Cocktail volume effects in 4πβ Liquid scintillation spectrometry with 3H-standard efficiency tracing for low-energy β-emitting radionuclides

Abstract

The effect of total liquid scintillation (LS) cocktail volume (or mass) in the 4πβ LS spectrometry of β-emitting radionuclides has been investigated. The magnitude of such possible volume effects on the apparent activity of low-energy β-emitting radionuclides, as determined by the CIEMAT/NIST 3H-standard efficiency tracing method, as well as any systematic trends in the relative detection efficiencies of low-ebergy β-emitters were studied. The radionuclides chosen for the study, 63Ni and 36Cl, were traced against 3H. Detection efficiency losses, as a function of total LS cocktail volume, were found to be energy-dependent. Because of the low E βmax for the decay of 3H and 63Ni, systematic losses in efficiency with increasing cocktail mass, as well as at extremely low (< 5 g) masses were observed. The determination of quench-indicating parameters, even for cocktails of uniform composition, were also found to be volume-dependent. Despite a relatively large (17%) variation in the 3H counting efficiency over the volume range 1–20 mL in 22 mL vials and a corresponding 7% varition in the 63Ni counting efficiency, the CIEMAT/NIST method was able to trace the observed 63Ni activity to about 0.1%. No volume effect on the observed relative efficiency of 36Cl was found. The best condition for repeatability in the determination of the massic activity of a single sample was found to be when the volume is 8–12 mL in a 22 mL vial. The effect of total LS cocktail volume on the efficiency-traced massic activities was found to have no clear, consistent trend and proved difficult to quantify. Nonetheless, two approaches to estimating the magnitude of the effect are presented and indicate that it is extremely small (< 0.1%) and was within the statistical variation of the measurements.