The evolution of the dipole response in nuclei with strong neutron excess is studied in the Hartree-Fock plus random phase approximation with Skyrme forces. We find that the neutron excess increases the fragmentation of the isovector giant dipole resonance, while pushing the centroid of the distribution to lower energies beyond the mass dependence predicted by the collective models, The radial separation of proton and neutron densities associated with a large neutron excess leads to non-vanishing isoscalar transition densities to the GDR states, which are therefore predicted to be excited also by isoscalar nuclear probes. The evolution of the isoscalar compression dipole mode as a function of the neutron excess is finally studied. We find that the large neutron excess leads to a strong concentration of the strength associated with the isoscalar dipole operator Sigma(i)r(i)(3)Y(10), that mainly originates from uncorrelated excitations of the neutrons of the skin. (C) 1997 Elsevier Science B.V

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PTHInternational audienc

Second RPA calculations with a Skyrme force are performed to describe both high-and low-lying excited states in 16 O. The coupling between 1 particle-1 hole and 2 particle-2 hole as well as that between 2 particle-2 hole configurations among themselves are fully taken into account and the residual interaction is never neglected, not resorting therefore to a generally used approximate scheme where only the first kind of coupling is considered. The issue of the rearrangement terms in the matrix elements beyond standard RPA is addressed and discussed. As a general feature of second RPA results, a several-MeV shift of the strength distribution to lower energies is systematically found with respect to RPA distributions. A much more important fragmentation of the strength is also naturally provided by second RPA due to the huge number of 2 particle-2 hole configurations.A better description of the excitation energies of the low-lying 0 + and 2 + states is obtained with second RPA with respect to RPA.

We study the spatial correlations between two particles (or two holes) around a closed shell core in terms of the probability distribution expressed as a function of the c.m. coordinate R of the two particles and of their relative coordinate r. We find that the mixing of configurations induced by the pairing force leads to a probability distribution centered in regions corresponding to larger values of R and smaller values of r, as compared to the case of a pure (j)02 configuration. This tendency to a "surface clustering" is mainly due to the interference of the contributions coming from levels with different parity. However, even with the inclusion of a large number of configurations, the size of the localized "cluster" is much larger than that of a free dinucleon system. NUCLEAR STRUCTURE Pairing correlations, correlation in space, two-particle transfer reactions.Two-particle transfer reactions are usually considered a typical tool for the study of particle-particle correlations in nuclei. In such reactions [we are thinking, in particular, of reactions induced by light ions, such as (p, t), (t,p), or (3He, n)] a dinucleon system with J = 0, very confined in space, is transferred on to (or from) the nuclear surface. It is, therefore, of interest to study to what extent the pairs of particles in the nucleus move closely in space toward each other, in particular in the surface region, where the Pauli principle is less effective and the probability of formation of few-nucleon correlated substructures should increase. We want to clarify the effect on this correlation in space of the particle-particle residual interaction, which is known to strongly enhance the two-particle transfer cross section. This should shed some light on the more general problem of the relation between correlations in spin, isospin, and angular momentum and clusterization in space.We consider the case of two identical particles (or two holes), coupled to angular momentum I = 0 and moving in single particle orbitals around a closed-shell core. The problem has been approached in Refs. 2 and 4 by selecting the S =0 part of the two-particle wave function and assuming the two particles at equal distance from the center of the nucleus, and then studying the behavior of the two-particle wave function as a function of the relative angle between I be the general antisymmetrized wave function describing the two-particle system. The index o. stands for the set [n I j ) of quantum numbers characterizing the single particle wave function +"t/ ( r, X) =@. ]/(r)[ I ]( f) X]/2( X)ljIn order to study the spatial correlations between the two particles we introduce the coordinates R =~r ]+ r q~/J2 and r =~r ] -r q[/v 2 associated with the center of mass and relative motion, respectively, and consider the probability distribution P(r, R) = &~' P( r ], X]', r q, Xq)~r R dr" dR dX]dXqIn the case of harmonic oscillator (HO) wave functions the coordinate transformation and the integration over the angular variables can be performed analytically and the probability di...

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We study, within a semiclassical coupled-channels approach, the possible effects of anharmonicities and non linearities on the excitation of the so-called pygmy resonances in several Sn isotopes and using two different Skyrme interactions. In the energy region of the pygmy resonances, there are a few low-lying multiphonon states. The question is whether they may contribute to the observed peak. Calculations show that the inelastic cross sections in the relevant energy region have an increase that varies from 3% to 21% depending on the isotope and on the kind of Skyrme force used. At the same time, we have studied the nature of the pygmy resonance state by means of a new criterion to establish whether this state can be considered as a collective one.