14C measurement of synthesized benzene from old tree rings

To measure at an accuracy of <0.2% in the variations of the ¹⁴C concentration in old tree rings, the authors have constructed a highly accurate ¹⁴C measuring system composed of a benzene synthesizer and a Quantulus 1220^TM. The detection efficiency decreased in proportion to the number of shift channels in the ¹⁴C spectrum due to the quenching effect. The decreasing rate was 0.092% for 1 channel shift. Analysis by ion-chromatography showed that the ion SO₄^2- in the benzene sample was a main factor quenching the spectrum. The counting stability of the Quantulus for long term measurement was tested using the synthesized benzene samples for a duration of 5 months. All the data were distributed according to Poisson statistics and were within the Poisson counting precision of 3σ_p around the average counts. The authors conclude that the counting fluctuation of the Quantulus is almost determined by the Poisson counting statistics. Moreover, from the data analysis for the trend of the counting rate, it is indicated that the Quantulus operates with a high stability and has the possibility to detect the systematic variations of the counting rate which are 0.3% for a duration of 5 months