101 research outputs found
Density Functional Theory studies of cluster states in nuclei
The framework of nuclear energy density functionals is applied to a study of
the formation and evolution of cluster states in nuclei. The relativistic
functional DD-ME2 is used in triaxial and reflection-asymmetric relativistic
Hartree-Bogoliubov calculations of relatively light and neutron-rich
nuclei. The role of deformation and degeneracy of single-nucleon states in the
formation of clusters is analysed, and interesting cluster structures are
predicted in excited configurations of Be, C, O, Ne, Mg, Si, S, Ar and Ca nuclei. Cluster phenomena in neutron-rich nuclei are discussed, and it is
shown that in neutron-rich Be and C nuclei cluster states occur as a result of
molecular bonding of -particles by the excess neutrons, and also that
proton covalent bonding can occur in C
Covariant energy density functionals with and without tensor couplings at the Hartree-Bogoliubov level
Background: The study of additional terms in functionals is relevant to
better describe nuclear structure phenomenology. Among these terms, the tensor
one is known to impact nuclear structure properties, especially in neutron-rich
nuclei. However, its effect has not been studied on the whole nuclear chart
yet.
Purpose: The impact of terms corresponding to the tensor at the Hartree
level, is studied for infinite nuclear matter as well as deformed nuclei, by
developing new density-dependent functionals including these terms. In
particular, we study in details the improvement such a term can bring to the
description of specific nuclear observables.
Methods: The framework of covariant energy density functional is used at the
Hartree-Bogoliubov level. The free parameters of covariant functionals are
optimized by combining Markov-Chain-Monte-Carlo and simplex algorithms.
Results: An improvement of the RMS binding energies, spin-orbit splittings
and gaps is obtained over the nuclear chart, including axially deformed ones,
when including tensors terms. Small modifications of the potential energy
surface and densities are also found. In infinite matter, the Dirac mass is
shifted to a larger value, in better agreement with experiments.
Conclusions: Taking into account additional terms corresponding to the tensor
terms in the vector-isoscalar channel at the Hartree level, improves the
description of nuclear properties, both in nuclei and in nuclear matter
Alpha-particle formation and clustering in nuclei
The nucleonic localization function has been used for a decade to study the
formation of alpha-particles in nuclei, by providing a measure of having
nucleons of a given spin in a single place. However, differences in
interpretation remain, compared to the nucleonic density of the nucleus. In
order to better understand the respective role of the nucleonic localization
function and the densities in the alpha-particle formation in cluster states or
in alpha-decay mechanism, both an analytic approximation and microscopic
calculations, using energy density functionals, are undertaken. The nucleonic
localization function is shown to measure the anti-centrifugal effect, and is
not sensitive to the level of compactness of the alpha-particle itself. It
probes the purity of the spatial overlap of four nucleons in the four possible
(spin, isospin) states. The density provides, in addition, information on the
compactness of an alpha-particle cluster.Comment: 8 pages, 5 figure
Relativistic Hartree-Fock-Bogoliubov model for deformed nuclei
The relativistic Hartree-Fock-Bogoliubov model for axially deformed nuclei (RHFBz) is introduced. The model is based on an effective Lagrangian with density-dependent meson-nucleon couplings in the particle-hole channel, and the central part of the Gogny force is used in the pairing channel. The RHFBz quasiparticle equations are solved by expansion in the basis of a deformed harmonic oscillator. Illustrative RHFBz calculations are performed for carbon, neon, and magnesium isotopes. The effect of explicitly including the pion field is investigated for binding energies, deformation parameters, and charge radii and has an impact on the nuclei’s shape
Rooting the EDF method into the ab initio framework. PGCM-PT formalism based on MR-IMSRG pre-processed Hamiltonians
Recently, ab initio techniques have been successfully connected to the
traditional valence-space shell model. In doing so, they can either explicitly
provide ab initio shell-model effective Hamiltonians or constrain the
construction of empirical ones. In the present work, the possibility to follow
a similar path for the nuclear energy density functional (EDF) method is
analyzed. For this connection to be actualized, two theoretical techniques are
instrumental: the recently proposed ab initio PGCM-PT many-body formalism and
the MR-IMSRG pre-processing of the nuclear Hamiltonian. Based on both formal
arguments and numerical results, possible new lines of research are briefly
discussed, namely to compute ab initio EDF effective Hamiltonians at low
computational cost, to constrain empirical ones or to produce them directly via
an effective field theory that remains to be invented.Comment: 20 pages, 7 figure
Localization and clustering in the nuclear Fermi liquid
Using the framework of nuclear energy density functionals we examine the
conditions for single-nucleon localization and formation of cluster structures
in finite nuclei. We propose to characterize localization by the ratio of the
dispersion of single-nucleon wave functions to the average inter-nucleon
distance. This parameter generally increases with mass and describes the
gradual transition from a hybrid phase in light nuclei, characterized by the
spatial localization of individual nucleon states that leads to the formation
of cluster structures, toward the Fermi liquid phase in heavier nuclei. Values
of the localization parameter that correspond to a crystal phase cannot occur
in finite nuclei. Typical length and energy scales in nuclei allow the
formation of liquid drops, clusters, and halo structures.Comment: 6 pages, 3 figure
Microscopic description of , , and cluster decays of Rn and Ra
Alpha and cluster decays are analyzed for heavy nuclei located above
Pb on the chart of nuclides: Rn and Ra, that
are also candidates for observing the decay mode. A microscopic
theoretical approach based on relativistic Energy Density Functionals (EDF), is
used to compute axially-symmetric deformation energy surfaces as functions of
quadrupole, octupole and hexadecupole collective coordinates. Dynamical
least-action paths for specific decay modes are calculated on the corresponding
potential energy surfaces. The effective collective inertia is determined using
the perturbative cranking approximation, and zero-point and rotational energy
corrections are included in the model. The predicted half-lives for
-decay are within one order of magnitude of the experimental values. In
the case of single emission, the nuclei considered in the present
study exhibit least-action paths that differ significantly up to the scission
point. The differences in alpha-decay lifetimes are not only driven by Q
values, but also by variances of the least-action paths prior to scission. In
contrast, the decay mode presents very similar paths from
equilibrium to scission, and the differences in lifetimes are mainly driven by
the corresponding Q values. The predicted C cluster decay half-lives are
within three orders of magnitudes of the empirical values, and point to a much
more complex pattern compared to the alpha-decay mode.Comment: 9 pages, 13 figure
Direct mass measurements of 19B, 22C, 29F, 31Ne, 34Na and other light exotic nuclei
We report on direct time-of-flight based mass measurements of 16 light
neutron-rich nuclei. These include the first determination of the masses of the
Borromean drip-line nuclei B, C and F as well as that of
Na. In addition, the most precise determinations to date for N
and Ne are reported. Coupled with recent interaction cross-section
measurements, the present results support the occurrence of a two-neutron halo
in C, with a dominant configuration, and a
single-neutron halo in Ne with the valence neutron occupying
predominantly the 2 orbital. Despite a very low two-neutron separation
energy the development of a halo in B is hindered by the 1
character of the valence neutrons.Comment: 5 page
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