AMS of ³⁶Cl with the VERA 3 MV tandem accelerator

Recent progress with compact ionization chambers has opened new possibilities for isobar suppression in accelerator mass spectrometry (AMS). Separation of ³⁶Cl (t_1/2 = 0.30 Ma) at natural isotopic levels from its stable isobar ³⁶S became feasible at particle energies of 24 MeV, which are also accessible for mediumsized tandem accelerators with 3 MV terminal voltage like VERA (Vienna Environmental Research Accelerator). Investigations with an ionization chamber revealed how physics favors isobar separation even at energies below the maximum of the Bragg curve. The strong energy focusing effect at high energy losses reduces energy straggling significantly and isobar separation steadily increases up to almost full energy loss. With an optimized detection setup, sulfur suppression factors of 2 × 10⁴ have been achieved.
Refraining from the additional use of degrader foils has the benefit of high transmission to the detector (~16%), but requires a low sulfur output from the ion source. Therefore several backing materials have been screened for sulfur content. The dependence of the sulfur output on the AgCl sample size has been investigated as well.
Precision and accuracy have been thoroughly assessed over the last two years. Since drifts in the spectra are efficiently corrected by monitoring the position of the ³⁶S peak, the reproducibility for high ratio samples (³⁶Cl/Cl > 10^-12) is better than 2%. Our blank value of ³⁶Cl/Cl ≈ (5 ± 5) × 10^-16 is competitive to other labs. ³⁶Cl has become a routine AMS-isotope at VERA.
Recently we also explored novel techniques for additional sulfur suppression already in the ion source. While results with a small gas reaction cell in front of the sputter target were discouraging, a decrease in the sulfur/chlorine ratio by one order of magnitude was achieved by directing 300mW continuous wave laser beam at 445 nm towards the cathode in the ion source.