Low-Energy E 1 Strength Distributions of 68 Ni
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ow-Energy E1 Strength Distributions of 68Ni N. N. Arsenyeva,*, A. P. Severyukhina, b, V. V. Voronova, and Nguyen Van Giaic aJoint
Institute for Nuclear Research, Dubna, Moscow region, 141980 Russia Dubna State University, Dubna, Moscow region, 141982 Russia c Institut de Physique Nucléaire, CNRS-IN2P3, Université Paris-Sud, F-91406 Orsay Cedex, France *e-mail: [email protected] b
Received March 4, 2019; revised March 20, 2019; accepted March 29, 2019
Abstract—We study the effects of the coupling between one- and two-phonon components of the wave functions on the low-energy electric dipole response of 68Ni in a microscopic model based on an effective Skyrme interaction SLy5. The finite rank separable approach for the quasiparticle random phase approximation is used. The effect of phonon-phonon coupling leads to the fragmentation of the E1 strength to a lower energy and improves the agreement with available experimental data. DOI: 10.1134/S1063779619050034
INTRODUCTION Recently, with the advent of advanced radioactive beam facilities and novel experimental techniques, unexplored regions of exotic nuclei and new phenomena became accessible for detailed spectroscopic studies. The electric dipole (E1) response of nuclei at energies around particle separation energy is presently attracting much attention, particularly for unstable neutron-rich nuclei produced as radioactive beams [1]. The structure and dynamics of the low-energy dipole strength, also referred to as pygmy dipole resonance (PDR), has extensively been investigated using a variety of theoretical approaches and models; see, e.g., [2, 3]. In analogy to the giant dipole resonance (GDR), the PDR has been interpreted as a collective oscillation of the neutron skin with respect to a N ≈ Z inert core. The total sum of the measured energyweighted sum rule (EWSR) of such E1 distributions is less than 1–2% of the Thomas–Reiche–Kuhn (TRK) sum rule value for stable nuclei and less than 5–6% for unstable neutron-rich nuclei [1]. The existence of the PDR mode near the neutron threshold has important astrophysical implications. The PDR study is expected to provide information on the symmetry energy term of the nuclear equation of state [3], is such an example. A description of the properties of the low-energy E1 strength distribution requires the including of the coupling between one- and two-phonon components of the wave functions [4, 5]. The quasiparticle random phase approximation (QRPA) with a self-consistent mean-field derived from a Skyrme energy density functionals (EDF) is one of the most successful methods for studying the low-energy dipole strength, see e.g., [2, 3]. The main difficulty is that the complexity
of calculations beyond standard QRPA increases rapidly with the size of the configuration space, so that one has to work within limited spaces. By making use of the finite rank separable approximation (FRSA) [6, 7] for the residual interaction, one can perform Skyrme-QRPA calculations in very large two-quasiparticle spaces. Following the basic ideas o
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