Pairing and nonaxial-shape correlations in N=150 isotones

Background: Rotational bands have been measured around Fm-250 associated with strong deformed-shell closures. The K-pi = 2(-) excited band emerges systematically in N = 150 isotones raging from plutonium to nobelium with even-Z numbers, and a sharp drop in energies was observed in californium. Purpose: I attempt to uncover the microscopic mechanism for the appearance of such a low-energy 2(-) state in Cf-248. Furthermore, I investigate the possible occurrence of the low-energy K-pi = 2(+) state, the. vibration, to elucidate the mechanism that prefers the simultaneous breaking of the reflection and axial symmetry to the breaking of the axial symmetry alone in this mass region. Method: I employ a nuclear energy-density functional (EDF) method: the Skyrme-Kohn-Sham-Bogoliubov and the quasiparticle random-phase approximation are used to describe the ground state and the transition to excited states. Results: The Skyrme-type SkM* and SLy4 functionals reproduce the fall in energy but not the absolute value of the K-pi = 2(-) state at Z = 98 where the proton two-quasiparticle excitation [633]7/2 circle times [521]3/2 plays a decisive role for the peculiar isotonic dependence. I find interweaving roles by the pairing correlation of protons and the deformed-shell closure at Z = 98. The SkM* model predicts the K-pi = 2(-) state appears lower in energy in Cf-246 than in 248Cf as the Fermi level of neutrons is located in between the [622]5/2 and the [734]9/2 orbitals. Except for Fm-250 in the SkM* calculation, the Kp = 2(+) state is predicted to appear higher in energy than the K-pi = 2(+) state because the quasiproton [521]1/2 orbital is located above the [633]7/2 orbital. Conclusions: A systematic study of low-lying collective states in heavy actinide nuclei provides a rigorous testing ground for microscopic nuclear models. The present paper shows a need for improvements in the EDFs to describe pairing correlations and shell structures in heavy nuclei, that are indispensable in predicting the heaviest nuclei.

Automated summary

This study aims to uncover the microscopic mechanism for the appearance of low-energy 2(-) state in Cf-248, as well as investigate the possible occurrence of the low-energy K-pi = 2(+) state, highlighting the need for improvements in EDFs to describe pairing correlations and shell structures in heavy nuclei.