Visual Examination (VE) gloveboxes are used at the Savannah River Site (SRS) to remediate and Bias of Large Container Gamma-Ray Assay Results

In the gamma-ray assay of containers such as 200 liter drums and large crates, several factors can contribute significantly to the total measurement uncertainty (TMU) and even give rise to a bias in the reported activity. A major contributor, and in many cases the leading contributor, in this arena is the nonuniformity of the distributions of activity and attenuating matrix within the container. This is because, lacking detailed information about these distributions, the assay of a given container is generally based around the assumption of uniformity. Differences between the uniform assumption and the actual distribution within a particular container can have very large effects on the final assay result depending on the energy of interest and the matrix density and degree of heterogeneity. A specialized Monte Carlo technique has been developed to model random spatial distributions of both source and matrix within typical containers and has been used to map out distributions of assay results (i.e. ratio of measured result to true activity) over a range of container types, gamma-ray energies, sample densities, and degrees of nonuniformity. These results will be presented along with simple semi-empirical formulae which parameterize these contributions to assay TMU and bias for common assay situations.