36Cl exposure dating with a 3-MV tandem

Abstract 36 Cl 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 36 Cl 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 36 S. Despite the good separation, a relatively high sulfur output from the ion source ( 36 S − / 35 Cl −  ≈ 4 × 10 −10 for samples prepared from chemically pure reagents), and a possibly cross contamination resulted in a background corresponding to 36 Cl/Cl ≈ 3 × 10 −14 . The method was applied to samples containing between 10 5 and 10 6 atoms 36 Cl/g rock from sites in Italy and Iran, which were already investigated by other laboratories for surface exposure dating. The 36 Cl/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 36 Cl exposure dating, which is presently the domain of larger facilities.