Suddenly shortened half-lives beyond ⁷⁸Ni : N = 50 magic number and high-energy nonunique first-forbidden transitions

Abstract

Background: β-decay rates play a decisive role in understanding the nucleosynthesis of heavy elements and are governed by microscopic nuclear-structure information. A sudden shortening of the half-lives of Ni isotopes beyond N = 50 was observed at the RIKEN-RIBF. This is considered due to the persistence of the neutron magic number N = 50 in the very neutron-rich Ni isotopes. Purpose: By systematically studying the β-decay rates and strength distributions in the neutron-rich Ni isotopes around N = 50, I try to understand the microscopic mechanism for the observed sudden shortening of the half-lives. Methods:The β-strength distributions in the neutron-rich nuclei are described in the framework of nuclear density-functional theory. I employ the Skyrme energy-density functionals (EDF) in the Hartree-Fock-Bogoliubov calculation for the ground states and in the proton-neutron quasiparticle random-phase approximation (pnQRPA) for the transitions. Not only the allowed but the first-forbidden (FF) transitions are considered. Results: The experimentally observed sudden shortening of the half-lives beyond N = 50 is reproduced well by the calculations employing the Skyrme SkM* and SLy4 functionals in contrast to the monotonic shortening predicted in the preceding calculation using the SkO' functional. Conclusions: The sudden shortening of the half-lives beyond N = 50 in the neutron-rich Ni isotopes is due to the shell gap at N = 50 and cooperatively with the high-energy transitions to the low-lying 0⁻ and 1⁻ states in the daughter nuclei. The onset of FF transitions pointed out around N = 82 and 126 is preserved in the lower-mass nuclei around N = 50. This study suggests that a microscopic calculation is needed, where the shell structure in neutron-rich nuclei and its associated effects on the FF transitions are self-consistenly taken into account for predicting β-decay rates of exotic nuclei in unknown region.