Zakopane 2026 Conference on Nuclear Physics

Rare-earth nuclei are known to exhibit a pronounced shape transition from spherical to well-deformed configurations in the vicinity of the N = 104 mid-shell region. Within the ytterbium isotopic chain, the neutron-rich isotopes $^{176}$Yb (N = 106) and $^{178}$Yb (N = 108) lie firmly on the deformed side of this transition, as evidenced by their R$_{4/2}$ = E(4$_1^+$) / E(2$_1^+$) ratios approaching the rigid-rotor limit of 3.33. To investigate the evolution of quadrupole deformation in this mass region, an experimental study employing γ-ray spectroscopy and fast-timing lifetime measurements was performed at the 10 MV FN Tandem accelerator of the Institut für Kernphysik, University of Cologne. The lifetime of the first excited 2$_1^+$ state in $^{176}$Yb was remeasured to improve its precision, while the corresponding lifetime in $^{178}$Yb was determined for the first time. Preliminary results obtained from the fast-timing analysis will be presented in this contribution alongside the observed γ-spectroscopic characteristics. The combined findings confirm the pronounced collective behaviour of both nuclei and indicate that $^{178}$Yb exhibits an even stronger rotational character, positioning it as the most rigid-rotor-like nucleus within the ytterbium isotopic chain.

*This work is supported by the German Research Foundation – 539757749.