Erchinger, Jennifer Lynn (2017-08). Development and Demonstration of an Ultra-Low-Background Liquid Scintillation Counter. Doctoral Dissertation.
The Ultra-Low Background Liquid Scintillation Counter (ULB LSC) constructed at Pacific Northwest National Laboratory further expands the capabilities of LSC by utilizing background reduction techniques to achieve lower minimum detectable activities. Hence, alpha and beta detection capabilities are enhanced for samples that have previously required extended count times, large sample volumes, and/or complex separation chemistry. The three-chamber ULB LSC system design includes layers of passive shielding in conjunction with active rejection of cosmic muon interactions with the goal of reaching background rates on the order of 10 to 100 counts per day. GEANT4 Monte Carlo radiation transport simulations for the full shield showed an expected count rate of 10 counts per day from the environmental radiation backgrounds, in addition to a systematic count rate of 15 counts per day expected from initial tests with a partial build. The observed chamber background is 11 counts per day for the left chamber and 2 counts per day for the right chamber, including systematic and environmental radiation backgrounds, over an energy range of 25-2500 keV. This is an improvement in reducing background count rates by approximately 2 orders of magnitude compared to commercially available systems. Initial test results of ^90Sr/^90Y samples with the ULB LSC show promising results for spectral capabilities, though the detection efficiencies of 15% and 60% for these samples in the left and right chambers, respectively, were lower than expected. Further testing will improve characterization of the light collection efficiency, spectral capabilities, and alpha/beta separation by pulse shape analysis. The ULB LSC broadens trace level measurement capabilities that will impact applications in nuclear nonproliferation, treaty verification, and environmental and geochemical science studies.