arXiv subjects

X. Vinas

Publications and source records attributed to X. Vinas.

At least 19 recordsLinked to original sources

arXiv2015

Unified equation of state for neutron stars on a microscopic basis

We derive a new equation of state (EoS) for neutron stars (NS) from the outer crust to the core based on modern microscopic Brueckner-Hartree-Fock (BHF) calculations using the Argonne $v_{18}$ potential plus three-body forces computed with the Urbana model. To deal with the inhomogeneous structures of matter in the NS crust, we use the recent Barcelona-Catania-Paris-Madrid (BCPM) nuclear energy density functional that is directly based on the same microscopic BHF calculations, and which is able to reproduce the ground-state properties of nuclei along the periodic table. The EoS of the outer crust requires the masses of neutron-rich nuclei, which are obtained through Hartree-Fock-Bogoliubov calculations with the BCPM functional when they are unknown experimentally. To compute the inner crust, Thomas-Fermi calculations in Wigner-Seitz cells are performed with the same functional. Existence of nuclear pasta is predicted in a range of average baryon densities between $\simeq$0.067 fm$^{-3}$ and $\simeq$0.0825 fm$^{-3}$, where the transition to the core takes place. The NS core is computed from the nuclear EoS of the BHF calculation assuming non-exotic constituents (core of $npeμ$ matter). In each region of the star, we discuss the comparison of the new EoS with previous EoSes for the complete NS structure, in particular, with the Lattimer-Swesty EoS and with the Shen et al. EoS widely used in astrophysical calculations. The new microscopically derived EoS fulfills at the same time a NS maximum mass of 2~$M_\odot$ with a radius of 10 km, and a 1.5~$M_\odot$ NS with a radius of 11.7 km.

nucl-th↗

arXiv2014

Influence of the single-particle structure on the nuclear surface and the neutron skin

We analyze the influence of the single-particle structure on the neutron density distribution and the neutron skin in Ca, Ni, Zr, Sn, and Pb isotopes. The nucleon density distributions are calculated in the Hartree-Fock+BCS approach with the SLy4 Skyrme force. A close correlation is found between the quantum numbers of the valence neutrons and the changes in the position and the diffuseness of the nuclear surface, which in turn affect the neutron skin thickness. Neutrons in the valence orbitals with low principal quantum number and high angular momentum mainly displace the position of the neutron surface outwards, while neutrons with high principal quantum number and low angular momentum basically increase the diffuseness of the neutron surface. The impact of the valence shell neutrons on the tail of the neutron density distribution is discussed.

nucl-th↗

arXiv2013

Simple effective interaction: Infinite nuclear matter and finite nuclei

The mean field properties and equation of state for asymmetric nuclear matter are studied by using a simple effective interaction which has a single finite range Gaussian term. The study of finite nuclei with this effective interaction is done by means of constructing a quasilocal energy density functional for which the single particle equations take the form of Skryme-Hartree-Fock equations. The predictions of binding energies and charge radii of spherical nuclei are found to be compatible with the results of standard models as well as experimental data.

nucl-th↗

arXiv2012

An effective Nuclear Model: from Nuclear Matter to Finite Nuclei

The momentum and density dependence of mean fields in symmetric and asymmetric nuclear matter are analysed using the simple density dependent finite range effective interaction containing a single Gaussian term alongwith the zero-range terms. Within the formalism developed, it is possible to reproduce the various diverging predictions on the momentum and density dependence of isovector part of the mean field in asymmetric matter. The finite nucleus calculation is formulated for the simple Gaussian interaction in the framework of quasilocal density functional theory. The prediction of energies and charge radii of the interaction for the spherical nuclei compares well with the results of other effective theories.

nucl-th↗

arXiv2010

Semi-Classical Description of the Average Pairing Properties in Nuclei

We present a new semi-classical theory for describing pairing in finite Fermi systems. It is based in taking the $\hbar \to 0$, i.e. Thomas-Fermi, limit of the gap equation written in the basis of the mean field (weak coupling). In addition to the position dependence of the Fermi momentum, the size dependence of the matrix elements of the pairing force is also taken into account in this theory. An example typical for the nuclear situation shows the improvement of this new approach over the standard Local Density Approximation. We also show that if in this approach some shell fluctuations are introduced in the level density, the arch structure displayed by the quantal gaps along isotopic chains is almost recovered. We also point out that in heavy drip line nuclei pairing is strongly reduced.

nucl-th↗

arXiv2010

Cooper pair sizes in superfluid nuclei in a simplified model

Cooper pair sizes are evaluated in a simple harmonic oscillator model reproducing the values of sophisticated HFB calculations. Underlying reasons for the very small sizes of 2.0-2.5 fm of Cooper pairs in the surface of nuclei are analysed. It is shown that the confining properties of the nuclear volume is the dominating effect. It is argued that for Cooper pair sizes LDA is particularly inadapted.

