New progress of a 2.45 GHz ECR ion source for carbon positive ion mass spectrometry

Abstract

Positive ion mass spectrometry (PIMS) is a trace element measurement technique based on positive and negative ion conversion, which is contrary to conventional accelerator mass spectrometry (AMS). To precisely measure 14C, interference of molecular isobars (such as 12CH2 and 13CH) need to be suppressed in the ion source by generating multi-charged carbon ions, such as C2+ or C3+. At present, a high-frequency (> 7 GHz) electron cyclotron resonance (ECR) ion source is commonly used to generate C2+ or C3+ ions through CO2 discharge. At Peking University (PKU), we developed a compact permanent magnet 2.45 GHz ECR ion source attempting to produce C2+ ions. In this work, systematical experiments are performed to further improve the performance of the ion source for the application in PIMS. The results show that it can deliver 1.0 emA C2+ ions with an alumina ceramic liner, and its C consumption is about 10∼20 ng/s. Meanwhile, it is also proved that a certain proportion of He and CO2 mixing can improve the production efficiency of C2+ ions, and the consumption of C can be lower. Furthermore, results confirm that this ECR ion source with alumina liner has a lower memory effect, and the influence of original CO2 discharge can be eliminated by Ar plasma cleaning for several minutes. The experimental results show that the 2.45 GHz ECR ion source has the potential to be used in the PIMS system.