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ABSTRACT: Recent improvements in the experimental determination of properties of the
Isovector Giant Quadrupole Resonance (IVGQR), as demonstrated in the A=208 mass
region, may be instrumental for characterizing the isovector channel of the
effective nuclear interaction. We analyze properties of the IVGQR in 208Pb,
using both macroscopic and microscopic approaches. The microscopic method is
based on families of non-relativistic and covariant Energy Density Functionals
(EDF), characterized by a systematic variation of isoscalar and isovector
properties of the corresponding nuclear matter equations of state. The
macroscopic approach yields an explicit dependence of the nuclear symmetry
energy at some subsaturation density, for instance S(\rho=0.1 fm^{-3}), or the
neutron skin thickness \Delta r_{np} of a heavy nucleus, on the excitation
energies of isoscalar and isovector GQRs. Using available data it is found that
S(\rho=0.1 fm{}^{-3})=23.3 +/- 0.6 MeV. Results obtained with the microscopic
framework confirm the correlation of the \Delta r_{np} to the isoscalar and
isovector GQR energies, as predicted by the macroscopic model. By exploiting
this correlation together with the experimental values for the isoscalar and
isovector GQR energies, we estimate \Delta r_{np} = 0.14 +/- 0.03 fm for 208Pb,
and the slope parameter of the symmetry energy: L = 37 +/- 18 MeV.
Physical Review C 12/2012; 87:034301. · 3.31 Impact Factor
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ABSTRACT: Although many random-phase approximation (RPA) calculations of the Gamow-Teller (GT) response exist, this is not the case for calculations going beyond the mean-field approximation. We apply a consistent model, that includes the coupling of the GT resonance to low-lying vibrations, to nuclei of the fp shell. Among other motivations, our goal is to see if the particle-vibration coupling can redistribute the low-lying GT+ strength that is relevant for electron-capture processes in core-collapse supernova. We conclude that the lowering and fragmentation of that strength are consistent with the experimental findings and validate our model. However, the particle-vibration coupling cannot account for the quenching of the total value of the low-lying strength.
Physical Review C 03/2012; 85(3):034314. · 3.31 Impact Factor
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ABSTRACT: 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 ${}^{68}$Ni,
${}^{132}$Sn and ${}^{208}$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.
12/2011;
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ABSTRACT: The multipole dependent effects induced by tensor correlations on the charge-exchange spin-dipole (SD) modes of the light nucleus 16O are studied within a Hartree-Fock (HF) plus self-consistent random-phase approximation (RPA), using Skyrme-type interactions. The role played by the triplet-even and triplet-odd tensor terms are examined separately to explain either the softening or hardening of the excitation energies of SD modes. The unperturbed and RPA summed strengths, calculated with and without the tensor force, are discussed and compared with the results of shell-model calculations.
Phys. Rev. C. 10/2011; 84(4).
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ABSTRACT: Gamow-Teller (GT) and charge-exchange spin-dipole (SD) excitation energies in 90Zr and 208Pb are systematically studied to determine the appropriate magnitude of the tensor terms of the Skyrme interactions. We have found that the centroid energies of GT and SD excitations are sensitive to the adopted strengths of the triplet-even and triplet-odd tensor interactions. Especially, the 1- SD state plays a crucial role in constraining the triplet-even part while the triplet-odd part is related rather to the GT peaks. Among the 36 TIJ parameter sets that include nonperturbatively the tensor terms, the four sets, T21, T32, T43, and T54, give reasonable results for the centroid energies in comparison with the experimental data. The sign and magnitude of the tensor terms are also discussed when these terms are added to the existing Skyrme interactions SGII and SLy5. The triplet-even strength can be constrained in a narrow range by using the available experimental data while further empirical data are needed to set a constraint on the triplet-odd term.
Phys. Rev. C. 05/2011; 83(5).
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ABSTRACT: Zero-range effective interactions are commonly used in nuclear physics and in other domains to describe many-body systems within the mean-field model. If they are used within a beyond-mean-field framework, contributions to the total energy that display an ultraviolet divergence are found. We propose a general strategy to regularize this divergence and we illustrate it in the case of the second-order corrections to the equation of state (EOS) of uniform symmetric matter. By setting a momentum cutoff Λ, we show that for every (physically meaningful) value of Λ it is possible to determine a new interaction such that the EOS with the second-order corrections reproduces the empirical EOS, with a fit of the same quality as that obtained at the mean-field level.
