127 research outputs found
Second random-phase approximation with the Gogny force. First applications
We present the first applications of the second random-phase-approximation
model with the finite-range Gogny interaction. We discuss the advantages of
using such an interaction in this type of calculations where 2 particle-2 hole
configurations are included. The results found in the present work confirm the
well known general features of the second random-phase approximation spectra:
we find a large shift, several MeV, of the response centroids to lower energies
with respect to the corresponding random-phase-approximation values. As known,
these results indicate that the effects of the 1 particle-1 hole/2 particle-2
hole and 2 particle-2 hole/2 particle-2 hole couplings are important. It has
been found that the changes of the strength distributions with respect to the
standard random-phase-approximation results are particularly large in the
present case. This important effect is due to some large neutron-proton matrix
elements of the interaction and indicates that these matrix elements (which do
not contribute in the mean-field calculations employed in the conventional fit
procedures of the force parameters) should be carefully constrained to perform
calculation
Microscopic calculations of double and triple Giant Resonance excitation in heavy ion collisions
We perform microscopic calculations of the inelastic cross sections for the
double and triple excitation of giant resonances induced by heavy ion probes
within a semicalssical coupled channels formalism. The channels are defined as
eigenstates of a bosonic quartic Hamiltonian constructed in terms of collective
RPA phonons. Therefore, they are superpositions of several multiphonon states,
also with different numbers of phonons and the spectrum is anharmonic. The
inclusion of (n+1) phonon configurations affects the states whose main
component is a n-phonon one and leads to an appreacible lowering of their
energies. We check the effects of such further anharmonicities on the previous
published results for the cross section for the double excitation of Giant
Resonances. We find that the only effect is a shift of the peaks towards lower
energies, the double GR cross section being not modified by the explicity
inclusion of the three-phonon channels in the dynamical calculations. The
latters give an important contribution to the cross section in the triple GR
energy region which however is still smaller than the experimental available
data. The inclusion of four phonon configurations in the structure calculations
does not modify the results.Comment: Revtex4, to be published in PR
Excitation of Pygmy Dipole Resonance in neutron-rich nuclei via Coulomb and nuclear fields
We study the nature of the low-lying dipole strength in neutron-rich nuclei,
often associated to the Pygmy Dipole Resonance. The states are described within
the Hartree-Fock plus RPA formalism, using different parametrizations of the
Skyrme interaction. We show how the information from combined reactions
processes involving the Coulomb and different mixtures of isoscalar and
isovector nuclear interactions can provide a clue to reveal the characteristic
features of these states.Comment: 9 Pages, 8 figures, contribution to International Symposium On
Nuclear Physics, December 8-12, 2009,Bhabha Atomic Research Centre, Mumbai,
Indi
Microscopic description of Coulomb and nuclear excitation of multiphonon states in Ca + Ca collisions
We calculate the inelastic scattering cross sections to populate one- and
two-phonon states in heavy ion collisions with both Coulomb and nuclear
excitations. Starting from a microscopic approach based on RPA, we go beyond it
in order to treat anharmonicities and non-linear terms in the exciting field.
These anharmonicities and non-linearities are shown to have important effects
on the cross sections both in the low energy part of the spectrum and in the
energy region of the Double Giant Quadrupole Resonance. By properly introducing
an optical potential the inelastic cross section is calculated semiclassically
by integrating the excitation probability over all impact parameters. A
satisfactory agreement with the experimental results is obtained.Comment: 20 pages, 2 figures, revtex, to be published in Phys. Rev.
Excitation of multiple giant dipole resonances: from spherical to deformed nuclei
The effect of deformation on the excitation of multiple giant dipole resonances is studied. Analytical expressions are derived in the framework of the interacting boson model for the energies and E1 properties of giant dipole resonances in spherical and deformed nuclei, and a numerical treatment of transitional nuclei is proposed. Coulomb-excitation cross sections are calculated in U and in the samarium isotopes
On "the authentic damping mechanism" of the phonon damping model
Some general features of the phonon damping model are presented. It is
concluded that the fits performed within this model have no physical content
Anharmonic vibrations in nuclei
In this letter, we show that the non-linearitites of large amplitude motions in atomic nuclei induce giant quadrupole and monopole vibrations. As a consequence, the main source of anharmonicity is the coupling with configurations including one of these two giant resonances on top of any state. Two-phonon energies are often lowered by one or two MeV because of the large matrix elements with such three phonon configurations. These effects are studied in two nuclei, 40Ca and 208Pb
- …