The French accelerator mass spectrometry facility ASTER: Improved performance and developments


The French accelerator mass spectrometry facility ASTER: Improved performance and developments

Arnold, M.; Merchel, S.; Bourlès, D. L.; Braucher, R.; Benedetti, L.; Finkel, R. C.; Aumaître, G.; Gottdang, A.; Klein, M.

A new 5 MV tandem accelerator has been installed at the Centre Européen de Recherche et d’Enseignement des Géosciences de l’Environnement (CEREGE). The machine is fully dedicated to accelerator mass spectrometry (AMS) in applied research like environmental and Earth sciences. For 10Be and 36Cl the troublesome isobar suppression is maintained by nuclear charge dependent energy loss in an absorber foil, subsequent energy selection by a 35° electrostatic deflector followed by a 30° vertical magnet, and final separation of the rare isotope in a high-resolution, 4-anode (ΔE1, ΔE2, ΔE3, E-final) gas ionisation chamber [1].

Since the acceptance test in March 2007, we have successfully established routine measurement conditions for the long-lived cosmogenic radionuclides 10Be and 26Al. Using 9Be carrier derived in our laboratory from phenakite crystals originating from a deep mine, we determined a background-level as low as 1.5x10-16 (10Be/9Be) [2]. Considerable time has been spent to investigate reasons for variability of ion currents and background. We have performed cross-calibrations for 26Al and 41Ca and took part into international round-robin exercises of 10Be and 36Cl to establish quality assurance at ASTER. For 41Ca (extracted as CaF3 -) and 129I, background levels are in the range of 2x10-14. The original Cs-sputter ion source SO110 [3], that produced sample-to-sample cross-contamination at the ‰-level and unacceptable long-term memory effects for volatile elements as chlorine or iodine, had been continuously replaced by a number of newly developed versions enhancing the performance step by step. Under optimum conditions, i.e. using exclusively low-level standards (36Cl/Cl=1.4x10-13) for calibration and cathode material containing the lowest isobar (36S) concentrations, the 36Cl background can reach 3x10-16 (36Cl/35Cl). The total transmission varies from 2% for 36Cl with the post-stripping absorber foil to 38% for 26Al.

The main focus of applications of ASTER is on geological and environmental topics with the broad goal of using isotopic techniques to help understand the timing and rates of processes in the earth system. Work so far has included reconstruction of past climate, determination of the rate and timing of seismic activity, volcanic eruptions and rock falls.

References: [1] M.G. Klein et al., Nucl. Instr. and Meth. B 266 (2008) 1828. [2] S. Merchel et al., Nucl. Instr. and Meth. B 266 (2008) 4921. [3] M.G. Klein et al., Nucl. Instr. and Meth. B 249 (2006) 764.

Acknowledgments: This work was partially funded by CRONUS-EU (Marie-Curie Action, 6th FP #511927).

Keywords: accelerator mass spectrometry; AMS; terrestrial cosmogenic nuclides

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    19th International Conference on Ion Beam Analysis, 07.-11.09.2009, Cambridge, UK

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