nucl-th↗

arXiv2010

Influence of the symmetry energy on the giant monopole resonance of neutron-rich nuclei

We analyze the influence of the density dependence of the symmetry energy on the average excitation energy of the isoscalar giant monopole resonance (GMR) in stable and exotic neutron-rich nuclei by applying the relativistic extended Thomas-Fermi method in scaling and constrained calculations. For the effective nuclear interaction, we employ the relativistic mean field model supplemented by an isoscalar-isovector meson coupling that allows one to modify the density dependence of the symmetry energy without compromising the success of the model for binding energies and charge radii. The semiclassical estimates of the average energy of the GMR are known to be in good agreement with the results obtained in full RPA calculations. The present analysis is performed along the Pb and Zr isotopic chains. In the scaling calculations, the excitation energy is larger when the symmetry energy is softer. The same happens in the constrained calculations for nuclei with small and moderate neutron excess. However, for nuclei of large isospin the constrained excitation energy becomes smaller in models having a soft symmetry energy. This effect is mainly due to the presence of loosely-bound outer neutrons in these isotopes. A sharp increase of the estimated width of the resonance is found in largely neutron-rich isotopes, even for heavy nuclei, which is enhanced when the symmetry energy of the model is soft. The results indicate that at large neutron numbers the structure of the low-energy region of the GMR strength distribution changes considerably with the density dependence of the nuclear symmetry energy, which may be worthy of further characterization in RPA calculations of the response function.

nucl-th↗

arXiv2009

Neutron skin thickness in droplet model with surface width dependence: indications of softness of the nuclear symmetry energy

We analyze the neutron skin thickness in finite nuclei with the droplet model and effective nuclear interactions. The ratio of the bulk symmetry energy J to the so-called surface stiffness coefficient Q has in the droplet model a prominent role in driving the size of neutron skins. We present a correlation between the density derivative of the nuclear symmetry energy at saturation and the J/Q ratio. We emphasize the role of the surface widths of the neutron and proton density profiles in the calculation of the neutron skin thickness when one uses realistic mean-field effective interactions. Next, taking as experimental baseline the neutron skin sizes measured in 26 antiprotonic atoms along the mass table, we explore constraints arising from neutron skins on the value of the J/Q ratio. The results favor a relatively soft symmetry energy at subsaturation densities. Our predictions are compared with the recent constraints derived from other experimental observables. Though the various extractions predict different ranges of values, one finds a narrow window L ~ 45-75 MeV for the coefficient L that characterizes the density derivative of the symmetry energy which is compatible with all the different empirical indications.

nucl-th↗

arXiv2009

Symmetry coefficients and incompressibility of clusterized supernova matter

The symmetry energy coefficients, incompressibility, and single-particle and isovector potentials of clusterized dilute nuclear matter are calculated at different temperatures employing the $S$-matrix approach to the evaluation of the equation of state. Calculations have been extended to understand the aforesaid properties of homogeneous and clusterized supernova matter in the subnuclear density region. Comparison of the results in the $S$-matrix and mean-field approach reveals some subtle differences in the density and temperature region we explore.

nucl-th↗

arXiv2009

Nuclear symmetry energy probed by neutron skin thickness of nuclei

We describe a relation between the symmetry energy coefficients c_sym(rho) of nuclear matter and a_sym(A) of finite nuclei that accommodates other correlations of nuclear properties with the low-density behavior of c_sym(rho). Here we take advantage of this relation to explore the prospects for constraining c_sym(rho) of systematic measurements of neutron skin sizes across the mass table, using as example present data from antiprotonic atoms. The found constraints from neutron skins are in harmony with the recent determinations from reactions and giant resonances.

nucl-th↗

arXiv2008

Theoretical study of elastic electron scattering off stable and exotic nuclei

Results for elastic electron scattering by nuclei, calculated with charge densities of Skyrme forces and covariant effective Lagrangians that accurately describe nuclear ground states, are compared against experiment in stable isotopes. Dirac partial-wave calculations are performed with an adapted version of the ELSEPA package. Motivated by the fact that studies of electron scattering off exotic nuclei are intended in future facilities in the commissioned GSI and RIKEN upgrades, we survey the theoretical predictions from neutron-deficient to neutron-rich isotopes in the tin and calcium isotopic chains. The charge densities of a covariant interaction that describes the low-energy electromagnetic structure of the nucleon within the Lagrangian of the theory are used to this end. The study is restricted to medium and heavy mass nuclei because the charge densities are computed in mean field approach. Since the experimental analysis of scattering data commonly involves parameterized charge densities, as a surrogate exercise for the yet unexplored exotic nuclei, we fit our calculated mean field densities with Helm model distributions. This procedure turns out to be helpful to study the neutron-number variation of the scattering observables and allows us to identify correlations of potential interest among some of these observables within the isotopic chains.

