The French accelerator mass spectrometry facility ASTER after 4 years: Status and recent developments on ³⁶Cl and ¹²⁹I

Since the acceptance tests of the French 5 MV accelerator mass spectrometry facility ASTER in 2007, routine measurement conditions for the long-lived radionuclides ¹⁰Be and ²⁶Al have been established. Yearly sample throughput as high as over 3300 unknowns has been reached for ¹⁰Be in 2010. Unacceptable cross-contamination for volatile elements has been largely solved by an ion source upgrade allowing ³⁶Cl measurements at ASTER. However, recent long-term tests using ³⁵Cl/³⁷Cl samples with strongly varying ratios have shown that identical targets lead to different ³⁵Cl/³⁷Cl results at the 2-4% level when being measured after a time gap of 24 hours while the source is running other samples. Besides time dependent mass fractionation, another very likely reason for this effect might be source memory, thus, asking for sophisticated measurement strategies and improved data evaluation and eventually further ion source improvement. Finally, after establishing quality assurance by cross-calibration of secondary in-house ²⁶Al and ⁴¹Ca standards and taking part in round-robin exercises of ¹⁰Be and ³⁶Cl, a two-step cross-calibration of secondary in-house ¹²⁹I standards has been performed. The NIST 3231 containing ¹²⁹I/¹²⁷I at 0.981 x 10^-6 has been used for step-wise dilution with NaI to produce gram-quantities of lower-level standards for every-day use. The resulting material SM-I-9 (¹²⁹I/¹²⁷I: ~1 x 10^-9) has been measured vs. AgI produced from the two NIST ampoules with (0.982+0.012) x 10^-8 solution using minimum chemistry. In a second stage, SM-I-10 and SM-I-11 with ratios of ~1 x 10^-10 and ~1 x 10^-11, respectively, have been cross-calibrated against SM-I-9. Individual uncertainties of the traceable secondary standards are 1.3-1.4 % (2σ), mainly originating from the given uncertainty of the primary NIST 3231 at 10^-8. The cross-contamination for iodine is in the range of 0.4-0.6% within the first 20 hours of running the source.