When an energetic highly charged ion (HCI) collides with an atom, the target electrons may be captured by theprojectile ion, either radiatively or non-radiatively. During a radiative electron capture (REC), a target electronis transferred to the projectile accompanying with a photon emission, which carries away the excess energy andmomentum. During a non-radiative electron capture (NRC), the energy and momentum conservations are ensuredby the target nucleus[1]. If the captured electrons are populated in excited states, photons maybe emitted duringthe following stabilization processes, and therefore the X-ray spectrum can provide information about the initialpopulation. However, if a solid target is employed, the single-collision condition cannot be ensured when consideringthe capture processes, as well as strong background will be produced[2]. With the development of heavy ioncooling storage rings, the experimental luminance is enhanced by the strong ion beams when a gaseous target isprerequisite[3].
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