Cocktail mismatch effects in 4πβ liquid scintillation spectrometry: Implications based on the systematics of 3H detection efficiency and quench indicating parameter variations with total cocktail mass (volume) and H2O fraction

Detection efficiency changes for 3H by 4πβ liquid scintillation (LS) spectrometry cannot be adequately monitored by quench indicating parameters when the quench changes are the result of multiple causal factors (e.g. simultaneously varying cocktail sizes and composition). In consequence, some kinds of cocktail mismatches (between LS counting sources) introduce errors that result from efficiency changes that cannot be fully accounted for by quench monitoring compensations. These cocktail mismatch effects are examined for comparative 3H measurements and for 3H-standard efficiency tracing methods for the assay of other β-emitting radionuclides. Inherent errors can occur in both types of radionuclidic assays, as demonstrated with realistic case examples, unless cocktails are very closely matched. The magnitude of the cocktail mismatch effect (and attendant errors) can range from being virtually negligible (particularly for high-energy β-emitting nuclides and for slight single-variable cocktail composition mismatches) to be being very significant for high-precision metrology and standardizations (particularly with easily quenched, low-energy β emitters and for mismatches due to both varying cocktail constituents and concentrations). The findins presented here support the need to understand fully the quenching systematics of a given LS system (combination of cocktails and spectrometer) and the need for very careful control of cocktail preparations.