Optimization of liquid scintillation counting techniques for the determination of carbon-14 in environmental samples

The goal of this work was to optimize the liquid scintillation counting tec hniques for the determination of C-14 in stack effluent gases and in enviro nmental samples such as biological and air samples. Carbon-14 activities in most environmental samples were measured with the direct CO2 absorption me thod. The highest figures of merit were found through the variation of Carb osorb E and Permafluor V ratio, and measurement windows. The best condition was an 1:1 volume ratio. Average 2.35 g of CO2 was reproducibly absorbed i n the 20 mi mixture within 40 minutes. The counting efficiency determined b y repeated analysis of NIST oxalic acid standard and the background count r ate were measured to be 58.8+/-1.44 and 1.88+/-0.06 cpm, respectively, in c ase of saturated solution. The correction curves of counting efficiency for partially saturated solutions and for saturated solutions with quenching w ere prepared, respectively. The overall uncertainty of the sample specific activity for near background levels was estimated to be about 7% for 4 hour s counting at 95% confidence level. Stack effluent gas samples were measure d by a gel suspension counting method. After precipitation of CO2 in the fo rm of BaCO3, 140 mg of which was mixed with 6 mi H2O and 12 mi of Instagel XF. The counting efficiency was measured to be 71.5+/-1.7% and the typical sensitivity of this technique was about 510 mBq/m(3) for a 100 min count at a background count rate of 4.7 cpm. For the benzene counting method measur ements were performed with a mixture of 3 mi benzene and 1 mi of scintillat ion cocktail (5 g of butyl-PBD in 100 mi of scintillation-grade toluene) in a low potassium 7 mi borosilicate glass vial. The counting efficiency and the background count rate were measured to be 64.3+/-1.0% and 0.51+/-0.05 c pm, respectively. The long-term stability of samples has been checked for a ll the counting techniques over a two week period, during which no apparent change in counting efficiency and background level was found.