14C measurement of synthesized benzene from old tree rings

To measure in the accuracy <0.2% the variations of the C-14 concentration in an old tree rings, we have constructed the high accurate C-14 measuring system composed of a benzene synthesizer and a Quantulus 1220(TM). The detection efficiency decreased in proportion to the number of shift channel in the C-14 spectrum due to the quenching effect. The decreasing rate was 0.092% for 1 channel shift. Analysis by ion-chromatography showed that the ion SO42- in the benzene sample was effective for quenching the spectrum. The counting stability of the Quantulus for a long term measurement is tested using the synthesized benzene samples for the duration of 5 months. All the data were within the Poisson counting precision 3 sigma(p). We conclude that the counting fluctuation of the Quantulus is almost determined by the counting statistics. Moreover, from the data analysis for the trend of the counting rate, it is indicated that the Quantulus operates with the high stability as well as possible to detect the systematic fluctuation of 0.3% in the duration of 5 months.