Source Term Quantification Using 3DCZT

The nuclear industry has long used germanium spectrometers to accurately quantify source terms. These systems may consist of a shielded germanium detector with a limited open angle and a modeling toolkit to determine the efficiency based on the source source geometry. These systems provide state-of-the-art quantification by using the excellent energy resolution to identify peaks, shielding to block out interfering sources, and a detailed model of the source geometry and detector efficiency. There is a strong motivation to perform these characterization measurements in situ; in the case of barrels it is possible to relocate them for counting, but in the case of pipes it is necessary to bring the detector to the source and often elevate them. For these reasons using a compact system that does not require cryogenic cooling, such as large-volume high-resolution CdZnTe spectrometers, would greatly ease the source term quantification challenges in the safeguards field where hold-up measurements of nuclear material in pipes are common. This work will discuss the combination of a radiation transport model and detector efficiency characterization toolkit with a shielded high resolution imaging spectrometer. This spectrometer was developed under SBIR funding for Materials Accounting and Control from the US Department of Energy consists of a single ~5cm3 CdZnTe crystal capable of 1% FWHM at 662 keV with 2.5cm of tungsten shielding on all sides except the forward direction. The results are focused on the development of the transport model and validation of the model using measured data and standard industry transport codes for benchmarking. The transport model calculates the activity per unit area or per unit volume for pipes or barrels given user-specified geometry and shielding terms. For each detected isotope in the user’s library the model will provide an estimated activity, for isotopes that are not detected it will provide a minimum detectable activity. The efficiency of the detector is calibrated as a function of direction using a robot and the correct sensitivity is applied for the angle of travel for each source term voxel to the detector. The quantification method is validated using industry standard general transport codes and measured data.