J. Erchinger “Development and Demonstration of an Ultra-Low-Background Liquid Scintillation Counter”, Ph.D. Dissertation, Nuclear Engineering, Texas A&M University, College Station, TX (2017).
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, enhancing alpha and beta detection for samples that have previously required extended count times, large sample volumes, and/or complex separation chemistry. The three-chamber system design includes layers of passive shielding in conjunction with an active veto and overburden with the goal of reaching background rates on the order of 10 to 100 counts per day. GEANT4 simulations for the full shield showed an expected count rate of 10 counts per day from the environmental physics backgrounds, in addition to a systematic count rate of 15 cpd expected from initial tests with a partial build. The observed chamber background is just above 50 cpd, including systematic and physics backgrounds, over an energy range of ~13- 2700 keV. While twice the initial expectation of 25 cpd, the observed background is still within the target range of 10-100 cpd. This is an improvement of approximately 2 orders of magnitude below the background environment of commercially available systems. Initial test results of 3H and 90Sr with the ULB LSC show promising results for ultra-low background detection with liquid scintillation counting. Further testing will improve characterization of the 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, environmental and geochemical science studies, and forensics.