Capture Gamma-Ray Spectroscopy and Related Topics, 2013
ABSTRACT We have revised an equation of motion method for generating a basis of microscopic multi... more ABSTRACT We have revised an equation of motion method for generating a basis of microscopic multiphonon states built of Tamm-Dancoff phonons. This new version removes the inconsistencies of the previous formulation and yields eigenvalue equations of simple structure. To test the method we have computed the electric dipole response of the neutron rich 20O in a space extending up to the three-phonon subspace.
The method outlined here consists in generating a multiphonon basis by constructing and solving i... more The method outlined here consists in generating a multiphonon basis by constructing and solving iteratively a set of equations of motion within a subspace spanned by states which are tensor products of n Tamm Dancoff phonons. In such a basis, the Hamiltonian takes on a simple form and can be easily diagonalized. For illustrative purposes, the method is applied to
The collective properties of excited superdeformed bands have been investigated in the framework ... more The collective properties of excited superdeformed bands have been investigated in the framework of self-consistent cranked Nilsson plus quasiparticle random-phase approximation. The expected octupole nature of some bands observed recently in some nuclei has been confirmed by a comparative analysis of their E1 decays to the yrast band and of the anomalous behavior of their dynamical moment of inertia. It
A method which generates a multiphonon basis through the construction and the iterative solution ... more A method which generates a multiphonon basis through the construction and the iterative solution of a set of equations of motion is adopted to investigate the negative parity spectrum and the structure of the giant dipole resonance in 16O.
Key Topics in Nuclear Structure - Proceedings of the 8th International Spring Seminar on Nuclear Physics, 2005
We show that a recently developed algorithm for generating a subset of eigenvalues and eigenvecto... more We show that a recently developed algorithm for generating a subset of eigenvalues and eigenvectors of large matrices may be used for an efficient definition of an importance sampling of the shell-model basis. The sampling so implemented allows for substantial reductions of configuration spaces, and, also, for extrapolations to exact eigenvalues and E2 strengths.
Challenges of Nuclear Structure - Proceedings of the 7th International Spring Seminar on Nuclear Physics, 2002
We investigate the properties of magnetic dipole and quadrupole excitations recently discovered i... more We investigate the properties of magnetic dipole and quadrupole excitations recently discovered in spherical and deformed heavy nuclei within a microscopic multiphonon scheme.
ABSTRACT Tamm-Dancoff and random-phase approximations are formulated in the canonical Hartree-Foc... more ABSTRACT Tamm-Dancoff and random-phase approximations are formulated in the canonical Hartree-Fock-Bogoliubov quasi-particle basis and adopted to compute the E1 response in Ca isotopes using an intrinsic Hamiltonian composed of a Vlowk potential, deduced from the CD-Bonn nucleon-nucleon interaction, corrected with phenomenological density dependent and spin-orbit terms. Attention is focused on the evolution of the dipole strength distribution, including the low-lying transitions associated to the pygmy resonance, in going from the N = Z 40Ca to Ca isotopes with neutron excess.
ABSTRACT A phenomenological analysis of low-lying positive and negative-parity bands of some heav... more ABSTRACT A phenomenological analysis of low-lying positive and negative-parity bands of some heavy deformed nuclei is carried out within a coherent-state model by making use of an interacting boson Hamiltonian where quadrupole bosons associated to the nucleon motion are coupled to scalar and dipole bosons introduced to describe the motion of α-clusters with respect to a deformed nuclear core. The deformation is induced by the use of an intrinsic ground state which is coherent in the quadrupole bosons only. The physical states are generated through angular-momentum projection from intrinsic wavefunctions obtained by the action of dipole operators of the simplest form on the coherent intrinsic ground state. Energy spectra and electromagnetic transition probabilities are calculated and compared to experiments. The good overall agreement between predictions and experiments is in support of the important role played by α-clustering in deformed heavy nuclei.
The extent of validity of schematic models in describing low lying magnetic dipole excitations kn... more The extent of validity of schematic models in describing low lying magnetic dipole excitations known as scissors mode is studied from a quite general microscopic point of view. Schematic RPA, taken as an illustrative example, is shown to be consistent with other schematic descriptions even if a sizeable part of the M1 strength induced by correlations of scissors type is predicted to go to the high energy Kpi = 1+ electric giant quadrupole mode. The general microscopic scheme is used as a guideline for a correct interpretation of the results obtained in realistic calculations. It is argued in particular that some recent RPA results are consistent with the scissors picture of the low energy mode.
