Pulse height distributions of beta rays in C-14 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 C-14 in an ancient tree, synthesizing 10 g benzene from the tree rings and using the,liquid scintillation countin g system Quantulus with ultra low background level. The background rate of the Quantulus was 1.2 cpm while the counting rate of C-14 was 91.0 cpm for the ancient tree The pulse height distribution of beta rays from C-14 was w ell fitted with theoretical beta rays spectrum. The Quantulus detected beta rays down to 7.5 keV with good detection efficiency. It was confirmed that the total counts from the C-14 in the sample benzene can be measured withi n 0.2% accuracy on the basis of the pulse height distribution of a standard benzene sample, using the least squares fitting method.