Development of a low background liquid scintillation counter for a shallow underground laboratory

• Graded-shielding can produce an ultra-low-background liquid scintillation counter.
• Location in a shallow underground cleanroom further enhances background reduction.
• A novel light collection design and selected low background materials are utilized.
• The background is predicted to be 10–100 times below typical commercial systems.
• Simulations tentatively predict a background rate of order 10 counts per day.

Pacific Northwest National Laboratory has recently opened a shallow underground laboratory intended for measurement of low-concentration levels of radioactive isotopes in samples collected from the environment. The development of a low-background liquid scintillation counter is currently underway to further augment the measurement capabilities within this underground laboratory. Liquid scintillation counting is especially useful for measuring charged particle (e.g., β and α) emitting isotopes with no (or very weak) gamma-ray yields. The combination of high-efficiency detection of charged particle emission in a liquid scintillation cocktail coupled with the low-background environment of an appropriately designed shield located in a clean underground laboratory provides the opportunity for increased-sensitivity measurements of a range of isotopes. To take advantage of the 35 m-water-equivalent overburden of the underground laboratory, a series of simulations have evaluated the scintillation counter's shield design requirements to assess the possible background rate achievable. This report presents the design and background evaluation for a shallow underground, low background liquid scintillation counter design for sample measurements.