C. Marianno, J. Erchinger, “Initial Background Evaluation of an Ultra-Low Background Liquid Scintillation Counter”, Poster. 2016 IEEE Symposium on Radiation Measurements and Applications, Berkeley, CA, 22-26 May 2016.
An Ultra-Low Background Liquid Scintillation Counter developed at Pacific Northwest National Laboratory is undergoing characterization and performance testing for low-level alpha and beta detection. This system is expected to reach lower minimum detectable activities and enhance 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 was designed with the goal of reaching background rates on the order of 10 to 100 counts per day. A partial build of the system around one of the measurement chambers has been used for initial background characterization and system performance characterization. This initial shield build lacks the lead plug directly above the LSC sample position allowing a line of sight directly to the LSC cocktail vial. However, background rates for non-rad scintillation cocktail samples are ~2400 counts per day over the full energy scale (up to ~3 MeV), which is already comparable to current above ground systems. The backgrounds observed are consistently within 10% of GEANT4 simulation results for this partial shield build using an external photon source term for the underground laboratory. The GEANT4 simulations with the full shield, including the lead plug above the sample, show an expected count rate of 14 counts per day from all remaining backgrounds. One planned demonstration of trace level analysis will involve tritium, an environmental radionuclide frequently analyzed by LSC. In typical environmental samples the tritium undergoes preconcentration through electrolytic enrichment in order to be in a range above the MDA. We expect the lower background will improve analysis of tritium samples as the need for enrichment steps is decreased. The ultra-low background liquid scintillation counter broadens trace level measurement capabilities applicable in nuclear nonproliferation, treaty verification, environmental and geochemical science studies, and forensics. The latest data available for the low background LSC system build will be presented.