A comparison of four aqueous-miscible liquid scintillation cocktails with an alpha/beta discriminating Wallac 1415 liquid scintillation counter.

This work was directed towards an assessment of alpha/beta separation, alpha detection efficiency, energy resolution, quench sensitivity, and background count rate for four commercially available liquid scintillation cocktails using a Wallac 1415 liquid scintillation counter. The four cocktails were Wallac Oy OptiPhase HiSafe 3 and HiSafe 2, and Packard Ultima Gold AB and Ultima Gold XR. Aqueous radioactive solutions were prepared in 0.1 N HCl and contained 241Am (5.49 MeV alpha) and 90Sr/90Y (0.46/2.28 MeV beta-). The radioactive solutions were loaded into the cocktails at a ratio of 1 mL of aqueous solution to 10 mL of cocktail in 20 mL high-density polyethylene (HDPE) liquid scintillation vials. The effect of three levels of chemical quench on the various properties of interest was assessed for each of the LS cocktails. Alpha/beta discrimination was quantified by the fraction of beta interactions that "spillover" into the alpha window and vice versa, at the working discriminator setting. The working discriminator setting was defined as the point where the spillover of beta interactions into the alpha window and alpha interactions into the beta window were equal. For the low-quench samples, Ultima Gold AB had the lowest spillover (0.25%) and Ultima Gold XR had the highest spillover (0.8%). For the high-quench samples, the spillover ranged from 4% (HiSafe 3) to 10.5% (Ultima Gold XR). The detection efficiency for 241Am was nearly 100% and decreased with increased quench because of loss from spillover. HiSafe 2 gave the best energy resolution, 500 keV (FWHM), for the low-quench sample. The lowest background count rate was achieved with Ultima Gold AB, 0.056 cpm in the 241Am window, for a low-quench blank.