The goal of this work was to optimize the liquid scintillation counting techniques for the determination of 14C in stack effluent gases and in environmental samples such as biological and air samples. Carbon-14 activities in most environmental samples were measured with the direct CO 2 absorption method. The highest figures of merit were found through the variation of Carbosorb E and Permafluor V ratio, and measurement windows. The best condition was an 1:1 volume ratio. Average 2.35 g of CO 2 was reproducibly absorbed in the 20 ml mixture within 40 minutes. 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. Stack effluent gas samples were measured by a gel suspension counting method. After precipitation of CO 2 in the form of BaCO 3, 140 mg of which was mixed with 6 ml H20 and 12 ml of lnstagel 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 measurements were performed with a mixture of 3 ml benzene and 1 ml of scintillation cocktail (5 g of butyl-PBD in 100 ml of scintillation-grade toluene) in a low potassium 7 ml borosilicate glass vial. The counting efficiency and the background count rate were measured to be 64.3+1.0% and 0.51_+0.05 cpm, respectively. The long-term stability of samples has been checked for all the counting techniques over a two week period, during which no apparent change in counting efficiency and background level was found. Introduction ExperimentalThe monitoring of 14C in the environment is important because of its relatively long half-life, high potential mobility in geological systems, and presence in the carbon cycle in food chains. The conventional techniques used for high precision 14C analysis are gas proportional counting, liquid scintillation counting with synthesized benzene and AMS (Accelerator Mass Spectrometry).However, these methods need complicated analytical procedures and expensive analytical equipment, and require considerable analytical skill. For general environmental monitoring the accuracy and/or the sensitivity of these techniques far exceed the requirement. Therefore, the development of simple and less expensive methods for monitoring of 14C is desired.The purpose of this paper was to establish simple and reliable methods for the measurement of 14C activity in environmental and stack effluent gas samples with a lowbackground liquid scintillation counter. Direct CO 2 absorption was investigated and applied to 14C measurements in environmental ...
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