The ANTECH G3800 Series of instruments utilises the Tomographic Gamma Scanner (TGS) approach (ASTM C1718) for the non-destructive assay of samples containing gamma emitting materials. Compliant with ASTM C1718, the ANTECH TGS divides the drum into cubic volume elements, known as voxels, employing maximum likelihood algorithms to determine the distribution of both activity and attenuation within the drum. This enables a 3D visual representation of the drum matrix and activity to be determined and a high accuracy assay of the drum to be completed. However, due to the high levels of collimation, typical measurement times are high and require a higher count rate drum to be effectively deployed.
The G3850 Tomographic Gamma Scanner (TGS) uses transmission corrected, single photon emission computerised axial tomography to determine the spatial distribution and quantity of radionuclides in a drum or can. This High Resolution Gamma Spectroscopy technique (HRGS) represents a considerable advance over the segmented gamma scanning technique by implementing a simple translation axis in addition to vertical scanning and rotation axes.
The TGS was developed by the Los Alamos National Laboratory (LANL) in the early 1990s for the United States’ Department of Energy. A transmission source allows the determination of a 3-D spatial map of the attenuation coefficient at any energy by interpolating between the gamma peaks of transmission source at several energies. Once the attenuation coefficient maps have been established for the sample, emission tomography is used to determine the distribution of selected radioisotopes within the sample. Two pass (transmission followed by emission) measurements are performed.
The segmented gamma scanning technique produces an average linear attenuation coefficient for each horizontal segment of the sample can or drum. The tomographic gamma scanning technique provides source (emission) images for each segment in addition to this average linear attenuation coefficient. The typical spatial resolution for the emission image from the TGS is
about 50 mm (1.97 in) for drums and 30 mm (1.18 in) for cans.
The TGS is capable of correcting for errors that arise from lumps below the resolution of the tomographic gamma scanning technique using a comprehensive multi energy lump correction algorithm. This is appropriate for samples such as plutonium pyrochemical salts that may contain pieces of plutonium metal approximately 2 mm (0.079 in) in diameter. The multi energy
lump correction algorithm is not available with the segmented gamma scanning technique.
The TGS uses a single calibration constant to determine the isotope mass of a wide range of material and matrix types. The attenuation and source distribution matrix is known more accurately for the TGS than the Segmented Gamma Scanner so biases due to matrix and source distribution are significantly reduced by the TGS.
Isotopic ratio analysis of plutonium is performed using PC/FRAM code. An 8k channel spectrum is obtained for isotopic analysis during the TGS scans. Typically, the ANTECH TGS is also able to perform SGS analysis. The TGS Drum Scanner meets both US Nuclear Safeguards, and DOE WIPP measurement and QA requirements using ANTECH GammaScan+ software.