Isobar suppression for 36Cl accelerator mass spectrometry at the University of Tsukuba

A 6 MV tandem accelerator at the University of Tsukuba, Japan, has been working on multi-nuclide (¹⁰Be, ¹⁴C, ²⁶Al, ³⁶Cl, ⁴¹Ca, and ¹²⁹I) accelerator mass spectrometry (AMS) since 2016. This paper reports improvement of isobar suppression techniques for ³⁶Cl AMS. Previous studies have shown that a Cu sample holder with AgBr backing is the most suitable for ³⁶Cl AMS to decrease isobaric ³⁶S interference. Therefore, the method of AgBr preparation was considered, and we found that preparation under acidic conditions was most appropriate to reduce ³⁶S in ³⁶Cl AMS. Cl charge states and their analysis method were considered to improve peak separation. 3⁶Cl⁸⁺ was chosen for detection because it allows clear separation of ³⁶S from the ³⁶Cl spectrum. Furthermore, ³⁶S spectrum tails could be removed using a third anode (dE3) signal. As a result, a ³⁶Cl/Cl background of ∼ 3 ×10^-15 was achieved.