Optimization and Calibration of a Low-background Liquid Scintillation Counter for the Simultaneous Determination of Alpha

The use of low-background liquid scintillation counting (ISC) techniques permits the simultaneous measurement of both alpha and beta emitters with efficiencies close to 100%. In this study, the counting parameters of a Wallac 1220TM Quantulus IS counter have been optimized for the best possible separation of the a and 3 spectra when measuring a sample containing 8 ml of water and 12 ml of Optiphase HiSafeTM 3 scintillation cocktail. For a normal quenching, minimum interference was observed when using an optimized a/3 pulse-shape discrimination value, PSA = 117 (which ranges from 1 to 256). As the detection limits were increased when using a pulse-amplitude comparator to reduce the background, this circuit was not activated during the experiment. In these conditions, the total interference between the a and 3 spectrum was -8%. This interference occurred only between the a window and the high-energy 3 window, and was little influenced by small variations of quenching. Better separation was achieved when the samples showed lower quenching. The figures of merit for the a and high-energy 3 windows were 144,000 and 1452 cpm-1, respectively, corresponding to efficiencies of 95% and 77%. The minimum detectable activities for a and high-energy f3 emitters in water samples were -0.15 Bq liter 1 and 1.36 Bq liter 1 for a total counting time of 360 min.