AMS 14C measurement and preparative techniques at NIES-TERRA

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Abstract

The AMS facility at the National Institute for Environmental Studies (NIES) has now been operating for five years since 1997. This AMS-dedicated facility named NIES-TERRA (Tandem accelerator for Environmental Research and Radiocarbon Analysis) consists of a 5MV tandem Pelletron (15SDH-2), a solid ion source with sequential injection system, a gas ion source with a simultaneous injection system, and a gas ionization detector. Typical precision with 14C measurements is around 0.5% and the background level has been achieved at around 50 kBP. Our main target is radiocarbon in environmental sciences, while other cosmogenic isotopes (10Be and 26Al) have been measured. Recently we have improved our preparation laboratory to increase sample throughput. CO2 production and purification using a commercial elemental analyzer connected to cryogenic traps (EA–CT) were investigated. We used a modified elemental analyzer with a chemical trap for separation and this is expected to reduce memory effect and background in comparison with an analyzer using gas-chromatography. This newly designed EA–CT system showed high precision and accuracy. Source of background contamination was also discussed based on the analysis of various amounts of standard materials.

Introduction

The Tandem accelerator for Environmental Research and Radiocarbon Analysis at the National Institute for Environmental Studies (NIES-TERRA) is the third Japanese facility for accelerator mass spectrometry (AMS) in routine operation. Development of this facility started in September 1995 and entered into full operation in February 1997 [1], [2]. This Pelletron accelerator (15SDH-2 made by National Electrostatics Corp., USA) has been exclusively used for AMS measurements of 14C, 10Be and 26Al, and negative ions can be produced from two different Cs-sputter sources; MC-SNICS for 40 solid samples and MGF-SNICS for 12 gas samples [3], [4], [5].

The mission of our facility is to expand the research field of environmental studies at NIES by using the AMS method. We have collaborated with researchers working in various fields including oceanography, environmental chemistry, atmospheric chemistry, geology, geophysics, archaeology and anthropology. Because our main target nuclide is 14C, details of this measurement and preparation techniques will be reported. Samples analyzed at NIES-TERRA consist of various kinds of materials and, as examples, results on the following samples are reported in this volume; DIC in sea water [6], atmospheric CO2[7], suspended particulate matters (SPM) [8], polycyclic aromatic hydrocarbons (PAHs) in urban pond sediment [9], living coral colony [10], technical progress in small sample preparation down to 10 μg [11] and for interspecies comparison of the marine reservoir effects [12]. NIES-TERRA has become one of the most active AMS laboratories in Japan.

Recently we started a project on the analysis of long coral cores from the Pacific monsoon area to reconstruct past environmental changes. For this project, we have established a new preparation system for the 14C measurement of organic and carbonate samples by using a commercially available elemental analyzer (EA). Similar systems have already been established at several laboratories by using EAs based on gas chromatographic columns [13], [14], [15], but we applied a modified EA with chemical trap for the separation of generated CO2 from other gases. This EA was expected to show lower background and easier to adjust trap timing than others. Here, the first results of the newly designed preparation system consisting of an EA and cryogenic traps (EA–CT) will be reported together with the data on coral cores and the results of background analysis.

Section snippets

Conventional method on pretreatment and measurement of samples

The typical number of total targets including standards and backgrounds analyzed by NIES-TERRA has become more than 1000 per year in recent years. This number is not limited by the AMS machine time but by sample preparation throughput. Typical series of samples on a wheel of MC-SNICS consist of one background (IAEA C-1 or commercial oxalic acid), 4 HOxII (new oxalic acid standard distributed by NIST; SRM-4990c) standards, 4 IAEA C-6 sucrose or other IAEA standards and 30 unknown samples.

Results and Discussion

In our preliminary study, the data on the coral core showed seemingly a small but pseudo-seasonal variation in 14C contents (Fig. 2). The variability of 14C content may reflect local oceanographic events such as water-mass movement and upwelling strength. In this sample from Western Australia, these two factors are related strongly to the existence of the Leeuwin Current originated from the Western Pacific Warm Pool, the strength of which is known to be affected by El Nino events [17], [18].

Conclusion

In this paper, we have showed the current status of NIES-TERRA, especially 14C sample preparation capability. In recent years, we have been involved with various collaborative studies with research in different fields. Currently, the average number of 14C measurements, including those of standard materials, is almost 1000 per year. Although we have established a preparative laboratory with enough precision and a background level about of 0.5% as one standard error and >50 000 BP as background

Acknowledgements

The authors thank Ms. C. Kitazaki, Ms. S. Minoura and Ms. M. Ihara for their preparative operations. Two anonymous reviews gave useful comments and suggestion on the manuscript of this paper. This work is partly supported by the global research coordination system program on “Coral Skeleton Climatology”, Ministry of the Environment, and also the Grant-in-Aids for Scientific Research (13740499, 14340271), Ministry of Education, Culture, Sports, Science and Technology, Japan.

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