector is determined by detector parameters such as dynamic range and integration/readout time. For higher-density objects, which need to be imaged at higher energies and for which there is a higher probability of Compton scatter, the linear diode array produces better contrast images of small voids in a scattering medium. A series of experiments designed to test the performance of each detector with and without a scattering medium will be presented.
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