Status of the French accelerator mass spectrometry facility ASTER after 4 years

Since the acceptance tests of ASTER in March 2007, routine measurement conditions for the long-lived radionuclides ¹⁰Be and ²⁶Al have been established. Sample throughput as high as over 3300 unknowns has been reached for ¹⁰Be in 2010. Steady numbers for ²⁶Al within the last three years settle down ~300 real samples.

Unacceptable cross-contamination for volatile elements has been largely solved by an ion source upgrade [1]. Thus, the second frequent nuclide measured at ASTER in 2010 is ³⁶Cl with ~480 sample targets. The enhancement with respect to immediate short-term sample to sample by the new ion source is impressive allowing minimal measurement time losses such as for initial burn-in periods of 5 min for virgin targets and waiting periods between data acquisition of two sample runs of 2 min. 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. Reasons for this such as source memory, time dependent mass fractionation, drift of the Faraday-cup measurement system or something else are not yet clear.

Finally, after establishing quality assurance at ASTER by cross-calibration of secondary in-house ²⁶Al and ⁴¹Ca standards [1] and taking part in round-robin exercises of ¹⁰Be and ³⁶Cl, we performed a two-step cross-calibration of secondary in-house ¹²⁹I standards. The two ampoules of NIST 3231 containing ¹²⁹I/¹²⁷I at 0.981x10^-6 have been used for step-wise dilution with NaI (MERCK, suprapur, 99.99 %) to get gram-quantities of lower-level standards for every-day use. The material SM-I-9 (~1x10^-9) has been measured vs. AgI produced from the two NIST ampoules with (0.982+0.012)x10^-8 solution using minimum chemistry. In a second stage, SM-I-10 and SM-I-11 with ratios of ~1x10^-10 and ~1x10^-11, respectively, have been cross-calibrated against SM-I-9. Individual uncertainties of the traceable secondary standards are around 1.5 %, mainly originating from the given uncertainty of the primary NIST 3231 at 10^-8.

References: [1] M. Arnold et al., NIMB 268 (2010) 1954.