L. M. Robledo

Studies of pear-shaped nuclei using accelerated radioactive beams

There is strong circumstantial evidence that certain heavy, unstable atomic nuclei are ‘octupole deformed’, that is, distorted into a pear shape. This contrasts with the more prevalent rugby-ball shape of nuclei with reflection-symmetric, quadrupole deformations. The elusive octupole deformed nuclei are of importance for nuclear structure theory, and also in searches for physics beyond the standard model; any measurable electric-dipole moment (a signature of the latter) is expected to be amplified in such nuclei. Here we determine electric octupole transition strengths (a direct measure of octupole correlations) for short-lived isotopes of radon and radium. Coulomb excitation experiments were performed using accelerated beams of heavy, radioactive ions. Our data on 220Rn and 224Ra show clear evidence for stronger octupole deformation in the latter. The results enable discrimination between differing theoretical approaches to octupole correlations, and help to constrain suitable candidates for experimental studies of atomic electric-dipole moments that might reveal extensions to the standard model.

Microscopic theory of the isovector dipole resonance at high angular momenta

Abstract The giant dipole resonance at large angular velocities and finite temperatures is studied within the framwork of temperature-dependent linear response theory for superfluid Fermi liquids. The peak energy of the resonance and its splitting is discussed as a function of angular momentum and temperature. The influence of the shape and gap parameters on the fine structure is investigated.

Self-consistent calculations of fission barriers in the Fm region

The fission barriers of the nuclei 254 Fm, 256 Fm, 258 Fm, 258 No, and 260 Rf are investigated in a fully microscopic way up to the scission point. The analysis is based on the constrained Hartree-Fock-Bogoliubov theory and Gogny’s D1S force. The quadrupole, octupole, and hexadecapole moments as well as the number of nucleons in the neck region are used as constraints. Two fission paths, corresponding to the bimodal fission, are found. The decrease with isotope mass of the half-life times of heavy Fm isotopes is also explained.

Correlations beyond the mean field in magnesium isotopes: angular momentum projection and configuration mixing

Abstract The quadrupole deformation properties of the ground and low-lying excited states of the even–even magnesium isotopes with N ranging from 8 to 28 have been studied in the framework of the angular momentum projected generator coordinate method with the Gogny force. It is shown that the N=8 neutron magic number is preserved (in a dynamical sense) in 20Mg leading to a spherical ground state. For the magic numbers N=20 and N=28 this is not the case and prolate deformed ground states are obtained. The method yields values of the two neutron separation energies which are in much better agreement with experiment than those obtained at the mean field level. It is also obtained that 40Mg is at the neutron dripline. Concerning the results for the excitation energies of the 2+ excited states and their transition probabilities to the ground state we observe a good agreement with the available experimental data. On the theoretical side, we also present a detailed justification of the prescription used for the density dependent part of the interaction in our beyond-mean-field calculations.

Intrinsic vs laboratory frame description of the deformed nucleus 48Cr.

The collective yrast band of the nucleus {sup 48}Cr is studied using the spherical shell model and the Hartree-Fock-Bogoliubov (HFB) method. Both approaches produce basically the same axially symmetric intrinsic state up to the---accurately reproduced---observed backbending. Agreement between both calculations extends to most observables. The only significant discrepancy comes from the static moments of inertia and can be attributed to the need of a more refined treatment of pairing correlations in the HFB calculation.

Relativistic Hartree+Bogoliubov description of the deformed N = 28 region

Ground-state properties of neutron-rich N≈28 nuclei are described in the framework of relativistic Hartree plus Bogoliubov (RH+B) theory. The model

Particle number projection with effective forces

Abstract The particle-number projection method is formulated for density-dependent forces and in particular for the finite-range Gogny force. Detailed formula for the projected energy and its gradient are provided. The problems arising from the neglection of any exchange term, which may lead to divergences, are thoroughly discussed and the possible inaccuracies estimated. Numerical results for the projection after variation method are shown for the nucleus 164Er and for the projection before variation approach for the nuclei 48,50Cr. We also confirm the Coulomb antipairing effect found in mean-field theories.

Charge radii and structural evolution in Sr, Zr, and Mo isotopes

The evolution of the ground-state nuclear shapes in neutron-rich Sr, Zr, and Mo isotopes, including both even-even and odd-A nuclei, is studied within a self-consistent mean-field approximati on based on the D1S Gogny interaction. Neutron separation energies and charge radii are calculated and compared with available data. A correlation between a shape transition and a discontinuity in those observables is found microscopically. While in Sr and Zr isotopes the steep behavior observed in the isotopic dependence of the charge radii is a consequence of a sharp prolate-oblate tran sition, the smooth behavior found in Mo isotopes has its origin in an emergent region of triaxiality.

Angular momentum projected analysis of quadrupole collectivity in 30,32,34Mg and 32,34,36,38Si with the Gogny interaction

A microscopic angular momentum projection after variation is used to describe quadrupole collectivity in (^{30,32,34}Mg) and (^{32,34,36,38}Si). The Hartree-Fock-Bogoliubov states obtained in the quadrupole constrained mean field approach are taken as intrinsic states for the projection. Excitation energies of the first (2^{+}) states and the (B(E2,0^{+}\to 2^{+})) transition probabilities are given. A reasonable agreement with available experimental data is obtained. It is also shown that the mean field picture of those nuclei is strongly modified by the projection.

Coulomb exchange and pairing contributions in nuclear Hartree-Fock-Bogoliubov calculations with the Gogny force

Abstract We present exact Hartree–Fock–Bogoliubov calculations with the finite-range density-dependent Gogny force using a triaxial basis. For the first time, all contributions to the pairing and Fock fields arising from the Gogny and Coulomb interactions as well as the two-body correction of the kinetic energy have been calculated in this basis. We analyze the relevance of these terms in different regions of the periodic table at zero and high angular momentum. The validity of commonly used approximations that neglect different terms in the variational equations is also checked. We find a decrease of the proton-pairing energies mainly due to a Coulomb antipairing effect.