Optical design considerations for efficient light collection from liquid scintillation counters

Abstract

Liquid scintillation counters measure charged particle-emitting radioactive isotopes and are used for environmental studies, nuclear chemistry, and life science. Alpha and beta emissions arising from the material under study interact with the scintillation cocktail to produce light. The prototypical liquid scintillation counter employs low-level photon-counting detectors to measure the arrival of the scintillation. For reliable operation, the counting instrument must convey the scintillation light to the detectors efficiently and predictably. Current best practices employ the use of two or more detectors for coincidence processing to discriminate true scintillation events from background events due to instrumental effects such as photomultiplier tube dark rates, tube flashing, or other light emission not generated in the scintillation cocktail vial. In low-background liquid scintillation counters, additional attention is paid to shielding the scintillation cocktail from naturally occurring radioactive material present in the laboratory and within the instrument's construction materials. Low-background design is generally at odds with optimal light collection. This study presents the evolution of a light collection design for liquid scintillation counting (LSC) in a low-background shield. The basic approach to achieve both good light collection and a low-background measurement is described. The baseline signals arising from the scintillation vial are modeled and methods to efficiently collect scintillation light are presented as part of the development of a customized low-background, high-sensitivity LSC system.

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

“…Each of these three types of consideration is presented in-turn within this section. The key innovation for the ULB LSC system is the implementation of low background shielding best practices around a novel approach to the scintillation light collection (Bernacki et al, (2015)). A hollow light guide cut out of the copper interior shielding and coated with a specularly reflective material will guide the photons emitted from the liquid scintillator around a 90-degree curve.…”