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^- = 5×10^-11) 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.