ABSTRACT A self-consistent calculation is performed within an equation of motion method to study ... more ABSTRACT A self-consistent calculation is performed within an equation of motion method to study the electric dipole response in the neutron rich $^{132}\mathrm{Sn}$ with special attention at the low-energy excitations associated with the pygmy dipole resonance. A basis of one- plus two-phonon states is constructed by solving iteratively a set of equations of motion and adopted to bring the nuclear Hamiltonian to diagonal form. The phonons are generated in a Tamm-Dancoff approximation using a Hartree-Fock basis. The Hamiltonian is composed of an intrinsic kinetic term and an optimized two-body chiral potential. If complemented with a weak phenomenological density dependent term, such a potential improves substantially the description of the dipole response as compared to other realistic interactions. The two-phonon states play an important role in determining the fine structure of the giant dipole resonance and in enhancing the fragmentation of the strength at low energy, in fair agreement with the available data. Insights into the nature of the excitations building up the pygmy resonance are gained from the analysis of the phonon composition and shell structure of the states.
Collective Motion and Phase Transitions in Nuclear Systems - Proceedings of the Predeal International Summer School in Nuclear Physics, 2007
We outline two methods for facing the nuclear eigenvalue problem, one suitable for low-lying spec... more We outline two methods for facing the nuclear eigenvalue problem, one suitable for low-lying spectroscopy and the other for describing collective modes. The first is an an important sampling algorithm which generates a subset of exact eigensolutions of the nuclear shell model Hamiltonian within a truncated space. The other is an equation of motion method which generates iteratively a microscopic
The low-lying 0+ states observed in large abundance in several deformed nuclei are investigated s... more The low-lying 0+ states observed in large abundance in several deformed nuclei are investigated systematically within the microscopic quasiparticle-phonon model. Attention is paid at their quadrupole and pairing collective properties.
Capture Gamma-Ray Spectroscopy and Related Topics, 2013
ABSTRACT We have revised an equation of motion method for generating a basis of microscopic multi... more ABSTRACT We have revised an equation of motion method for generating a basis of microscopic multiphonon states built of Tamm-Dancoff phonons. This new version removes the inconsistencies of the previous formulation and yields eigenvalue equations of simple structure. To test the method we have computed the electric dipole response of the neutron rich 20O in a space extending up to the three-phonon subspace.
The method outlined here consists in generating a multiphonon basis by constructing and solving i... more The method outlined here consists in generating a multiphonon basis by constructing and solving iteratively a set of equations of motion within a subspace spanned by states which are tensor products of n Tamm Dancoff phonons. In such a basis, the Hamiltonian takes on a simple form and can be easily diagonalized. For illustrative purposes, the method is applied to
The collective properties of excited superdeformed bands have been investigated in the framework ... more The collective properties of excited superdeformed bands have been investigated in the framework of self-consistent cranked Nilsson plus quasiparticle random-phase approximation. The expected octupole nature of some bands observed recently in some nuclei has been confirmed by a comparative analysis of their E1 decays to the yrast band and of the anomalous behavior of their dynamical moment of inertia. It
A method which generates a multiphonon basis through the construction and the iterative solution ... more A method which generates a multiphonon basis through the construction and the iterative solution of a set of equations of motion is adopted to investigate the negative parity spectrum and the structure of the giant dipole resonance in 16O.
Key Topics in Nuclear Structure - Proceedings of the 8th International Spring Seminar on Nuclear Physics, 2005
We show that a recently developed algorithm for generating a subset of eigenvalues and eigenvecto... more We show that a recently developed algorithm for generating a subset of eigenvalues and eigenvectors of large matrices may be used for an efficient definition of an importance sampling of the shell-model basis. The sampling so implemented allows for substantial reductions of configuration spaces, and, also, for extrapolations to exact eigenvalues and E2 strengths.
