Mass and lifetime measurements of exotic nuclei at storage rings

I will introduce nuclear physics programs ongoing at storage rings worldwide. Currently, three heavy-ion storage rings are operational, which can store radioactive ion beams for an extended period at relativistic energies; the Experimental Storage Ring (ESR) at GSI/FAIR, Darmstadt, Cooler Storage Ring for Experiments (CSRe) at the Institute of Modern Physics, Lanzhou, China and Rare-RI Ring (R3) at RIKEN, Japan.

Under such unique experimental conditions, many atomic masses of unstable nuclei far from stability have been precisely measured, contributing to the developments of nuclear structure theory and astrophysical nucleosynthesis [1]. A mass resolving power at ESR is approaching ~10⁶, which enables to separate low-lying excited states in flight.

Lifetime measurements of stored ions are often unique, because half-lives of highly charged ions can dramatically change compared to those of neutral atoms. For example, fully stripped, bare ²⁰⁵Tl ions become radioactive by bound-state beta decay, whereas neutral ones are stable on the earth [2]. Note that bound-state beta decay refers to a new decay mode that the emitting electron is captured into an open atomic orbital of daughter nucleus. The decay phenomena of highly charged ions can simulate those in extreme stellar conditions of high temperature and high pressure.

Recently, we have constructed a new storage ring called Rare-RI Ring at RIKEN, where the highest intensities of radioactive ion beams have been available up to now. To extend mass measurements further away from stability, a new technique has been established, that an exotic nucleus of interest is in-flight selected and injected one-by-one with a newly-developed fast kicker magnet. Masses of neutron-rich nuclei near astrophysical rapid-neutron capture process are successfully updated [3]. In this seminar, I will focus on the highlights performed at the GSI and RIKEN storage ring facilities.

References

[1] T. Yamaguchi, et al., Prog. Part. Nucl. Phys. 120 (2023) 103882.

[2] G. Leckenby, et al., Nature 635 (2024) 321; R.S. Sidhu, et al., Phys. Rev. Lett. 133 (2024) 232701.

[3] H.F. Li, et al., Phys. Rev. Lett. 128 (2022) 152701; D. Nagae, et al., Phys. Rev. C 110 (2024) 014310.