Low-energy dipole excitations in neon isotopes andN=16isotones within the quasiparticle random-phase approximation and the Gogny force

Abstract

Low-energy dipole excitations in neon isotopes and $N=16$ isotones are calculated with a fully consistent axially-symmetric-deformed quasiparticle random phase approximation (QRPA) approach based on Hartree-Fock-Bogolyubov (HFB) states. The same Gogny D1S effective force has been used both in HFB and QRPA calculations. The microscopical structure of these low-lying resonances, as well as the behavior of proton and neutron transition densities, are investigated in order to determine the isoscalar or isovector nature of the excitations. It is found that the $N=16$ isotones $^{24}\mathrm{O}$, $^{26}\mathrm{Ne}$, $^{28}\mathrm{Mg}$, and $^{30}\mathrm{Si}$ are characterized by a similar behavior. The occupation of the $2s$${}_{1/2}$ neutron orbit turns out to be crucial, leading to nontrivial transition densities and to small but finite collectivity. Some low-lying dipole excitations of $^{28}\mathrm{Ne}$ and $^{30}\mathrm{Ne}$, characterized by transitions involving the $\ensuremath{\nu}1d$${}_{3/2}$ state, present a more collective behavior and isoscalar transition densities. A collective proton low-lying excitation is identified in the $^{18}\mathrm{Ne}$ nucleus.