Speaker
Description
The neutron-rich rare-earth isotopes pose a challenge for both experimentalists trying to produce and measure them, as well as for the theorists trying to describe them due to their large valence spaces. Towards the double mid-shell at $Z=66$ and $N=104$, we find some of the most well-deformed isotopes between the $^{132}$Sn and $^{208}$Pb shell closures, as evidenced by their low $2^+$ excitation energies $E(2^+)$ [1]. Detailed studies reveal a more complicated evolution with proton and neutron numbers, however, where local minima at $N = 98$ in Dy, Gd and Sm have been ascribed to a "deformed shell-gap" at $N=98$ [2], stabilizing the deformation. Later, evidence from measurements of $\beta$ decays of $^{160,162}$Eu was used to point to a shell gap at $N=100$ to describe the $E(2^+)$ features [3]. Recent progress in configuration mixing calculations are also advancing in the region and proposing alternative views on traditional pictures of shapes and deformations in the region [4]. Spectroscopy on these nuclei is therefore critical to provide input to theoretical ventures and to explain phenomena in the region, where rotational-, vibrational- and single-particle-like states all appear at low excitation energies.
As an effort to study the nuclear structure in this region, we performed the first ever projectile fragmentation of $^{170}$Er with an energy of $1\mathrm{GeV/u}$ at the GSI Helmholtzzentrum für Schwerionenforschung GmbH in Darmstadt, Germany. The subsequent reaction products were cleanly separated and identified on an ion-by-ion basis using the GSI Fragment Separator before being implanted in the decay spectroscopy setup of the DESPEC collaboration [5]. The combination of HPGe, LaBr$_3$ and DSSD detectors allowed us to access significant spectroscopic information of the delayed decays of the exotic ions that were produced.
This contribution will focus on the isotope $^{168}$Dy where the significant gain in statistics compared to earlier works allow for a new view of its nuclear structure in terms of both single-particle and vibrational excitations.
[1] A. Bohr and B. Mottelson, Nuclear structure volume 2: Nuclear deformations (World Scientific Publishing, New York,
1975).
[2] Z. Patel et al., PRL 113, 262502 (2014).
[3] D. Hartley et al., PRL 120, 182502 (2018).
[4] T. Otsuka et al., EPJ A 61, 126 (2025).
[5] A. Mistry et al., NIM A 1033, 166662 (2022).