石英中のC-14の抽出法の検討(第20回名古屋大学年代測定総合研究センターシンポジウム平成19(2007)年度報告)

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  • The extraction of carbon-14 from quartz samples(Proceedings of the 20^<th> Symposium on Chronological Studies at the Nagoya University Center for Chronological Research in 2007)

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Cosmogenic radionuclides are produced in rocks depending on the thick of surface ground covering them, which derives that erosion rate can be evaluated from the relation of accumulation and half life of cosmogenic radionuclide. Among rocks, quartz is chemically stable and abundant in the environment and hence it is useful material for evaluation of erosion rate and dating. The analysis of Al-26, Be-10 and C-14 in quartz yields the erosion rate from 5×10^<-6> to 0.1 cm/y. Unlike to beryllium and aluminum, extraction of carbon is required to treat gaseous species. In this paper we show the recovery of carbon from quartz and graphite mixture. Because the amount of carbon in quartz is far from enough to make AMS target pieces, we have added some graphite as carbon carrier to quartz samples. Among simple and stable carbon compounds (materials), natural graphite is quite low C-14 content and hard to be contaminated from atmospheric C-14. All samples were preheated at 450℃ for 12 h and then volatile contamination on sample surface was decreased. CO gas generated from quartz melting reaction with graphite was oxidized to CO_2 through heated Pt and CuO mixture, after which CO_2 gas was purified by a liquid nitrogen/ethanol trap. A small aliquot of CO_2 was used to measured stable isotope ratio and the remains were reduced in hydrogen atmosphere to graphite on iron powder which was subject to AMS analysis. The result that the carbon recovery ratio was less than 3% and was decreased with additive carbon amount suggests that some reagent, such as platinum and iron, is required to enhance the melting reaction. Because the carbon recovery ratio was evaluated here on the basis of additive carbon amount, it is unknown that the exact amount of C-14 from inside quartz crystal. The problem would be clarified by using quartz samples doped with C-14 produced through low-flux neutron or photon irradiation to them. All samples indicated that the value of δC-13 in about 5 permil. The background C-14 was evaluated to at most 2.8×10^<-6> atoms. In order to evaluate erosion rates, sources of contamination will be investigated to decrease background in carbon extraction system.

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