DELAYED GAMMA-RAY SPECTROSCOPY WITH LANTHANUM BROMIDE DETECTOR FOR NON-DESCTRUCTIVE ASSAY OF NUCLEAR MATERIAL

摘要

High-energy delayed gamma-ray (DG) spectroscopy is a potential technique for directly assaying spent fuel assemblies and achieving the safeguards goal of quantifying nuclear material inventories for spent fuel handling, interim storage, reprocessing facilities, repository sites, and final disposal. Requirements for the gamma-ray detection system, in the energy range up to ～7 MeV, can be summarized as follows: high efficiency at high gamma-ray energies, high energy resolution, good linearity between gamma-ray energy and output signal amplitude, ability to operate at very high count-rates, and ease of use in industrial environments such as nuclear facilities. High Purity Germanium Detectors (HPGe) are the state of the art and provide excellent energy resolution but are limited in their count rate capability. Note that fully-burned spent-fuel assemblies produce over 10~(15) gamma-rays/s, thus making high count-rate capability being an important driving criterion among the requirements. Lanthanum Bromide (LaBr_3) scintillation detectors offer significantly higher count-rate capability than HPGe detectors, although at lower energy resolution, and they do not need a cooling system. Thus LaBr_3 detectors may be an effective alternative for nuclear spent-fuel applications. The performance of a 2" (length) × 2" (diameter) of LaBr_3 scintillation detector system was evaluated. Spectroscopy characteristics of the detector system were measured up to count rates of ～3 Mcps. Further, experimental measurements were conducted at the Idaho Accelerator Center, Idaho State University, where ～3g of ~(235)U and ～3g of ~(239)Pu samples were irradiated with neutrons from a photon-neutron source and DG spectra collected. Potential capabilities and limitations of LaBr_3 scintillation detectors for high count rate DG spectroscopy for assaying nuclear material were investigated.