Year
2003
Abstract
The Tomographic Gamma Scanner (TGS) can be used to produce transmission images of opaque voxels. The detection of opaque voxels in a heterogeneous matrix is used to evaluate the validity of assay measurements. The TGS produces images that are reconstructed using an array of voxels in a three-dimensional space. The threedimensional space can be constructed using various resolutions, which improves the identification of the opaque voxels. An opaque voxel is one that contains a dense region that completely attenuates the transmission source. Opaque voxels themselves do not create measurement errors if the material being measured is located outside the dense region. However, any material located within this dense region will be inaccurately assayed due to the complete attenuation of the transmission source. This illustrates one way to hide nuclear material; the material’s gamma rays can be shielded with a highly attenuating material such as tungsten or lead, and its neutrons moderated with a layer of low-density polyethylene. This eliminates the ability of both gamma and neutron measurements to accurately assay the material. Also, non-destructive assay (NDA) measurements are susceptible to opaque voxel problems because dense regions may contain material that cannot be accurately quantified, thereby giving erroneous assay results. Using the TGS to detect opaque voxels provides useful evaluation techniques to validate the NDA results for the quantification of special nuclear material in transuranic waste.