Date of Award
Master of Science
Department of Engineering Physics
Jeffrey B. Martin, PhD
An analytical method is developed and applied to find the activities of two radioisotopes based on measurements influenced by true coincidence summing. The method incorporates the solid angle subtended by the detector, the macroscopic cross sections of the materials present, the absolute peak and total efficiencies of the detector, and the modes and probabilities of decay of the radioisotope. With this information, the method corrects for both summing-in and summing-out events. Summing events affect peak counts and cause the calculated activity to differ from the true activity. Thin disk sources of Mo-99 and Cs-136 on the face of a closed-end, coaxial high purity germanium detector have been studied. For Mo-99, the analytical method shows there is a 29% reduction in the 740 keV peak counts due to summing events. This factor adjusts the no-coincidence-assumed activity to within 4.0% of the correct value. As for Cs-136, the analytical method shows a 41% reduction in the 1048 keV peak counts. This factor corrects the simplistic activity to within 0.5% of the correct value. Hence, the results indicate that coincidence summing is the primary cause of activity discrepancies for the given configuration.
DTIC Accession Number
Popovich, Anthony P., "An Analytical Method to Calculate Activity from Measurements Affected by Coincidence Summing" (1997). Theses and Dissertations. 5744.