Impact of Source Definitions in Efficiency Calibrations for Segmented Gamma Non-Destructive Ass

Segmented gamma scanners (SGS) are often employed to provide an accurate determination of gamma-emitting nuclide activity for containerized waste and scrap. In this system, the detector is vertically collimated to view only a portion of the container to account for density variations. During the course of an assay, the container is scanned through its full height while being rotated. The efficiency calibration or calibration factor that converts a measured peak rate to an activity typically assumes a uniform source distribution within a given segment. If calibrated with known sources, the uniform distribution is achieved by placing a number of sources, either point sources or rod sources, in a distribution that approximates uniformity. For a given container segment, the vertical extent of the source term can be defined in two ways: a source term that is defined only in the viewing solid angle of the collimated detector, or a source term that spans the entire container vertically. In the first method, the total activity is determined by adding the individual segment activities. In the second method, the total activity is determined by the weighted average of the segment activity results. The differences between the two methods depend on several factors including the distribution of the source within the sample, the definition of the source term in the calibration, the construction and geometry of the collimation, and the container-to-detector distance. The effects of cross-talk among adjacent segments also will be dependent on energy and matrix attenuation. These subtle effects, coupled with knowledge of the expected item stream, influence the choice of SGS calibration and analysis methodology. In this work, we assess the benefits and drawbacks of each method taking into account the various factors that can impact the final activity determination. This work also illustrates the benefits of calibrating with mathematical methods, due to the flexibility in being able to alter the calibration to suit the desired methodology.