The underlying structure of low-lying collective bands of atomic nuclei is discussed from a novel perspective on the interplay between single-particle and collective degrees of freedom, by utilizing state-of-the-art configuration interaction calculations on heavy nuclei. Besides the multipole components of the nucleon-nucleon interaction that drive collective modes forming those bands, the monopole component is shown to control the resistance against such modes. The calculated structure of ${}^{154}\mathrm{Sm}$ corresponds to the coexistence between prolate and triaxial shapes, while that of ${}^{166}\mathrm{Er}$ exhibits a deformed shape with a strong triaxial instability. Both findings differ from traditional views based on $\beta /\gamma$ vibrations. The formation of collective bands is shown to be facilitated from a self-organization mechanism.

Hyperfine structure and isotope shifts have been measured for the ground and isomeric states in the neutron-deficient isotopes ${}^{179–184}\mathrm{Tl}$ using the 276.9-nm transition. The experiment has been performed at the CERN-Isotope Separator On-Line facility using the in-source resonance-ionization laser spectroscopy technique. Spins for the ground states in ${}^{179,181,183}\mathrm{Tl}$ have been determined as $I=1/2$. Magnetic moments and changes in the nuclear mean-square charge radii have been deduced. By applying the additivity relation for magnetic moments of the odd-odd Tl nuclei the leading configuration assignments were confirmed. A deviation of magnetic moments for isomeric states in ${}^{182,184}\mathrm{Tl}$ from the trend of the heavier Tl nuclei is observed. The charge radii of the ground states of the isotopes ${}^{179–184}\mathrm{Tl}$ follow the trend for isotonic (spherical) lead nuclei. The noticeable difference in charge radii for ground and isomeric states of ${}^{183,184}\mathrm{Tl}$ has been observed, suggesting a larger deformation for the intruder-based $9/2^{-}$ and ${10}^{-}$ states compared to the ground states. An unexpected growth of the isomer shift for ${}^{183}\mathrm{Tl}$ has been found.

The changes in mean-squared charge radii of neutron-deficient gold nuclei have been determined using the in-source, resonance-ionization laser spectroscopy technique, at the ISOLDE facility (CERN). From these new data, nuclear deformations are inferred, revealing a competition between deformed and spherical configurations. The isotopes ${}^{180,181,182}\mathrm{Au}$ are observed to possess well-deformed ground states and, when moving to lighter masses, a sudden transition to near-spherical shapes is seen in the extremely neutron-deficient nuclides, ${}^{176,177,179}\mathrm{Au}$. A case of shape coexistence and shape staggering is identified in ${}^{178}\mathrm{Au}$ which has a ground and isomeric state with different deformations. These new data reveal a pattern in ground-state deformation unique to the gold isotopes, whereby, when moving from the heavy to light masses, a plateau of well-deformed isotopes exists around the neutron midshell, flanked by near-spherical shapes in the heavier and lighter isotopes—a trend hitherto unseen elsewhere in the nuclear chart. The experimental charge radii are compared to those from Hartree-Fock-Bogoliubov calculations using the D1M Gogny interaction and configuration mixing between states of different deformation. The calculations are constrained by the known spins, parities, and magnetic moments of the ground states in gold nuclei and show a good agreement with the experimental results.

The in-gas-jet laser spectroscopy method relies on the production of uniform and low-temperature gas jets to fully resolve the atomic hyperfine structure and efficiently determine fundamental nuclear properties of short-lived isotopes from, e.g., the hardly accessible actinide and transactinide elements. In this article we present the studies devoted to designing, producing, and characterizing the flow properties of a convergent-divergent (de Laval) hypersonic nozzle with a superior performance for laser spectroscopy applications. A novel flow mapping technique, based on resonance ionization spectroscopy (RIS), has been employed to characterize the local flow properties of an argon gas jet formed by this nozzle, revealing a 61.5-mm long, highly collimated atomic jet at a uniform low temperature of 16.6(5) K [Mach 8.11(12)] that will enable laser spectroscopy experiments on heavy-exotic nuclei with an unprecedented spectral resolution and a high efficiency. These results have been compared with those obtained by planar laser induced fluorescence spectroscopy (PLIFS) studies and show a good agreement between the two techniques and a significant improvement in efficiency of the RIS mapping method with respect to PLIFS. The data are compared to state-of-the-art fluid-dynamics calculations that were carried out to obtain the nozzle contour and simulate its performance, as well as to explain the observation of a possible onset of argon nucleation.