3,815 research outputs found
Relativistic mean-field description of the dynamics of giant resonances
The relativistic mean-field theory provides a framework in which the nuclear
many-body problem is described as a self-consistent system of nucleons and
mesons. In the mean-field approximation, the self-consistent time evolution of
the nuclear system describes the dynamics of collective motion: nuclear
compressibility from monopole resonances, regular and chaotic dynamics of
isoscalar and isovector collective vibrations.Comment: LaTeX, 10 pages, 5 figures, Invited Talk, Topical Conference on Giant
resonances, Varenna, May 1998, to be published in Nucl. Phys.
The Proton Electric Pygmy Dipole Resonance
The evolution of the low-lying E1 strength in proton-rich nuclei is analyzed
in the framework of the self-consistent relativistic Hartree-Bogoliubov (RHB)
model and the relativistic quasiparticle random-phase approximation (RQRPA).
Model calculations are performed for a series of N=20 isotones and Z=18
isotopes. For nuclei close to the proton drip-line, the occurrence of
pronounced dipole peaks is predicted in the low-energy region below 10 MeV
excitation energy. From the analysis of the proton and neutron transition
densities and the structure of the RQRPA amplitudes, it is shown that these
states correspond to the proton pygmy dipole resonance.Comment: 7 pages, 4 figures, to be published in Phys. Rev. Let
Accuracy of BCS-based approximations for pairing in small Fermi systems
We analyze the accuracy of BCS-based approximations for calculating
correlation energies and odd-even energy differences in 2-component fermionic
systems with a small number of pairs. The analysis is focused on comparing BCS
and projected BCS treatments with the exact solution of the pairing
Hamiltonian, considering parameter ranges appropriate for nuclear pairing
energies. We find that the projected BCS is quite accurate over the entire
range of coupling strengths in spaces of up to about 20 doubly degenerate
orbitals. It is also quite accurate for two cases we considered with a more
realistic Hamiltonian, representing the nuclei around 117Sn and 207Pb. However,
the projected BCS significantly underestimates the energies for much larger
spaces when the pairing is weak.Comment: 10 pages; 14 figure
Relativistic description of exotic collective excitation phenomena in atomic nuclei
The low-lying dipole and quadrupole states in neutron rich nuclei, are
studied within the fully self-consistent relativistic quasiparticle
random-phase approximation (RQRPA), formulated in the canonical basis of the
Relativistic Hartree-Bogoliubov model (RHB), which is extended to include the
density dependent interactions. In heavier nuclei, the low-lying E1 excited
state is identified as a pygmy dipole resonance (PDR), i.e. as a collective
mode of excess neutrons oscillating against a proton-neutron core. Isotopic
dependence of the PDR is characterized by a crossing between the PDR and
one-neutron separation energies. Already at moderate proton-neutron asymmetry
the PDR peak is calculated above the neutron emission threshold, indicating
important implications for the observation of the PDR in (gamma,gamma')
scattering, and on the theoretical predictions of the radiative neutron capture
rates in neutron-rich nuclei. In addition, a novel method is suggested for
determining the neutron skin of nuclei, based on measurement of excitation
energies of the Gamow-Teller resonance relative to the isobaric analog state.Comment: 8 pages, 3 figures, invited talk at the international workshop
"Blueprints for the nucleus: From First Principles to Collective Motion", May
17-22. 2004, Istanbul, Turkey; to appear in Int. J. Mod. Phys.
Toroidal dipole resonances in the relativistic random phase approximation
The isoscalar toroidal dipole strength distributions in spherical nuclei are
calculated in the framework of a fully consistent relativistic random phase
approximation. It is suggested that the recently observed "low-lying component
of the isoscalar dipole mode" might in fact correspond to the toroidal giant
dipole resonance. Although predicted by several theoretical models, the
existence of toroidal resonances has not yet been confirmed in experiment. The
strong mixing between the toroidal resonance and the dipole compression mode
might help to explain the large discrepancy between theory and experiment on
the position of isoscalar giant dipole resonances.Comment: 10 pages, 3 figures; Phys.Rev.C, in prin
Pygmy dipole resonances in relativistic random phase approximation
The isovector dipole response in Pb is described in the framework of
a fully self-consistent relativistic random phase approximation. The NL3
parameter set for the effective mean-field Lagrangian with nonlinear meson
self-interaction terms, used in the present calculations, reproduces ground
state properties as well as the excitation energies of giant resonances in
nuclei. In addition to the isovector dipole resonance in Pb, the
present analysis predicts the occurrence of low-lying E1 peaks in the energy
region between 7 and 11 MeV. In particular, a collective state has been
identified whose dynamics correspond to that of a dipole pygmy resonance: the
vibration of the excess neutrons against the inert core composed of equal
number of protons and neutrons.Comment: LaTex 7 pages, 4 eps Figs, submitted to Phys. Lett.
Microscopic calculation of 240Pu scission with a finite-range effective force
Hartree-Fock-Bogoliubov calculations of hot fission in
have been performed with a newly-implemented code that uses the D1S
finite-range effective interaction. The hot-scission line is identified in the
quadrupole-octupole-moment coordinate space. Fission-fragment shapes are
extracted from the calculations. A benchmark calculation for
is obtained and compared to results in the literature. In
addition, technical aspects of the use of HFB calculations for fission studies
are examined in detail. In particular, the identification of scission
configurations, the sensitivity of near-scission calculations to the choice of
collective coordinates in the HFB iterations, and the formalism for the
adjustment of collective-variable constraints are discussed. The power of the
constraint-adjustment algorithm is illustrated with calculations near the
critical scission configurations with up to seven simultaneous constraints.Comment: 18 pages, 24 figures, to be published in Physical Review
Nuclear Halos and Drip Lines in Symmetry-Conserving Continuum HFB Theory
We review the properties of nuclear halos and nuclear skins in drip line
nuclei in the framework of the spherical Hartree-Fock-Bogoliubov theory with
continuum effects and projection on good particle number with the Gogny force.
We first establish the position of the un-projected HFB drip lines for the two
most employed parametrizations of the Gogny force and show that the use of
finite-range interactions leads almost always to small-sized halos, even in the
least bound nuclei, which is in agreement with most mean-field predictions. We
also discuss the size of the neutron skin at the drip line and its relation to
neutron asymmetry. The impact of particle-number projection and its conceptual
consequences near the drip line are analyzed in detail. In particular, we
discuss the role of the chemical potential in a projected theory and the
criteria required to define the drip line. We show that including particle
number projection can shift the latter, in particular near closed shells. We
notice that, as a result, the size of the halo can be increased due to larger
pairing correlations. However, combining the most realistic pairing
interaction, a proper treatment of the continuum and particle number projection
does not permit to reproduce the very large halos observed in very light
nuclei.Comment: Re-submitted to Phys. Rev. C after Referee's review. Layout of
figures changed to cope with editor's requirement
- …