75 research outputs found
Low-lying dipole response: isospin character and collectivity in Ni, Sn and Pb
The isospin character, the collective or single-particle nature, and the
sensitivity to the slope of the nuclear symmetry energy of the low-energy
isovector dipole response (known as pygmy dipole resonance) are nowadays under
debate. In the present work we study, within the fully self-consistent
non-relativistic mean field (MF) approach based on Skyrme Hartree-Fock plus
Random Phase Approximation (RPA), the measured even-even nuclei Ni,
Sn and Pb. To analyze the model dependence in the
predictions of the pygmy dipole strength, we employ three different Skyrme
parameter sets. We find that both the isoscalar and the isovector dipole
responses of all three nuclei show a low-energy peak that increases in
magnitude, and is shifted to larger excitation energies, with increasing values
of the slope of the symmetry energy at saturation. We highlight the fact that
the collectivity associated with the RPA state(s) contributing to this peak is
different in the isoscalar and isovector case, or in other words it depends on
the external probe. While the response of these RPA states to an isovector
operator does not show a clear collective nature, the response to an isoscalar
operator is recognizably collective, for {\it all} analyzed nuclei and {\it
all} studied interactions.Comment: Submitted to Phys. Rev.
The pygmy dipole strength, the neutron radius of Pb and the symmetry energy
The accurate characterization of the nuclear symmetry energy and its density
dependence is one of the outstanding open problems in nuclear physics. A
promising nuclear observable in order to constrain the density dependence of
the symmetry energy at saturation is the neutron skin thickness of medium and
heavy nuclei. Recently, a low-energy peak in the isovector dipole response of
neutron-rich nuclei has been discovered that may be correlated with the neutron
skin thickness. The existence of this correlation is currently under debate due
to our limited experimental knowledge on the microscopic structure of such a
peak. We present a detailed analysis of Skyrme Hartree-Fock (HF) plus random
phase approximation (RPA) predictions for the dipole response in several
neutron-rich nuclei and try to elucidate whether models of common use in
nuclear physics confirm or dismiss its possible connection with the neutron
skin thickness. Finally, we briefly present theoretical results for parity
violating electron scattering on Pb at the conditions of the PREx
experiment and discuss the implications for the neutron skin thickness of
Pb and the slope of the symmetry energy.Comment: Contribution to the 2nd Iberian Nuclear Astrophysics Meeting on
Compact Stars proceeding
RPA calculations with Gaussian expansion method
The Gaussian expansion method (GEM) is extensively applied to the
calculations in the random-phase approximation (RPA). We adopt the
mass-independent basis-set that has been tested in the mean-field calculations.
By comparing the RPA results with those obtained by several other available
methods for Ca isotopes, using a density-dependent contact interaction and the
Woods-Saxon single-particle states, we confirm that energies, transition
strengths and widths of their distribution are described by the GEM bases to
good precision, for the , and collective states. The GEM is
then applied to the self-consistent RPA calculations with the finite-range
Gogny D1S interaction. The spurious center-of-mass motion is well separated
from the physical states in the response, and the energy-weighted sum
rules for the isoscalar transitions are fulfilled reasonably well. Properties
of low-energy transitions in Ca are argued in some detail.Comment: 30 pages including 12 figure
Continuum particle-vibration coupling method in coordinate-space representation for finite nuclei
In this paper we present a new formalism to implement the nuclear particle-vibration coupling (PVC) model. The key issue is the proper treatment of the continuum that is allowed by the coordinate space representation. Our formalism, based on the use of zero-range interactions such as the Skyrme forces, is microscopic and fully self-consistent. We apply it to the case of neutron single-particle states in Ca-40, Pb-208, and O-24. The first two cases are meant to illustrate the comparison with the usual (i.e., discrete) PVC model. However, we stress that the present approach allows one to calculate properly the effect of PVC on resonant states. We compare our results with those from experiments in which the particle transfer in the continuum region has been attempted. The latter case, namely O-24, is chosen as an example of a weakly-bound system. Such a nucleus, being double magic and not displaying collective low-lying vibrational excitations, is characterized by quite pure neutron single-particle states around the Fermi surface
Pairing collectivity in medium-mass neutron-rich nuclei near drip-line
We look for collective excitations originating from the strong surface
pairing in unstable nuclei near the neutron drip-line. The soft dipole
excitation is such a pairing mode as it exhibits a character of
core-vs-dineutron motion. Possibility of the hydrodynamic phonon mode (the
Anderson-Bogoliubov mode) is also discussed.Comment: 9 pages, a talk presented at Collective Motion in Nuclei under
Extreme Conditions (COMEX2), June 20-23, 2006, St. Goar, German
Linear response strength functions with iterative Arnoldi diagonalization
We report on an implementation of a new method to calculate RPA strength
functions with iterative non-hermitian Arnoldi diagonalization method, which
does not explicitly calculate and store the RPA matrix. We discuss the
treatment of spurious modes, numerical stability, and how the method scales as
the used model space is enlarged. We perform the particle-hole RPA benchmark
calculations for double magic nucleus 132Sn and compare the resulting
electromagnetic strength functions against those obtained within the standard
RPA.Comment: 9 RevTeX pages, 11 figures, submitted to Physical Review
Instabilities in the Nuclear Energy Density Functional
In the field of Energy Density Functionals (EDF) used in nuclear structure
and dynamics, one of the unsolved issues is the stability of the functional.
Numerical issues aside, some EDFs are unstable with respect to particular
perturbations of the nuclear ground-state density. The aim of this contribution
is to raise questions about the origin and nature of these instabilities, the
techniques used to diagnose and prevent them, and the domain of density
functions in which one should expect a nuclear EDF to be stable.Comment: Special issue "Open Problems in Nuclear Structure Theory" of
Jour.Phys.G - accepted. 7 pages, 2 figure
Error analysis of nuclear mass fits
We discuss the least-square and linear-regression methods, which are relevant
for a reliable determination of good nuclear-mass-model parameter sets and
their errors. In this perspective, we define exact and inaccurate models and
point out differences in using the standard error analyses for them. As an
illustration, we use simple analytic models for nuclear binding energies and
study the validity and errors of models' parameters, and uncertainties of its
mass predictions. In particular, we show explicitly the influence of
mass-number dependent weights on uncertainties of liquid-drop global
parameters.Comment: 10 RevTeX pages, 9 figures, submitted to Physical Review
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