A Tomographic Segmented Gamma Scanner for the Measurement of Decommissioning Wastes
Authors: Mason J. A. , Tolchard A. C. , Towner A. C. N. , Burke K. , Price R. A. , Dittrich S. , Zurey F. , Walraven D. 
Published: March 2009

The ANTECH Tomographic Segmented Gamma Scanner (TSGS) combines: a) Conventional Segmented Gamma Scanner (SGS) analysis (compliant to ASTM C1133-96), b) Tomographic Gamma Scanner (TGS) analysis providing both attenuation and source distribution maps (effectively 3D images) of the interior of drums, and c) Isotopic ratio analysis for uranium and transuranic elements using PC-FRAM. In SGS mode the drum is rotated and scanned segment by segment along its height. A two-pass measurement, one for transmission and one for emission, results in two spectra for each segment. An assay is made by measuring the intensity of a characteristic gamma ray from each nuclide. Corrections are made for count rate-related losses and attenuation by the item (using a transmission source). Calibration standards are used to provide the relationship between observed gamma-ray intensity and nuclide content. On completion, count rates are summed, and mass values for the nuclides of interest in the entire drum are calculated based on comparisons to appropriate calibration materials. In the case of SGS, the matrix is assumed to be homogeneous on a segment by segment basis. TGS involves measuring drums in segments as for SGS. However, in the case of TGS, while the drum is rotated, it is also moved in the horizontal direction (translated). Also, instead of taking a single large spectrum for each segment, 150 separate spectra are taken as the drum rotates and is translated. These 150 spectra are obtained both for transmission and for emission measurements. The 150 spectra taken for transmission constitute a set of data that can be solved to yield the distribution, or map of attenuation coefficients throughout the segment of the sample or drum. The measurement equations are over specified and the solution uses a maximum likelihood analysis. This results in the determination of a map (after a geometric transformation) of attenuation coefficients in a rectangular grid suitably superimposed on each segment. The attenuation map enables the operator to ‘visulise’ the variation of the density (governed by the collimator size and voxe resolution) in regions of the drum. This serves a non-destructive examination function similar to ‘real time radiography’ but with lower resolution. For the analysis of the emission data, the additional information obtained from the transmission data allows the emission data to be corrected for attenuation. This attenuation correction is the essential and important characteristic of TGS measurements not present in other gamma-ray measurement systems. For the first time in the case of the TGS, the map of attenuation is used to correct the measured source distribution in the matrix (segment by segment). The TSGS extends the range of gamma-ray measurement technology, as it is able to correctly determine the attenuation corrected radionuclide inventory in heterogeneous matrices where previous techniques such as the SGS are only applicable to homogeneous matrices. In the case of TGS a single calibration based on a non-interfering or empty matrix is made and then corrections relating back to this non-interfering matrix are made using the attenuation information determined from the transmission scan.