Alpha spectroscopy of purified beams of exotic nuclei at the FRS Ion Catcher

Abstract

The FRS Ion Catcher (FRS-IC) is located at the final focal plane of the Fragment Separator FRS at GSI. The FRS-IC setup is well known for high-precision experiments with stopped exotic nuclei produced by projectile fragmentation and fission. The facility consists of the cryogenic gas-filled stopping cell (CSC), an RFQ-based beamline (DISTRICT), and a multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS). This paper illustrates how alpha spectroscopy performed at this facility has emerged as a promising tool to unveil the nuclear structure of exotic nuclei, i.e., half-live and decay energy measurements. First studies of that kind were performed on the decay chains of ²¹⁸Rn, ²¹⁹Rn, ²²¹Ac, ²²⁰Fr, and ^223,224Th produced by projectile fragmentation of ²³⁸U. The α decay energy measurements performed and the deduced Q_α values confirm the known maximum at N=128 and the values of Q_α at N$=132-133$ follow the predicted increasing in Q_α values compared to the values for At isotopes at the same neutron number N. Further, the production rate ratio of the isomer to the ground state of ²¹¹Po was measured. It allows an estimate of the angular momentum distribution of ²¹¹Po fragments following fragmentation of ²³⁸U in a ⁹Be target at relativistic energies. In addition, the potential of mass-selected decay spectroscopy behind the MR-TOF-MS was demonstrated with short-lived ²¹⁵Po ions ($t_{1/2}=1.78$ ms). This demonstrates that the FRS-IC is a reliable setup for α spectroscopy studies and related nuclear structure studies.