nucl-th↗

arXiv2008

Density dependence of symmetry free energy of hot nuclei

The density and excitation energy dependence of symmetry energy and symmetry free energy for finite nuclei are calculated microscopically in a microcanonical framework taking into account thermal and expansion effects. A finite-range momentum and density dependent two-body effective interaction is employed for this purpose. The role of mass, isospin and equation of state (EoS) on these quantities is also investigated; our calculated results are in consonance with the available experimental data.

nucl-th↗

arXiv2008

Khon-Sham Density Functional Inspired Approach to Nuclear Binding

A non-relativisitic nuclear density functional theory is constructed, not as usual, from an effective density dependent nucleon-nucleon force but directly introducing in the functional results from microscopic nuclear and neutron matter Bruckner G-matrix calculations at various densities. A purely phenomenological finite range part to account for surface properties is added. The striking result is that only four to five adjustable parameters, spin-orbit included, suffice to reproduce nuclear binding energies and radii with the same quality as obtained with the most performant effective forces, containing on the order of ten parameters. In this pilot work, for the pairing correlations, simply a density dependent zero range force is adopted from the literature. Possible future extensions of this approach are pointed out.

nucl-th↗

arXiv2007

Excitation energy dependence of symmetry energy of finite nuclei

A finite range density and momentum dependent effective interaction is used to calculate the density and temperature dependence of the symmetry energy coefficient Csym(rho,T) of infinite nuclear matter. This symmetry energy is then used in the local density approximation to evaluate the excitation energy dependence of the symmetry energy coefficient of finite nuclei in a microcanonical formulation that accounts for thermal and expansion effects. The results are in good harmony with the recently reported experimental data from energetic nucleus-nucleus collisions.

nucl-th↗

arXiv2007

Density reorganization in hot nuclei

The density profile of a hot nuclear system produced in intermediate energy heavy ion collisions is studied in a microcanonical formulation with a momentum and density dependent finite range interaction. The caloric curve and the density evolution with excitation are calculated for a number of systems for the equilibrium mononuclear configuration; they compare favorably with the recent experimental data. The studied density fluctuations are seen to build up rapidly beyond an excitation energy of 8 MeV/u indicating the instability of the system towards nuclear disassembly. Explicit introduction of deformation in the expansion path of the heated nucleus, however, shows that the system might fragment even earlier. We also explore the effects of the nuclear equation of state and of the mass and isospin asymmetry on the nuclear equilibrium configuration and the relevant experimental observables.

nucl-th↗

arXiv2006

Average ground-state energy of finite Fermi systems

Semiclassical theories like the Thomas-Fermi and Wigner-Kirkwood methods give a good description of the smooth average part of the total energy of a Fermi gas in some external potential when the chemical potential is varied. However, in systems with a fixed number of particles N, these methods overbind the actual average of the quantum energy as N is varied. We describe a theory that accounts for this effect. Numerical illustrations are discussed for fermions trapped in a harmonic oscillator potential and in a hard wall cavity, and for self-consistent calculations of atomic nuclei. In the latter case, the influence of deformations on the average behavior of the energy is also considered.

cond-mat.other↗

arXiv2006

Nuclear expansion with excitation

The expansion of an isolated hot spherical nucleus with excitation energy and its caloric curve are studied in a thermodynamic model with the SkM* force as the nuclear effective two-body interaction. The calculated results are shown to compare well with the recent experimental data from energetic nuclear collisions. The fluctuations in temperature and density are also studied. They are seen to build up very rapidly beyond an excitation energy of 9 MeV/u. Volume-conserving quadrupole deformation in addition to expansion indicates, however, nuclear disassembly above an excitation energy of 4 MeV/u

nucl-th↗

arXiv2006

Thomas-Fermi theory for atomic nuclei revisited

The recently developed semiclassical variational Wigner-Kirkwood (VWK) approach is applied to finite nuclei using external potentials and self-consistent mean fields derived from Skyrme interactions and from relativistic mean field theory. VWK consists of the Thomas-Fermi part plus a pure, perturbative hbar^2 correction. In external potentials, VWK passes through the average of the quantal values of the accumulated level density and total energy as a function of the Fermi energy. However, there is a problem of overbinding when the energy per particle is displayed as a function of the particle number. The situation is analyzed comparing spherical and deformed harmonic oscillator potentials. In the self-consistent case, we show for Skyrme forces that VWK binding energies are very close to those obtained from extended Thomas-Fermi functionals of hbar^4 order, pointing to the rapid convergence of the VWK theory. This satisfying result, however, does not cure the overbinding problem, i.e., the semiclassical energies show more binding than they should. This feature is more pronounced in the case of Skyrme forces than with the relativistic mean field approach. However, even in the latter case the shell correction energy for e.g. 208Pb turns out to be only -6 MeV what is about a factor two or three off the generally accepted value. As an ad hoc remedy, increasing the kinetic energy by 2.5%, leads to shell correction energies well acceptable throughout the periodic table. The general importance of the present studies for other finite Fermi systems, self-bound or in external potentials, is pointed out.

nucl-th↗