Physical Review Letters 12/2010; 105(26):262501. · 7.37 Impact Factor
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ABSTRACT: The charge exchange spin-dipole (SD) excitations of 208Pb are studied by using a fully self-consistent Skyrme Hartree-Fock plus random phase approximation formalism which includes the tensor interaction. It is found, for the first time, that the tensor correlations have a unique, multipole-dependent effect on the SD excitations; that is, they produce a softening of 1{-} states, but a hardening of 0{-} and 2{-} states. This paves the way to a clear assessment of the strength of the tensor terms. We compare our results with a recent measurement, showing that our choice of tensor terms improves the agreement with experiment. The robustness of our results is supported by the analytic form of the tensor matrix elements.
Physical Review Letters 08/2010; 105(7):072501. · 7.37 Impact Factor
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ABSTRACT: We discuss different approaches to the problem of reproducing the observed features of nuclear single-particle (s.p.) spectra. In particular, we analyze the dominant energy peaks, and the single-particle strength fragmentation, using the example of neutron states in 208Pb. Our main emphasis is the interpretation of that fragmentation as due to particle-vibration coupling (PVC). We compare with recent Energy Density Functional (EDF) approaches, and try to present a critical perspective. Comment: 7 pages. Contribution to the "Focus issue on Open Problems in Nuclear Structure", Journal of Physics G
01/2010;
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ABSTRACT: We study the effect of time-odd components of the Skyrme energy density functionals on the ground state of finite nuclei and in nuclear matter. The spin-density-dependent terms, which have been recently proposed as an extension of the standard Skyrme interaction, are shown to change the total binding energy of odd nuclei by only few tenths of keV, while the time-odd components of standard Skyrme interactions give an effect that is larger by one order of magnitude. The HFB-17 mass formula based on a Skyrme parametrization is adjusted including the new spin-density-dependent terms. A comprehensive study of binding energies in the whole mass table of 2149 nuclei gives a root mean square (rms) deviation of 0.575 MeV between experimental data and the calculated results, which is as good as the original HFB-17 mass formula. From the analysis of the spin instabilities of nuclear matter, restrictions on the parameters governing the spin-density-dependent terms are evaluated. We conclude that with the extended Skyrme interaction, the Landau parameters G0 and G'0 could be tuned with a large flexibility without changing the ground-state properties in nuclei or nuclear matter.
Journal of Physics G Nuclear and Particle Physics 10/2009; 36(12):125103. · 4.18 Impact Factor
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ABSTRACT: The tensor terms of the Skyrme effective interaction are included in the
self-consistent Hartree-Fock plus Random Phase Approximation (HF+RPA) model.
The Gamow-Teller (GT) strength function of 90Zr and 208Pb are calculated with
and without the tensor terms. The main peaks are moved downwards by about 2 MeV
when including the tensor contribution. About 10% of the non-energy weighted
sum rule is shifted to the excitation energy region above 30 MeV by the RPA
tensor correlations. The contribution of the tensor terms to the energy
weighted sum rule is given analytically, and compared to the outcome of RPA.
11/2008;
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Physics Letters B 09/2008; 668:457-457. · 3.95 Impact Factor
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ABSTRACT: In the present volume, several lectures are devoted to the
implement- ations of the Density Functional Theory (DFT) in atomic nuclei,
either in the non-relativistic or covariant formalism. By restricting
ourselves to the non-relativistic case, in this contribution we deal
with two aspects of the theory: firstly, its time-dependent extension
and the study of the nuclear vibrational excitations, and secondly,
the use of effective interactions beyond the mean field framework.
Few recent applications, involving both stable and
unstable nuclei, are presented.
The European Physical Journal Special Topics 03/2008; 156(1):229-236. · 1.56 Impact Factor
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ABSTRACT: Microscopic calculations of nuclear states excited by means of charge-exchange reactions and involving spin and isospin degrees
of freedom, in particular, of the Gamow—Teller and the spin—dipole resonances, are discussed. The framework is a fully self-consistent
nonrelativistic spherical quasiparticle random-phase approximation constructed on top of the Hartree—Fock—Bardeen—Cooper—Schrieffer
approach. Our results are compared with available experimental data, and a critical discussion is attempted.
Physics of Atomic Nuclei 07/2007; 70(8):1415-1422. · 0.57 Impact Factor
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ABSTRACT: The role the exchange of phonons between pairs of fermions moving in time reversal states lying close to the Fermi energy on the superfluidity and on the superconductivity of atomic nuclei and fullerenes respectively is discussed in terms of mean field theory and of ab-initio calculations.
