IMPLICATIONS OF BETA-ENERGY AND QUENCH LEVEL FOR ALPHA BETA LIQUID SCINTILLATION SPECTROMETRY CALIBRATION/

Alpha/beta separation is achieved by the use of pulse shape discrimina tion (PSD), calibrated by quantifying event misclassification at any g iven setting for pure alpha and beta emitters. Previous studies have s hown that the degree of misclassification is affected by quenching, bu t with no attempt made to understand the causes for this phenomenon. T his study examines the potential effects of beta energy End quench on PSD calibration. PSD was shown to be energy dependent, with misclassif ication increasing with beta event energy. Therefore, PSD calibration requires the use of a beta emitter with the same energy distribution a s is present in the sample, or a restricted region of interest should be employed. For gross alpha/gross beta analysis of samples containing unknown beta emitters, a stepwise calibration procedure is proposed f or both PSD and efficiency calibration. Quenching by carbon tetrachlor ide, nitromethane and 9 M hydrochloric acid was shown to affect PSD by suppression of the delayed component of the scintillation pulse, alth ough to variable extents, and therefore having a range of effects on m isclassification. Acetone quenching had little impact on PSD. Standard quench calibration procedures using either carbon tetrachloride or ni tromethane are inappropriate for applications utilising alpha/beta sep aration by PSD. Instead, a quench calibration procedure based on overs piking samples encompassing a range of quench conditions is recommende d. The influence of oxygen quenching on PSD was investigated through p urging samples with either oxygen or nitrogen. PSD was found to be una ffected by the small amounts of oxygen normally dissolved in the diiso propylnaphthalene based cocktail used in this study.