"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.