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 Si₃N₄ postacceleration 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 very 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 ubsequent electrostatic deflector is then able to suppress ³⁶S by a factor of ~5x10^-3. Differential energy loss in the detector further suppresses ³⁶S by about 10^-4, for an overall suppression of 5x10^-7. ³⁶S count rates are typically equivalent to a background ³⁶Cl/Cl of <10^-15. At typical ³⁵Cl currents of 20 μA samples with ³⁶Cl/³⁵Cl of 5x10^-14 can be measured to ± 6% statistical uncertainty with one hour of analysis time. Typical machine blanks have ³⁶Cl/Cl ~2x10^-15.