³⁶Cl exposure dating with a 3-MV tandem

³⁶Cl AMS measurements at natural isotopic concentrations have yet been performed only at tandem accelerators with 5 MV terminal voltage or beyond. We have developed a method to detect ³⁶Cl at natural terrestrial isotopic concentrations with a 3-MV system, operated above specifications at 3.5 MV. An effective separation was obtained with an optimized split-anode ionization chamber design (adopted from the ETH/PSI Zurich AMS group), providing a suppression factor of up to 30,000 for the interfering isobar ³⁶S. Despite the good separation, a relatively high sulfur output from the ion source (³⁶S^-/³⁵Cl^- ≈ 4 × 10^-10 for samples prepared from chemically pure reagents), and a possibly cross contamination resulted in a background corresponding to ³⁶Cl/Cl ≈ 3 × 10^-14. The method was applied to samples containing between 10⁵ and 10⁶ atoms ³⁶Cl/g rock from sites in Italy and Iran, which were already investigated by other laboratories for surface exposure dating. The ³⁶Cl/Cl ratios in the range from 2 × 10^-13 to 5 × 10^-12 show a generally good agreement with the previous results. These first measurements demonstrate that also 3-MV tandems, constituting the majority of dedicated AMS facilities, are capable of ³⁶Cl exposure dating, which is presently the domain of larger facilities.