EXTENDING CALIBRATIONS OF THE TOMOGRAPHIC GAMMA SCANNER

Tomographic Gamma Scanning (TGS) is a means of nondestructively assaying gamma-emitting radionuclides in waste containers. The TGS technique involves an assay protocol with three degrees of freedom (rotational, translational, and vertical) and image reconstruction algorithms are used to develop three-dimensional maps of the source distribution and matrix attenuation within the waste item. This permits a refined estimate of the matrix attenuation corrections to be made based on a finer spatial scale than other gamma ray scanning methods used in waste assay. Traditionally, the TGS is calibrated using radionuclide sources of known activities (or masses). For TGS systems assaying Special Nuclear Material (SNM), the usual practice has been to perform an isotope specific calibration, that is, to calibrate using the same radionuclides that are expected in item assays. But recently, the application of the TGS technique has extended to the assay of nuclear power plant wastes containing a wide variety of fission and activation product radionuclides. To accommodate such applications, a new approach has been successfully employed whereby a TGS system can be calibrated using radionuclide sources that need not be the same as those expected in item assays. In addition, the energy range of calibration can be extended carefully beyond energy range confines of the gamma rays present in the experimental data. The methodology is based on exploiting the similarities in the shape of the intrinsic efficiency curve of the High Purity Germanium (HPGe) detector to the shape of the TGS calibration factor as a function of energy. Extrapolation beyond the range of calibration data is done by determining a scaling factor (which may also account for residual energy dependence) based on relative efficiencies for a simple source-detector geometry. The scaling factor can be determined experimentally or by using a mathematical method such as Canberra’s In-Situ Object Calibration Software (ISOCS). The ISOCS methodology involves using a detector whose response has been mathematically characterized. We illustrate these methods of calibration using data taken on TGS systems intended for general purpose use on nuclear power plant wastes.