v 2 1 6 A ug 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(RRPA) based on the novel density-dependent meson-exchange interactions, and Continuum Random-Phase Approximation(CRPA) 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 CRPA analysis indicates that the escape width for the proton PDR is rather large, as a result of the coupling to the continuum. One of the major challenges in the region of unstable nuclei is the understanding of soft modes of excitations which involve loosely bound nucleons, nucleon halo, or skin. In particular, in neutron-rich nuclei, nucleons from the neutron skin may give rise to a low-energy dipole mode, known as a pygmy dipole resonance (PDR) [1]. Some experimental evidence about the low-lying dipole excitation phenomena in neutron-rich nuclei is available from (i) electromagnetic excitations in heavy-ion collisions [2], and (ii) nuclear resonance fluorescence experiments for nuclei with moderate neutron to proton number ratios, e.g. On the other side, a variety of theoretical models have in recent years been employed in studies of the low-lying E1 strength: continuum quasiparti-cle RPA formulated in the coordinate-space Hartree-Fock-Bogoliubov framework [8], continuum RPA with Woods-Saxon potential for the ground state and Landau-Migdal force in the residual interaction [9], the time-dependent density-matrix theory [10], Skyrme Hartree-Fock + quasiparticle RPA with phonon coupling [11,12], and the quasiparticle-phonon model [3,5,13]. In the relativistic framework, properties of low-energy excitations have been systematically studied in the relativistic RPA (RRPA) [14], and the relativistic quasi-particle RPA (RQRPA) [15,16,17,18]. Both experimental and theoretical studies qualitatively agree on the global properties of the isovector dipole response, i.e., as the number of neutrons increases along the isotope chain, the transition strength distribution is characterized by the appearance of pronounced low-lying E1 strength. Within the relativistic R(Q)RPA studies, it has been shown in a fully self-consistent way that the low-lying pygmy state represents a genuine structure effect: the neutron skin oscillates against the core, exhibiting a collective nature from medium towards heavier nuclei [14,15]. A detailed quantitative description of the low-lying E1 strength in neutron-rich nuclei is essential for calculations of radiative neutron capture rates …