46 research outputs found
Mode-coupling and the pygmy dipole resonance in a relativistic two-phonon model
A two-phonon version of the relativistic quasiparticle time blocking
approximation (RQTBA-2) represents a new class of many-body models for nuclear
structure calculations based on the covariant energy density functional. As a
fully consistent extension of the relativistic quasiparticle random phase
approximation (RQRPA), the two-phonon RQTBA implies a fragmentation of nuclear
states over two-quasiparticle and two-phonon configurations. This leads, in
particular, to a splitting-out of the lowest 1 state as a member of the
two-phonon quintuplet from the RQRPA pygmy dipole mode, thus
establishing a physical mixing between these three modes. The inclusion of the
two-phonon configurations in the model space allows to describe the positions
and the reduced transition probabilities of the lowest 1 states in isotopes
Sn as well as the low-energy fraction of the dipole strength
without any adjustment procedures. The model is also applied to the low-lying
dipole strength in neutron-rich Ni isotopes. Recent experimental
data for Ni are reproduced fairly well
Description of the Giant Monopole Resonance in the Even-A Sn Isotopes within the Microscopic Model Including Quasiparticle-Phonon Coupling
We have calculated the strength distributions of the giant monopole resonance
in the even-A tin isotopes (A = 112-124) which were recently measured in
inelastic -scattering. The calculations were performed within two
microscopic models: the quasiparticle random phase approximation (QRPA) and the
quasiparticle time blocking approximation which is an extension of the QRPA
including quasiparticle-phonon coupling. We used a self-consistent
calculational scheme based on the HF+BCS approximation. The single-particle
continuum was exactly included on the RPA level. The self-consistent mean field
and the effective interaction were derived from the Skyrme energy functional.
In the calculations, two Skyrme force parametrizations were used. The T5
parametrization with comparatively low value of the incompressibility of
infinite nuclear matter ( = 202 MeV) gives theoretical results in
good agreement with the experimental data including the resonance widths.Comment: 21 pages, 2 figures; the figures have been modified: experimental
data are shown in Fig.
Excitations of the unstable nuclei ^{48}Ni and ^{49}Ni
The isoscalar E1 and E2 resonances in the proton-rich nuclei ^{48,49}Ni and
the {f_{7/2}3^-} multiplet in ^{49}Ni have been calculated taking into account
the single-particle continuum exactly. The analogous calculations for the
mirror nuclei ^{48}Ca and ^{49}Sc are presented. The models used are the
continuum RPA for ^{48}Ni, ^{48}Ca and the Odd RPA for ^{49}Ni, ^{49}Sc, the
latter has been developed recently and describes both single-particle and
collective excitations of an odd nucleus on a common basis. In all four nuclei
we obtained a distinct splitting of the isoscalar E1 resonance into 1 h-bar
omega and 3 h-bar omega peaks at about 11 MeV and 30 MeV, respectively. The
main part of the isoscalar E1 EWSR is exhausted by the 3 h-bar omega
resonances. The 1 h-bar omega resonances exhaust about 35% of this EWSR in
^{48,49}Ni and about 22% in ^{48}Ca and ^{49}Sc. All seven {f_{7/2}3^-}
multiplet members in ^{49}Ni are calculated to be in the (6-8) MeV energy
region and have noticeable escape widths.Comment: 11 pages, 3 Postscript figure
Elimination of spurious states in the quasiparticle time blocking approximation
The quasiparticle time blocking approximation (QTBA) is considered as a model
for the description of excitations in open-shell nuclei. The QTBA is an
extension of the quasiparticle random phase approximation that includes
quasiparticle-phonon coupling. In the present version of the QTBA, the pairing
correlations are included within the framework of the BCS approximation. Thus,
in this model, the spurious states appear, which are caused by the
breaking of the symmetry related to the particle-number conservation. In this
work, the method is described which solves the problem of the spurious
states in the QTBA with the help of the projection technique. The method is
illustrated by calculations of excitations in Sn nucleus.Comment: 12 pages, 3 figures - To appear in the proceedings of the 59-th
International Meeting on Nuclear Spectroscopy and Nuclear Structure, June
15-19, 2009, Cheboksary, Russi
Relativistic quasiparticle time blocking approximation. Dipole response of open-shell nuclei
The self-consistent Relativistic Quasiparticle Random Phase Approximation
(RQRPA) is extended by the quasiparticle-phonon coupling (QPC) model using the
Quasiparticle Time Blocking Approximation (QTBA). The method is formulated in
terms of the Bethe-Salpeter equation (BSE) in the two-quasiparticle space with
an energy-dependent two-quasiparticle residual interaction. This equation is
solved either in the basis of Dirac states forming the self-consistent solution
of the ground state or in the momentum representation. Pairing correlations are
treated within the Bardeen-Cooper-Schrieffer (BCS) model with a
monopole-monopole interaction. The same NL3 set of the coupling constants
generates the Dirac-Hartree-BCS single-quasiparticle spectrum, the static part
of the residual two-quasiparticle interaction and the quasiparticle-phonon
coupling amplitudes. A quantitative description of electric dipole excitations
in the chain of tin isotopes (Z=50) with the mass numbers A = 100, 106, 114,
116, 120, and 130 and in the chain of isotones with (N=50) 88-Sr, 90-Zr, 92-Mo
is performed within this framework.
The RQRPA extended by the coupling to collective vibrations generates spectra
with a multitude of '2q+phonon' (two quasiparticles plus phonon) states
providing a noticeable fragmentation of the giant dipole resonance as well as
of the soft dipole mode (pygmy resonance) in the nuclei under investigation.
The results obtained for the photo absorption cross sections and for the
integrated contributions of the low-lying strength to the calculated dipole
spectra agree very well with the available experimental data.Comment: 43 pages, 3 figure
Extended Theory of Finite Fermi Systems: Application to the collective and non-collective E1 strength in Pb
The Extended Theory of Finite Fermi Systems is based on the conventional
Landau-Migdal theory and includes the coupling to the low-lying phonons in a
consistent way. The phonons give rise to a fragmentation of the single-particle
strength and to a compression of the single-particle spectrum. Both effects are
crucial for a quantitative understanding of nuclear structure properties. We
demonstrate the effects on the electric dipole states in Pb (which
possesses 50% more neutrons then protons) where we calculated the low-lying
non-collective spectrum as well as the high-lying collective resonances. Below
8 MeV, where one expects the so called isovector pygmy resonances, we also find
a strong admixture of isoscalar strength that comes from the coupling to the
high-lying isoscalar electric dipole resonance, which we obtain at about 22
MeV. The transition density of this resonance is very similar to the breathing
mode, which we also calculated. We shall show that the extended theory is the
correct approach for self-consistent calculations, where one starts with
effective Lagrangians and effective Hamiltonians, respectively, if one wishes
to describe simultaneously collective and non-collective properties of the
nuclear spectrum. In all cases for which experimental data exist the agreement
with the present theory results is good.Comment: 21 figures corrected typos in author fiel
Self-consistent calculations of the electric giant dipole resonances in light and heavy mass nuclei
While bulk properties of stable nuclei are successfully reproduced by
mean-field theories employing effective interactions, the dependence of the
centroid energy of the electric giant dipole resonance on the nucleon number A
is not. This problem is cured by considering many-particle correlations beyond
mean-field theory, which we do within the "Quasiparticle Time Blocking
Approximation". The electric giant dipole resonances in O, Ca,
and Pb are calculated using two new Skyrme interactions.Comment: 4 pages, 4 figure