A COMPARISON OF 4 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, an d background count rate for four commercially available liquid scintil lation cocktails using a Wallac 1415 liquid scintillation counter. The four cocktails were Wallac Oy OptiPhase HiSafe 3 and HiSafe 2, and Pa ckard Ultima Gold AB and Ultima Gold XR, Aqueous radioactive solutions were prepared in 0.1 N HCl and contained Am-241 (5.49 MeV alpha) and Sr-90/Y-90 (0.46/2.28 R/LeV beta(-)). The radioactive solutions were l oaded 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 scinti llation vials. The effect of three levels of chemical quench on the va rious properties of interest was assessed for each of the LS cocktails . Alpha/beta discrimination was quantified by the fraction of beta int eractions that ''spillover'' into the alpha window and vice versa, at the working discriminator setting. The working discriminator setting w as defined as the point where the spillover of beta interactions into the alpha window and alpha interactions into the beta window were equa l. 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 th e high-quench samples, the spillover ranged from 4 % (HiSafe 3) to 10. 5 % (Ultima Gold XR). The detection efficiency for Am-241 was nearly 1 00% and decreased with increased quench because of loss from spillover . HiSafe 2 gave the best energy resolution, 500 keV (FWHM), for the lo w-quench sample. The lowest background count rate was achieved with Ul tima Gold AB, 0.056 cpm in the Am-241 window, for a low-quench blank.