Isoscalar and neutron modes in the E1 spectra of Ni isotopes and the relevance of shell effects and the continuum

Abstract

We study theoretically the electric dipole transitions of even Ni isotopes at low energies, using the self-consistent quasi-particle random-phase approximation (RPA) with the D1S Gogny interaction and a continuum-RPA model with the SLy4 Skyrme force. We analyze isoscalar states, isovector states, and the dipole polarizability. We define a reference value for the polarizability, to remove a trivial dependence on the mass number. We compare our results with data and other calculations, with a focus on collective states, shell effects, and threshold transitions. Our results support the presence of a strong isoscalar transition, with little or moderate E1 strength, as a universal feature of ordinary nuclei. In moderately neutron-rich Ni isotopes, namely 68Ni and neighboring isotopes, this transition is found bimodal due to couplings with surface neutrons. An adequate treatment of the continuum states appears essential for describing suprathreshold E1 strength, especially beyond 68Ni. Very exotic isotopes (N>50) are found highly polarizable, with practically all their E1 strength in the continuum. The dipole polarizability and the neutron skin thickness are influenced by shell structure in different ways, so they can appear anticorrelated. A comparison with existing results for lighter (Ca) and heavier (Sn) nuclei suggests that the so-called pygmy dipole strength is influenced strongly by shell effects and that, partly for that reason, its isospin structure depends on the mass region.

14 pages, incl. 10 figures (v2) and 1 table; (v2: clarifications and minor restructuring to improve readability; one extra figure)

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Cited by in corpus (10)

• Nuclear Equation of State from ground and collective excited state properties of nuclei
• The neutron skin thickness from the measured electric dipole polarizability in ${}^{68}$Ni, ${}^{120}$Sn, and ${}^{208}$Pb
• Reference Database for Photon Strength Functions
• Toward an accurate strongly-coupled many-body theory within the equation of motion framework
• Accessing the Single-Particle Structure of the Pygmy Dipole Resonance in ${}^{208}$Pb
• Dipole response in neutron-rich nuclei with new Skyrme interactions
• Excitation spectra of exotic nuclei in a self-consistent phonon-coupling model
• Low-lying electric-dipole strengths of Ca, Ni, and Sn isotopes imprinted on total reaction cross sections
• Possible existence of pygmy dipole resonance built on excited states in a neutron-rich ${}^{80}$Ge nucleus
• Proton pygmy resonances: predictions for N=20 isotones