E. Maglione

Microscopic structure of monopole and quadrupole bosons

Abstract Within the pairing-plus-quadrupole model the wave function of the intrinsic state will be described as a product of basic fermion pairs, which are a linear combination of correlated monopole and quadrupole pairs only. The microscopic structure of these S- and D-pairs, as well as the mixture coefficient between them, are determined by minimizing the total energy within the field approximation. While the mixture between monopole and quadrupole pairs changes drastically to account for the transition from sphericity to static deformation, their internal structure remains fairly constant when varying the number of particles.

A representation to describe nuclear processes in the continuum

Bound single-particle states, outgoing resonances and a limited number of scattering states along a path in the complex k-plane (Berggren space) are used as a representation to describe the eigenstates of a realistic nuclear Hamiltonian. It is found that these eigenvectors are contained in the Berggren space, i.e. that the diagonalization of the Hamiltonian provides all the bound states and resonances while the rest of the eigenvalues still remain on the same complex path.

Exact and approximate calculation of giant resonances

Abstract Energies, sum rules and partial decay widths of giant resonances in 208 Pb are calculated solving exactly the continuum RPA equations corresponding to a central Woods-Saxon potential. For comparison an approximate treatment of those quantities in terms of pole expansions of the Green function (Berggren and Mittag-Leffler) is also performed. It is found that the approximated results agree well with the exact ones. Comparison with experimental data is made and a search for physically meaningful resonances is carried out.

Absolute cross sections of two-nucleon transfer reactions induced by heavy ions

Abstract Two-nucleon transfer reactions are calculated for a variety of reactions induced by 14 C and 16 O. The contributions due to the successive and the simultaneous transfer of nucleons and the correction connected with the non-orthogonality of the basis states are included in the calculations. They reproduce the experimental cross sections within a factor of 3. The successive transfer of nucleons alone accounts for the major fraction of the predicted cross section.

Role of high multipole pairs in the description of deformed nuclei

Abstract The intrinsic wave function associated to the ground state rotational band of axially symmetric deformed nuclei is described as a condensate of a basic collective pair which includes components with all possible angular momenta. The microscopic structure of this pair is determined by a variational procedure within the pairing plus quadrupole model. The effect of the truncation to the components with angular momentum Λ =0,2 only, as suggested by the Interacting Boson Model, is tested. It is found that this truncation emphasizes the pairing effects with respect to the shape distortion, with major consequences on different physical observables.

Test of the validity of the SD truncation for deformed systems

Abstract We have tested the basic assumption of the interacting boson model stating that only pairs of particles coupled to angular momentum 0 and 2 are important to the description of low-lying nuclear spectra, even for deformed systems. Variational solutions of the pairing plus quadrupole hamiltonian are obtained for a system of particles moving in a realistic set of single-particle orbitais. Two calculations are carried out based on the field approximation. The first corresponds to the quasiparticle Nilsson model, in which pairs of particles can couple to all values of Λ allowed by the angular momentum selection rule. In the second, pairs of particles are restricted to couple to angular momentum Λ = 0 and Λ = 2. Although most of the predicted values of the different observables are qualitatively similar in the two models, the underlying many-body pictures are very different. Restricting pairs of particles to couple to angular momentum Λ = 0 and Λ = 2 leads to a system in which pairing correlations are as strong as quadrupole correlations. On the other hand, quadrupole correlations are dominant in the Nilsson plus BCS model. Pairs of particles coupled to angular momentum Λ > 2 thus seem to play a decisive role in obtaining the many-body correlations appropriate to strongly deformed systems.

Calculation of Alpha-Decay Widths for Light Lead Isotopes

The absolute α-decay width of light lead isotopes is calculated within a spherical BCS formalism. Clustering features induced by the nuclear interaction appear by considering a large configuration space. Good agreement with experimental data is obtained.

On the radial dependence of the pair transition density in superfluid nuclei

Abstract The radial dependence of the pair transition density in superfluid nuclei is studied. An approximate expression is derived which relates the transition density with the variation of the nuclear density with the number of particles. The proportionality constant is the measure of the collectivity of the system and is directly gven in terms of the pairing gap Δ. The above proportionality was also found to hold on the nuclear surface when using a macroscopic parametrized expression for the density, leading to a description of the excitation of the pairing modes that formally parallels the standard one used in the case of surface variations.

Two-and four-particle surface clusterization in heavy deformed nuclei

Abstract We study the problem of the influence of the pairing interaction on two- and four-particle surface correlations in deformed nuclei. Taking the overlap between the two- or four-particle wave function and a di-nucleon or alpha-particle wave function, respectively, one obtains cluster probability distributions, as functions of the center-of-mass coordinate of the considered particles. For particles moving in pure Nilsson orbits the probability is localized in the intrinsic frame in different region of the nuclear surface, according to the K -quantum numbers of the considered orbitals. The inclusion of the pairing interaction leads to a probability distributed over the entire surface but at the same time to values of the total spectroscopic factors orders of magnitude larger than those associated with pure Nilsson orbits.

Microscopic description of β-band in the collective pair approximation

Abstract The collective pair approximation, previously used in the description of the ground-state band in deformed nuclei, is extended to the description of the β-band. This is obtained by promoting to an excited state one of the basic pairs defining the ground-state condensate in the intrinsic frame. The microscopic structure of the excited pair is determined variationally by minimizing the energy associated with the pairing-plus-quadrupole hamiltonian with the constraint of orthogonality to the ground state. We found that while the collective pair defining the ground-state condensate was dominated by the monopole and quadrupole components and exhibited a microscopic structure smoothly varying with the number of particles, high multipole pairs are dominant in the β-pair and its structure is strongly dependent on the movement of the Fermi surface. This casts serious doubts on the nature of the excited bands obtained in the framework of the interacting boson model.