Measurement of low 14C activities in a liquid scintillation counter in the Zagreb Radiocarbon Laboratory

Two methods of chemical preparation of radiocarbon samples are implemented in the Zagreb Radiocarbon Laboratory for measurement by a new liquid scintillation counter (LSC), Quantulus 1220(TM) : a CO2 absorption method (LSC-A) and a benzene synthesis method (LSC-B). For samples prepared by both methods, the optimal counting windows for measurement in LSC were determined. The total efficiency of LSC-A is 65% and that of the LSC-B is 83%, while the corresponding C-14 dating limits are 31,800 yr and 52,160 yr, respectively. C-14 activities measured by the LSC-A and LSC-B methods were compared with those measured by the gas proportional counter (GPC) method (efficiency 75%, C-14 dating limit 37,500 yr). The results obtained by the LSC-A method have larger errors than those measured by the GPC method, but LSC-A is quick, inexpensive, simple, and requires less carbon than the GPC method. Thus, LSC-A is suitable for C-14 measurements of geological, hydrological, and environmental samples. On the other hand, the results obtained by the LSC-B method give smaller errors and a larger C-14 dating range. Therefore, LSC-B is more suitable for C-14 dating of archaeological samples.