Low-lying dipole resonance in neutron-rich Ne isotopes

Abstract

Microscopic structure of the low-lying isovector dipole excitation mode in neutron-rich $^{26,28,30}$Ne is investigated by performing deformed quasiparticle-random-phase-approximation (QRPA) calculations. The particle-hole residual interaction is derived from a Skyrme force through a Landau-Migdal approximation. We have obtained the low-lying resonance in $^{26}$Ne at around 8.5 MeV. It is found that the isovector dipole strength at $E_{x}<10$ MeV exhausts about 6.0% of the classical Thomas-Reiche-Kuhn dipole sum rule. This excitation mode is composed of several QRPA eigenmodes, one is generated by a $\nu(2s^{-1}_{1/2} 2p_{3/2})$ transition dominantly, and the other mostly by a $\nu(2s^{-1}_{1/2} 2p_{1/2})$ transition. The neutron excitations take place outside of the nuclear surface reflecting the spatially extended structure of the $2s_{1/2}$ wave function. In $^{30}$Ne, the deformation splitting of the giant resonance is large, and the low-lying resonance is overlapping with the giant resonance.Comment: 8 pages, 9 figures and 5 table

Canonical-basis time-dependent Hartree-Fock-Bogoliubov theory and linear-response calculations

“…The similar structure is seen in the neutron-rich nucleus 32 Ne. However, despite the fact that the magnitude of deformation is roughly the same as that of 20,22 Ne, the position of the higher peak (K = 1) is lowered, and the splitting is not as prominent as that in 20,22 Ne. In oblate nuclei, such as 24 Ne, the deformation splitting is not clearly seen in the total strength distribution S(E; E1) because the high-energy peak becomes much smaller than the lower peak.…”

“…The QRPA calculations have been performed with a computer program for axially deformed nuclei developed in Ref. [22], which diagonalizes the QRPA matrix of large dimensions in the quasiparticle basis. This is based on the HFB ground state calculated in the twodimensional coordinate-space representation with the Skyrme functional SkM* but with the density-dependent contact interaction for the pairing channel.…”

“…1, we show the isoscalar quadrupole strength distributions (K = 0 and 2) for 34 Mg, [22], and (d) deformed QRPA calculation [23]. The smoothing parameter of = 1 MeV is used.…”

“…Note that a renormalization factor, which was used in Ref. [22], is set to be unity in the present QRPA calculation. The peak energies in these calculations are approximately identical, however, the height of the lowest peak is noticeably different.…”

“…Here, for axially symmetric nuclei, the symmetry axis is chosen as the z axis. The ground states of 20,22 Ne are deformed in a prolate shape with = 0 for both protons and neutrons. Thus, the calculation is identical to the small-amplitude TDHF.…”