AixMICADAS, the accelerator mass spectrometer dedicated to ¹⁴C recently installed in Aix-en-Provence, France

详细信息    查看全文

• 作者：Edouard Bard^a ; ^{bard@cerege.fr" class="auth_mail" title="E-mail the corresponding author} ; Thibaut Tuna^a ; Yoann Fagault^a ; Lise Bonvalot^a ; Lukas Wacker^b ; Simon Fahrni^b ; Hans-Arno Synal^b
• 关键词：Radiocarbon ; Geological graphite ; Gas ion source ; Carbonates ; Sequential leaching
• 刊名：Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
• 出版年：2015
• 出版时间：15 October 2015
• 年：2015
• 卷：361
• 期：Complete
• 页码：80-86
• 全文大小：1589 K

文摘

A compact AMS system dedicated to measuring ¹⁴C in ultra-small samples was installed at the CEREGE in Aix-en-Provence at the end of March 2014, together with an automated graphitization system. AixMICADAS operates at around 200 kV with carbon ion stripping in helium leading to a transmission of about 47%. The hybrid ion source works with graphite targets and CO₂ gas. It is coupled to a versatile gas interface system that ensures stable gas measurements from different sources: a cracker for CO₂ in glass ampoules, an elemental analyzer for combusting organic matter and an automated system to handle carbonate by wet chemistry.

The analyses performed during the first half-year of operation show that a precision of about 2‰ is reached on modern samples of about 1 mg of carbon. Measurements of IAEA reference materials of various ¹⁴C ages show a good agreement with consensus values. Direct measurements of geological graphites indicate a machine background equivalent to an age of 68,000 years BP. AixMICADAS is thus limited solely by the ¹⁴C contamination of samples in the field and in the laboratory.

The performances of the gas ion source and its gas interface system were tested with two CO₂ production units: the elemental analyzer and the automated carbonate hydrolysis unit. These tests show that samples ranging between 10 and 100 渭g C can produce a ¹²C⁻ ion beam of the order of 10–15 渭A during time spans ranging from 3 to 30 min depending on the sample mass.

Coupling the automated hydrolysis system to the gas ion source of AixMICADAS, enables us to develop a method involving sequential leaching of carbonate samples with direct ¹⁴C measurements of the leached fractions and the residual sample. The main advantage is that all of steps leaching and hydrolysis are performed in the same vial for a particular sample.

A sequential leaching was applied to a young carbonate sample (ca. 6600 years BP) whose ¹⁴C age agrees with previous determination and which shows no sign of significant surface contamination. By contrast, the tests also show that the leached CO₂ from very old carbonates is significantly “younger” than the residual sample. This study, though preliminary, already confirms that pretreatment by acid leaching of old carbonates is essential if we are to accurately measure the true age of such samples.