Separation of atomic and molecular ions by ion mobility with an RF carpet

TL;DR

This paper presents a novel RF carpet technique for separating atomic ions from molecular contaminants in gas-filled stopping cells, significantly reducing molecular background and improving ion extraction efficiency.

Contribution

The study introduces and demonstrates a new RF carpet method for ion separation based on ion mobility differences, enhancing purity and efficiency in ion beam experiments.

Findings

Molecular contaminants suppressed by three orders of magnitude.

Achieved background-free measurements with less than 1% background events.

Enhanced ion transport efficiency by reducing space-charge effects.

Abstract

Gas-filled stopping cells are used at accelerator laboratories for the thermalization of high-energy radioactive ion beams. Common challenges of many stopping cells are a high molecular background of extracted ions and limitations of extraction efficiency due to space-charge effects. At the FRS Ion Catcher at GSI, a new technique for removal of ionized molecules prior to their extraction out of the stopping cell has been developed. This technique utilizes the RF carpet for the separation of atomic ions from molecular contaminant ions through their difference in ion mobility. Results from the successful implementation and test during an experiment with a 600~MeV/u $^{124}$Xe primary beam are presented. Suppression of molecular contaminants by three orders of magnitude has been demonstrated. Essentially background-free measurement conditions with less than $1~\%$ of background events within a mass-to-charge range of 25 u/e have been achieved. The technique can also be used to reduce the space-charge effects at the extraction nozzle and in the downstream beamline, thus ensuring high efficiency of ion transport and highly-accurate measurements under space-charge-free conditions.