Determination of gamma emitters in radioactive waste containers by advanced gamma scanning

Gamma scanning gives information about the activity inventory of gamma emitting nuclides in radioactive waste containers. To extend investigations to non-homogeneous waste containers two advanced segmented gamma scanner systems were set up and improved methods for the evaluation of the measured data were developed. In non-segmented gamma scanning the inventory of a radioactive waste container is determined by measuring the emitted gamma spectrum with a collimated high purity germanium detector which is moved vertically while rotating the container (spiral scan). The radioactive nuclides present in the container are determined from this spectrum by their characteristic lines. Since there is a correlation between the net peak count rate and the specific activity of a nuclide the activity can be calculated assuming a homogeneous density within the container. This measuring method was extended by subdividing the scan area into a preselectable number of segments, typically 24 segments for each rotation of the container. For each segment the full energy spectrum (8 kbyte) is stored by the computer. The evaluation of these segment spectra results in a count rate distribution for every nuclide detected. Considering this information the accuracy in quantification of the gamma emitting nuclides for inhomogeneous activity distributions can be increased compared to the results of non-segmented gamma scanning. One main problem for the quantification of the radioactive inventory is the influence of matrix effects on the gamma attenuation. Using digital radiography additional information about the gamma attenuation is available. Correlating them with the count rate distributions the accuracy in quantification can be increased further