Pulse height distributions of /spl beta/ rays in /sup 14/C measurement with liquid scintillation counting system Quantulus

To investigate the variations of the solar activity over a long time scale, we have started the measurements of /sup 14/C in an ancient tree, synthesizing 10 g benzene from the tree rings and using the liquid scintillation counting system Quantulus with ultra low background level. The background rate of the Quantulus was 1.2 cpm while the counting rate of /sup 14/C was 91.0 cpm for the ancient tree. The pulse height distribution of /spl beta/ rays from /sup 14/C was well fitted with theoretical /spl beta/ rays spectrum. The Quantulus detected /spl beta/ rays down to 7.5 keV with good detection efficiency. It was confirmed that the total counts from the /sup 14/C in the sample benzene can be measured within 0.2% accuracy on the basis of the pulse height distribution of a standard benzene sample, using the least squares fitting method.