1,319 research outputs found
Self-consistent Continuum Random Phase Approximation calculations with finite-range interactions
We present a technique which allows us to solve the Random Phase
Approximation equations with finite-range interactions and treats the continuum
part of the excitation spectrum without approximations. The interaction used in
the Hartree-Fock calculations to generate the single particle basis is also
used in the Continuum Random Phase Approximation calculations. We present
results for the electric dipole and quadrupole excitations in the O,
O, O, Ca, Ca and Ca nuclei. We compare our
results with those of the traditional discrete Random Phase Approximation, with
the continuum mean-field results and with the results obtained by a
phenomenological approach. We study the relevance of the continuum, of the
residual interaction and of the self-consistency. We also compare our results
with the available total photoabsorption cross section data. We compare our
photoabsorption cross section in He with that obtained by a calculation
which uses a microscopic interaction.Comment: 25 pages, 14 figs., 4 tables, accepted for publication in Physical
Review
Charge-exchange excitations with finite range interactions including tensor terms
We study charge-exchange excitations in doubly magic-nuclei by using a
self-consistent Hartree-Fock plus Random Phase Approximation model. We use four
Gogny-like finite-range interactions, two of them containing tensor forces. We
investigate the effects of the various parts of the tensor forces in the two
computational steps of our model, and we find that their presence is not
negligible and improves the agreement with the experimental data.Comment: 17 pages, 5 figures, 8 table
Coulomb and spin-orbit interactions in random phase approximation calculations
We present a fully self-consistent computational framework composed by
Hartree-Fock plus ran- dom phase approximation where the spin-orbit and Coulomb
terms of the interaction are included in both steps of the calculations. We
study the effects of these terms of the interaction on the random phase
approximation calculations, where they are usually neglected. We carry out our
investigation of excited states in spherical nuclei of oxygen, calcium, nickel,
zirconium, tin and lead isotope chains. We use finite-range effective
nucleon-nucleon interactions of Gogny type. The size of the effects we find is,
usually, of few hundreds of keV. There are not simple approximations which can
be used to simulate these effects since they strongly depend on all the
variables related to the excited states, angular momentum, parity, excitation
energy, isoscalar and isovector characters. Even the Slater approximation
developed to account for the Coulomb exchange terms in Hartree-Fock is not
valid in random phase approximation calculations.Comment: 14 pages, 5 figures; accepted for publication in Phys. Rev.
A study of self-consistent Hartree-Fock plus Bardeen-Cooper-Schrieffer calculations with finite-range interactions
In this work we test the validity of a Hartree-Fock plus
Bardeen-Cooper-Schrieffer model in which a finite-range interaction is used in
the two steps of the calculation by comparing the results obtained to those
found in a fully self-consistent Hartree-Fock-Bogoliubov calculations using the
same interaction.Specifically, we consider the Gogny-type D1S and D1M forces.
We study a wide range of spherical nuclei, far from the stability line, in
various regions of the nuclear chart, from oxygen to tin isotopes. We calculate
various quantities related to the ground state properties of these nuclei, such
as binding energies, radii, charge and density distributions and elastic
electron scattering cross sections. The pairing effects are studied by direct
comparison with the Hartree-Fock results. Despite of its relative simplicity,
in most of the cases, our model provides results very close to those of the
Hartree-Fock-Bogoliubov calculations, and it reproduces rather well the
empirical evidences of pairing effects in the nuclei investigated.Comment: 28 pages, 13 figures. Accepted for publication in J. Phys.
