Improved 36Cl performance at the ASTER HVE 5 MV accelerator mass spectrometer national facility

The HVE 5 MV ASTER AMS national facility at CEREGE was accepted in 2007. Since then we have continued to optimize performance for 36Cl. Cl-36 analyses use AgCl, a Cs negative ion sputter source, Ar stripping to +5 in the terminal of the Tandetron™ accelerator at 5 MV and a silicon nitride post-acceleration stripper foil to enhance suppression of 36S relative to 36Cl. The major challenges to obtaining the desired performance for Earth science applications are ion source memory, mass fractionation effects, 36S isobar suppression and sensitivity. Redesign of the SO110 ion source by HVE to change the size of the aperture and the shape of cathode significantly reduced ion source memory to less than ∼0.1%, a level that can be compensated for by matching standards to samples. We observe small systematic drifts in 35Cl/37Cl ratios over time, the source of which is not yet determined. Measurement of standards indicates that this effect limits the precision of 35Cl/37Cl ratio determination to about 2%. 36S is suppressed in several ways. First, the sample chemistry has been designed to reduce S to low levels. Second, cathodes are constructed of low-S nickel, enabling direct target loading without the use of AgBr pre-packing. Third, a post-acceleration Si3N4 stripper foil differentially absorbs energy from 36Cl and 36S. A subsequent electrostatic deflector is then able to suppress 36S by a factor of ∼240 relative to 36Cl. Differential energy loss in the detector further suppresses 36S by about 10−4, for an overall suppression factor of 4 × 10−7. 36S count rates are typically equivalent to a background 36Cl/Cl of ∼10−15. At typical 35Cl currents of ∼20 μA Cl5+ samples with 36Cl/35Cl of 6 × 10−14 can be measured to ±5% statistical uncertainty with 1 h of analysis time. Typical machine blanks have 36Cl/Cl ∼2 × 10−15.