1,856 research outputs found
Neutron and spin--orbit splittings in Ca, S, and Si isotones: tensor--induced and pure spin--orbit effects
Neutron and spin--orbit splittings were recently measured in the
isotones S and Si by transfer reactions. Values were
reported by using the major fragments of the states. An important reduction of
the splitting was observed, from S to Si, associated to a
strong modification of the spin--orbit potential in the central region of the
nucleus Si. We analyze and neutron spin--orbit splittings in
the isotones Ca, S, and Si. We employ several
Skyrme and Gogny interactions, to reliably isolate pure spin--orbit and
tensor--induced contributions, within the mean--field approximation. We use
interactions (i) without the tensor force; (ii) with the tensor force and with
tensor parameters adjusted on top of existing parametrizations; (iii) with the
tensor force and with tensor and spin--orbit parameters adjusted simultaneously
on top of existing parametrizations. We predict in cases (ii) and (iii) a non
negligible reduction of both and splittings, associated to
neutron--proton tensor effects, from Ca to S. The two splittings
are further decreased for the three types of interactions, going from S
to Si. This reduction is produced by the spin--orbit force and is not
affected by tensor--induced contributions. For both reductions, from Ca
to S and from S to Si, we predict in all cases that the
modification is more pronounced for than for splittings. The
measurement of the centroids for neutron and states in the nuclei
S and Si would be interesting to validate this prediction
experimentally. We show the importance of using interactions of type (iii),
because they provide and splittings in the nucleus Ca which are
in agreement with the corresponding experimental values
Two Proton Emission with Electromagnetic Probes
A model to study two-proton emission from nuclei induced by electromagnetic
probes is developed. The process is due to one-body electromagnetic operators,
acting together with short-range correlations, and two-body currents.
The model includes all the diagrams containing a single correlation function.
The sensitivity of the cross section to the details of the correlation function
is studied by using realistic and schematic correlations. Results for the
O nucleus are presented.Comment: 9 pages, 8 figures, Contribution to the 6th. Workshop on
"Electromagnetic Induced Two-Hadron Emission",Pavia, 200
Photo-emission of two protons from nuclei
The photo-emission of two protons from the C, O and Ca
nuclei is investigated. Aim of the work is the study of the possibilities
offered by this probe to obtain information about the characteristics of the
short-range correlations. We have also evaluated the effects of the two-body
-currents which, in this processes, compete with those produced by the
short-range correlations. Our results show that (,pp) processes could
be more useful than (e,e'pp) for the study of the short-range correlations.Comment: 21 pages, 15 figures, 3 table
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
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
Mean-field Based Approaches to Pairing Correlations in Atomic Nuclei
The evolution of the pairing correlations from closed shell to middle shell
nuclei is analyzed with a Finite Range Density Dependent interaction in the Sn
isotopes. As theoretical approaches we use the Hartree-Fock-Bogoliubov, the
Lipkin-Nogami, their particle number projected counterparts and the full
variation after particle number projection method. We find that whereas all
approaches succeed rather well in the description of the total energy they
differ significantly in the pairing correlation content of the wave functions.
The description of the evolution from the weak to the strong pairing regime is
also approach dependent, specially at shell closure.Comment: 14 pages, 5 figure
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