Matter distribution and spin-orbit force in spherical nuclei
We investigate the possibility that some nuclei show density distributions
with a depletion in the center, a semi-bubble structure, by using a
Hartree-Fock plus Bardeen-Cooper-Schrieffer approach. We separately study the
proton, neutron and matter distributions in 37 spherical nuclei mainly in the
shell region. We found a relation between the semi-bubble structure and
the energy splitting of spin-orbit partner single particle levels. The presence
of semi-bubble structure reduces this splitting, and we study its consequences
on the excitation spectrum of the nuclei under investigation by using a
quasi-particle random-phase-approximation approach. The excitation energies of
the low-lying states can be related to the presence of semi-bubble
structure in nuclei.Comment: 15 pages, 7 tables, 11 figures. Version accepted for publication in
Phys. Rev. C; the number of nuclei analysed has been reduced; some figure
have bee redrawn, and a new figure and some references have been adde
Pygmy and giant electric dipole responses of medium-heavy nuclei in a self-consistent Random Phase Approximation approach with finite-range interaction
The pygmy dipole resonance (PDR) is studied in various medium-heavy nuclei by
using a Gogny interaction in a self-consistent Hartree-Fock plus Random Phase
Approximation method. We compare the details of the PDR structure with those of
the giant dipole resonance (GDR). In the PDR protons and neutrons vibrate in
phase, and the main contributions are given by particle-hole excitations
involving the neutrons in excess. On the contrary, in the GDR protons and
neutrons vibrate out of phase, and all the nucleons are involved in the
excitation. The values of the parameters we used to define the collectivity of
an excitation indicate that the PDR is less collective than the GDR. We also
investigate the role of the residual interaction in the appearance of the PDR
and we find a subtle interplay with the shell structure of the nucleus.Comment: 14 pages, 9 figures, 2 tables, to appear in Physical Review
Pairing in spherical nuclei: quasi-particle random phase approximation calculations with the Gogny interaction
We investigate the effects of the pairing in spherical nuclei. We use the
same finite-range interaction of Gogny type in the three steps of our approach,
Hartree-Fock, Bardeen, Cooper and Schrieffer, and quasi-particle random phase
approximation calculations. We study electric and magnetic dipole, quadrupole
and octuple excitations in oxygen and calcium isotopes and also in isotopes
with 20 neutrons. We investigate the pairing effects on single particle
energies and occupation probabilities, on the excitation energies, -values
and collectivity of low-lying states including the isoscalar electric dipole
and the magnetic dipole excitations, and also the giant resonances. The
inclusion of the pairing increases the values of the excitation energies in all
the cases we have studied. In general, the effects of the pairing are too small
to remarkably improve the agreement with the available experimental data.Comment: 18 pages, 6 tables, 8 figures, accepted for publication in Physical
Review
Nuclear proton and neutron distributions in the detection of weak interacting massive particles
In the evaluation of weak interacting massive particles (WIMPs) detection
rates, the WIMP-nucleus cross section is commonly described by using form
factors extracted from charge distributions. In this work, we use different
proton and neutron distributions taken from Hartree-Fock calculations. We study
the effects of this choice on the total detection rates for six nuclei with
different neutron excess, and taken from different regions of the nuclear
chart. The use of different distributions for protons and neutrons becomes more
important if isospin-dependent WIMP-nucleon interactions are considered. The
need of distinct descriptions of proton and neutron densities reduces with the
lowering of the detection energy thresholds.Comment: 15 pages, 10 figures 4 table
Evolution of the pygmy dipole resonance in nuclei with neutron excess
The electric dipole excitation of various nuclei is calculated with a Random
Phase Approximation phenomenological approach. The evolution of the strength
distribution in various groups of isotopes, oxygen, calcium, zirconium and tin,
is studied. The neutron excess produces strength in the low energy region.
Indexes to measure the collectivity of the excitation are defined. We studied
the behavior of proton and neutron transition densities to determine the
isoscalar or isovector nature of the excitation. We observed that in
medium-heavy nuclei the low-energy excitation has characteristics rather
different that those exhibited by the giant dipole resonance. This new type of
excitation can be identified as pygmy dipole resonance.Comment: 14 pages, 12 figures, 7 table
Evolution of the pygmy dipole resonance in nuclei with neutron excess
The electric dipole excitation of various nuclei is calculated with a Random
Phase Approximation phenomenological approach. The evolution of the strength
distribution in various groups of isotopes, oxygen, calcium, zirconium and tin,
is studied. The neutron excess produces strength in the low energy region.
Indexes to measure the collectivity of the excitation are defined. We studied
the behavior of proton and neutron transition densities to determine the
isoscalar or isovector nature of the excitation. We observed that in
medium-heavy nuclei the low-energy excitation has characteristics rather
different that those exhibited by the giant dipole resonance. This new type of
excitation can be identified as pygmy dipole resonance.Comment: 14 pages, 12 figures, 7 table
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