Challenges of Nuclear Structure - Proceedings of the 7th International Spring Seminar on Nuclear Physics, 2002
We investigate the properties of magnetic dipole and quadrupole excitations recently discovered i... more We investigate the properties of magnetic dipole and quadrupole excitations recently discovered in spherical and deformed heavy nuclei within a microscopic multiphonon scheme.
ABSTRACT Tamm-Dancoff and random-phase approximations are formulated in the canonical Hartree-Foc... more ABSTRACT Tamm-Dancoff and random-phase approximations are formulated in the canonical Hartree-Fock-Bogoliubov quasi-particle basis and adopted to compute the E1 response in Ca isotopes using an intrinsic Hamiltonian composed of a Vlowk potential, deduced from the CD-Bonn nucleon-nucleon interaction, corrected with phenomenological density dependent and spin-orbit terms. Attention is focused on the evolution of the dipole strength distribution, including the low-lying transitions associated to the pygmy resonance, in going from the N = Z 40Ca to Ca isotopes with neutron excess.
ABSTRACT A phenomenological analysis of low-lying positive and negative-parity bands of some heav... more ABSTRACT A phenomenological analysis of low-lying positive and negative-parity bands of some heavy deformed nuclei is carried out within a coherent-state model by making use of an interacting boson Hamiltonian where quadrupole bosons associated to the nucleon motion are coupled to scalar and dipole bosons introduced to describe the motion of α-clusters with respect to a deformed nuclear core. The deformation is induced by the use of an intrinsic ground state which is coherent in the quadrupole bosons only. The physical states are generated through angular-momentum projection from intrinsic wavefunctions obtained by the action of dipole operators of the simplest form on the coherent intrinsic ground state. Energy spectra and electromagnetic transition probabilities are calculated and compared to experiments. The good overall agreement between predictions and experiments is in support of the important role played by α-clustering in deformed heavy nuclei.
The extent of validity of schematic models in describing low lying magnetic dipole excitations kn... more The extent of validity of schematic models in describing low lying magnetic dipole excitations known as scissors mode is studied from a quite general microscopic point of view. Schematic RPA, taken as an illustrative example, is shown to be consistent with other schematic descriptions even if a sizeable part of the M1 strength induced by correlations of scissors type is predicted to go to the high energy Kpi = 1+ electric giant quadrupole mode. The general microscopic scheme is used as a guideline for a correct interpretation of the results obtained in realistic calculations. It is argued in particular that some recent RPA results are consistent with the scissors picture of the low energy mode.
ABSTRACT A self-consistent calculation is performed within an equation of motion method to study ... more ABSTRACT A self-consistent calculation is performed within an equation of motion method to study the electric dipole response in the neutron rich $^{132}\mathrm{Sn}$ with special attention at the low-energy excitations associated with the pygmy dipole resonance. A basis of one- plus two-phonon states is constructed by solving iteratively a set of equations of motion and adopted to bring the nuclear Hamiltonian to diagonal form. The phonons are generated in a Tamm-Dancoff approximation using a Hartree-Fock basis. The Hamiltonian is composed of an intrinsic kinetic term and an optimized two-body chiral potential. If complemented with a weak phenomenological density dependent term, such a potential improves substantially the description of the dipole response as compared to other realistic interactions. The two-phonon states play an important role in determining the fine structure of the giant dipole resonance and in enhancing the fragmentation of the strength at low energy, in fair agreement with the available data. Insights into the nature of the excitations building up the pygmy resonance are gained from the analysis of the phonon composition and shell structure of the states.
Collective Motion and Phase Transitions in Nuclear Systems - Proceedings of the Predeal International Summer School in Nuclear Physics, 2007
We outline two methods for facing the nuclear eigenvalue problem, one suitable for low-lying spec... more We outline two methods for facing the nuclear eigenvalue problem, one suitable for low-lying spectroscopy and the other for describing collective modes. The first is an an important sampling algorithm which generates a subset of exact eigensolutions of the nuclear shell model Hamiltonian within a truncated space. The other is an equation of motion method which generates iteratively a microscopic
The low-lying 0+ states observed in large abundance in several deformed nuclei are investigated s... more The low-lying 0+ states observed in large abundance in several deformed nuclei are investigated systematically within the microscopic quasiparticle-phonon model. Attention is paid at their quadrupole and pairing collective properties.
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