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 Qα values confirm the known maximum at N=128 and the values of Qα at N=132−133 follows the predicted increasing in Qα 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 (t1/2=1.78 ms). This demonstrates that the FRS-IC is a reliable setup for α spectroscopy studies and related nuclear structure studies.