0 50 60 10 v 1 3 J un 2 00 5 Low - energy dipole excitations towards the proton drip - line : doubly magic 48

The properties of the low-energy dipole response are investigated for the proton-rich doubly magic nucleus 48 Ni, in a comparative study of two microscopic models: fully self-consistent Relativistic Random-Phase Approximation based on the novel density-dependent meson-exchange interactions, and self-consistent Continuum Random-Phase Approximation using Skyrme-type interactions with the continuum properly included. Both models predict the existence of the low-energy soft mode, i.e. the proton pygmy dipole resonance (PDR), for which the transition densities and RPA amplitudes indicate the dynamics of loosely bound protons vibrating against the rest of the nucleons. The escape width for the proton PDR is rather large, as a result of the coupling to the continuum. Nuclei away from the valley of β-stability exhibit a wealth of unique nuclear structure phenomena. The weak binding of the outermost nucleons results in modifications of the bulk nuclear properties, new shell structures, appearance of the nucleon halo and skin structures. One of the major challenges in the region of unstable nuclei is the understanding of soft modes of exci-tations which involve loosely bound nucleons. In particular, in neutron-rich nuclei, nucleons from the neutron skin may give rise to a soft low-energy dipole resonance with excitation energy below 10 MeV. This mode, known as a pygmy dipole resonance (PDR) [1], has recently been the subject in numerous experimental and theoretical studies. The low-lying excitations which might be related to the PDR have also been observed in nuclei with more moderate neutron to proton number ratios. The onset of low-energy E1 strength has been observed with electromagnetic excitations in heavy-ion collisions for oxygen isotopes [2], and with photon scattering in 116,124 Sn [3], and 138 Ba [4]. More recently, low-lying dipole transitions have also extensively been studied at S-DALINAC with nuclear resonance fluorescence experiments for Pb [5] and Ca isotopes [6], and the N=82 isotone chain [7]. A variety of theoretical models have in recent years been employed in studies of low-lying E1 strength: continuum quasiparticle RPA formulated in the coordinate-space Hartree-Fock-Bogoliubov framework [8], continuum RPA with Woods-Saxon potential for the singler-particle spectra and Landau-Migdal force in the residual interaction [13], the time-dependent density-matrix theory [12], Skyrme Hartree-Fock + quasiparticle RPA with phonon coupling [11,10], and the quasiparticle-phonon model [3,5,14]. In the relativistic framework, properties of low-energy excitations have been systematically studied in the relativistic RPA (RRPA) [15], and the relativistic quasiparticle RPA (RQRPA) [16,17,18]. Both experimental and theoretical studies qualitatively agree on …