Physica Scripta 09/2006; 2000(T88):173. · 1.20 Impact Factor
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ABSTRACT: The bare nucleon–nucleon interaction is essential for the production of pair correlations in nuclei, but the induced interaction due to phonon exchange also contributes. In this paper, we shall present examples of the interplay between these two sources of pairing interaction in the case of finite nuclei and of the inner crust of neutron stars.
Physica Scripta 06/2006; 2006(T125):94. · 1.20 Impact Factor
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ABSTRACT: We calculate, for the first time, the state-dependent pairing gap of a finite nucleus (120Sn) diagonalizing the bare nucleon-nucleon potential (Argonne v
14) in a Hartree-Fock basis. The resulting gap accounts for about half of the experimental gap. Going beyond the mean field
in the particle-particle channel, the combined effect of the bare nucleon-nucleon potential and of the induced pairing interaction
arising from the exchange of low-lying surface vibrations between nucleons moving in time reversal states close to the Fermi
energy accounts for the experimental gap. Examples for light, halo nuclei are also reported. The more studied effects of the
particle-vibration coupling in the particle-hole channel are discussed for the low-lying quadrupole vibration in 120Sn and the giant dipole resonance in the unstable oxygen isotopes and 132Sn.
Physics of Atomic Nuclei 08/2004; 67(9):1640-1644. · 0.57 Impact Factor
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ABSTRACT: The study of the isoscalar giant monopole resonance (ISGMR) should allow extracting a value for the nuclear incompressibility
coefficient K
∞. In this contribution, we review the most recent attempts along this line. While the nonrelativistic (Skyrme, Gogny) models
predict K
∞ to be around 220–235 MeV, the values obtained from the relativistic calculations are significantly larger (250–270 MeV).
We argue that the most plausible reason for this discrepancy lies in the different behavior of the symmetry energy in the
two classes of models. We also discuss the role of the isoscalar giant dipole resonance (ISGDR). We conclude that a number
of experimental ambiguities still prevent us from deducing K
∞ from the ISGDR with a comparable accuracy as from the ISGMR.
Physics of Atomic Nuclei 01/2004; 67(9):1731-1736. · 0.57 Impact Factor
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ABSTRACT: If neutrons are progressively added to a normal nucleus, the Pauli principle forces them into states of higher momentum. When
the core becomes neutron saturated, the nucleus expels most of the wave function of the last neutrons outside to form a halo,
which, because of its large size, can have a lower momentum. It is an open question how nature stabilizes such a fragile system
and provides the glue needed to bind the halo neutrons to the core. Here, we show that this problem is similar to that of
the instability of the normal state of an electron system at zero temperature solved by Cooper, a solution which is at the
basis of BCS theory of superconductivity. By mimicking this approach using, aside from the bare nucleon-nucleon interaction,
the long wavelength vibrations of the nucleus 11Li, the paradigm of halo nuclei, as tailored glues of the least bound neutrons, we are able to obtain a unified and quantitative
picture of the observed properties of 11Li.
European Physical Journal A 09/2001; 11(4):385-392. · 2.19 Impact Factor
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ABSTRACT: In this paper we apply Nuclear Field Theory techniques to describe the 123Te K-electron capture, whose half-life is reported to be the longest one ever measured in a single decay. The theoretical half-life we obtain varies in the range (6.0 1019 – 8.9 1019)yr, which overestimates by a factor 3–4 the experimental one.
Czechoslovak Journal of Physics 03/2000; 50(4):509-517. · 0.42 Impact Factor
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ABSTRACT: The quadrupole strength function of $^{28}O$ is calculated making use of the SIII interaction, within the framework of continuum-RPA and taking into account collisions among the nucleons (doorway coupling). The centroid of the giant resonance is predicted at $\approx 14$ MeV, that is much below the energy expected for both isoscalar and isovector quadrupole resonances in nuclei along the stability valley. About half of this width arises from the coupling of the resonance to the continuum and about half is due to doorway coupling. This result is similar to that obtained in the study of giant resonances in light, $\beta$-stable nuclei, and shows the lack of basis for the expectation, entertained until now in the literature, that continuum decay was the main damping mechanism of giant resonances in halo nuclei. Comment: LaTeX file, 7 pages, figures not included but available if requested at colo@mi.infn.it, accepted for publication in Phys. Rev. C
09/1996;
