Suddenly shortened half-lives beyond Ni78: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 neutronrich 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 firstforbidden (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 needed is a microscopic calculation where the shell structure in neutron-rich nuclei and its associated effects on the FF transitions are selfconsistenly taken into account for predicting β-decay rates of exotic nuclei in unknown region.

Isovector spin susceptibility: Isotopic evolution of collectivity in spin response

“…The Ca and Ni isotopes are all spherical in the ground state within the present model. The neutrons are paired in all the isotopes except 70 Ca, 56 Ni, and 78 Ni while the protons are unpaired except in 52 Ni.…”

“…One sees an onset of the low-energy state in [50][51][52][53][54][55][56] Ca. This is due to the νp 3/2 → νp 1/2 excitation.…”

Charge-exchange dipole excitations in deformed nuclei

“…Importance of the higher multipole spin-isospin responses beyond the allowed transitions is recognized for the weak processes in stellar environment [4,19]. The forbidden transitions are also involved at zero temperature such as the neutrinoless double β decay [6,19] if any and the single β decay of heavy neutron-rich nuclei [20][21][22][23][24][25][26]. From a nuclear structure point of view, the spin-dipole resonance (SDR) with L = 1, S = 1, T = 1 have been studied to understand the mechanism for the collectivity of a giant resonance and the spin-isospin part of the interaction in nuclear medium [1,10] besides that the Gamow-Teller and M1 resonances have been extensively studied as the IV magnetic L = 0 transitions [7].…”

Spin-triplet proton-neutron pair in spin-dipole excitations

“…One can see a prominent peak around E T = −12 MeV and several states with a small transition strength. The prominent state corresponds to the low-lying 1 − state appearing at −11.5 MeV in the self-consistent calculation based on the nuclear EDF method [34]; see Fig. 5(b) there.…”