hrvatski

Mjerenje niskih 14C aktivnosti uzoraka u obliku benzena u tekućinskom scintilacijskom brojaču

In the Laboratory for Low-level Radioactivity, Ruđer Bošković Institute, archaeological, geological and hydrological samples have been dated by the 14C method. Gas proportional counter (GPC) has been used for more than thirty years for measurement of the 14C activity, as well as a liquid scintillation counter (LSC) Quantulus 1220 since 2001. Two methods of chemical samples preparation have been developed: a benzene synthesis method (LSC B) and a CO2 absorption method (LSC-A). The benzene synthesis method is presented and compared with the GPC method. CO2 obtained by combustion of organic samples or by dissolving inorganic samples in hydrochloric acid is the basic reactant for all three methods. The benzene synthesis from CO2 consists of several stages: reaction of CO2 with Li (production of Li2C2), Li2C2 hydrolysis (production of C2H2) and C2H2 trimerization into C6H6. Optimization of the individual processes resulted in the total reaction yield of 80%. Optimal parameters for measurement in LSC were also determined: counting in 45-50 cycles of 30 minutes, the counting window between 219 and 525 channels, the total efficiency of 14C measurement of 82%. 14C activities measured by LSC are compared with those measured by GPC and a good agreement is achieved (slope of the regression line is 1.0, and the correlation coefficient r = 0.997). When compared with the GPC and LSC-A methods, the LSC-B method has the highest sensitivity and precision, as well as the lowest detection limit (i.e., the maximal age that can be determined is about 52000 years). Therefore, the LSC-B method is suitable for dating of archaeological samples, and monitoring environmental 14C fluctuations. The disadvantage of the LSC-B method is relatively complex sample preparation.

benzen; 14C; LSC

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