Ion-optical design of a novel compact low-energy radiocarbon positive ion mass spectrometry

Positive-ion mass spectrometry (PIMS) is a new mass spectrometric technique for gaseous radiocarbon measurements of small samples, which employs the opposite charge exchange process compared to conventional accelerator mass spectrometry (AMS). Ion optics is a crucial aspect in the design of mass spectrometer. In this work, we propose a novel design scheme for a compact low-energy 14 C-PIMS ion-optical system without an accelerator. The designed system offers a simple and fast way for 14 C measurements by directly injecting gaseous CO 2 samples, thus avoiding the need for sample graphitization. The interference of molecular isobars ( 12 CH 2 , 13 CH and 7 Li 2 ) can be well suppressed by utilizing a 14.5 GHz electron cyclotron resonance (ECR) ion source that produces multiply-charged carbon ions. Additionally, a differentially pumped charge exchange cell (CXC), which operates with non-metallic gas targets to form negative ions, can effectively eliminate the atomic isobar ( 14 N). The ion-optical design and simulation were conducted using the GICOSY and TraceWin codes, and optimized beam envelopes and achromatic beam transport were achieved. The details of ion optical modeling are presented.