Optimization of CO2 direct absorption method for the determination of carbon-14 in environmental samples

The goal of this work was to optimize the liquid scintillation counting techniques for the determination of C-14 in environmental samples such as biological and air samples. Carbon-14 activities in most environmental samples were measured with direct CO2 absorption method. The highest figure of merit was found through the variation of Carbosorb ETM and Permafluor VTM ratio, in the measurement windows. The best condition was 1:1 volume ratio. Average 2.35 g of CO2 was reproducibly absorbed in the 20 ml mixture within 40 min. The counting efficiency determined by repeated analysis of NIST oxalic acid standard and the background count rate were measured to be 58.8±1.4 % and 1.88±0.06 cpm, respectively in case of saturated solution. The correction curves of counting efficiency for partially saturated solutions and for saturated solutions with quenching were prepared, respectively. The overall uncertainty of the sample specific activity for near background levels was estimated to be about 7 % for 4 hours counting at 95% confidence level. The long-term stability of samples has been checked for all the counting techniques over a two week periods, and no apparent change in counting efficiency and background level was found at that time