engleski

Measurement of 14C activity by liquid scintillation counting

Measurement of 14C activity in various types of samples, including radiocarbon dating, has been performed in the Radiocarbon Laboratory of the Rudjer Bošković Institute since 1968 by proportional counting measurement technique, since 2001 by liquid scintillation (LSC) technique and AMS (Accelerator Mass Spectrometry) technique has been under implementation. Here we describe our preparation lines and protocols for LSC measurement by LSC Quantulus 1220. Finally, we give some examples of environmental 14C measurements (14C activity in atmospheric CO2 and in biological samples). Organic samples are pre-treated by standard acid-base-acid method, combusted in a stream of pure oxygen and the obtained CO2 is used for benzene synthesis. If appropriate, samples can be carbonized at 600º C and then used for benzene synthesis. From carbonate samples CO2 is obtained by HCl hydrolysis. Two lines for benzene synthesis have been constructed: for “ large” samples containing about 4 g (or more) and for “ small” samples with 2 g (or less) of carbon. A separate line for absorption of CO2 in an absorption-scintillation cocktail (“ direct absorption” method) has also been constructed. Various types of vials have been tested and used for measurement: for samples prepared by the absorption method we use low-potassium glass vials of 20 mL volume, and for benzene samples low-potassium glass vials of 7 mL, Teflon-copper vials (7 mL) and Teflon vials (3 mL). We prepared individual measurement and data evaluation protocols for each of the measurement geometries. Routine measurements are performed by using 4 g benzene samples in 7-mL glass vials ; very old and/or high-precision 4-g benzene samples are measured in Teflon-copper vials, and 2-mL benzene samples are measured in Teflon vials. The maximal age is 54 000 years obtained by routine measurements of benzene samples and somewhat better (58 000 years) by using Teflon-copper vials. The simplest preparation method, absorption of CO2 shows lower 14C dating limit (28 000 years) and larger errors, but it is accurate enough for certain applications, e.g., geological, hydrological, and some environmental samples. Acknowledgement: Project 098-0982709-2741 from the Ministry of Science and EU FP6 043584 AMS-14C project.

14C; nuclear power plant; biological samples; monitoring; atmospheric CO2

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