Fine structure of proton-neutron mixed symmetry states in some N=80 isotones

A microscopic multiphonon approach is adopted to investigate the structure of some low-lying states observed experimentally in the N = 80 isotones 134Xe, 136Ba, and 138Ce. The calculation yields levels and electromagnetic transition strengths in good agreement with experiments and relates the observed selection rules to the neutron proton symmetry and phonon content of the observed states. Moreover, it ascribes the splitting of theM1 strength in 138Ce to the proton subshell closure which magnifies the role of pairing in the excitation mechanism

Spurious finite-size instabilities in nuclear energy density functionals: spin channel

It has been recently shown, that some Skyrme functionals can lead to non-converging results in the calculation of some properties of atomic nuclei. A previous study has pointed out a possible link between these convergence problems and the appearance of finite-size instabilities in symmetric nuclear matter (SNM) around saturation density. We show that the finite-size instabilities not only affect the ground state properties of atomic nuclei, but they can also influence the calculations of vibrational excited states in finite nuclei. We perform systematic fully-self consistent Random Phase Approximation (RPA) calculations in spherical doubly-magic nuclei. We employ several Skyrme functionals and vary the isoscalar and isovector coupling constants of the time-odd term $\mathbf{s}\cdot \Delta \mathbf{s}$ . We determine critical values of these coupling constants beyond which the RPA calculations do not converge because RPA the stability matrix becomes non-positive.By comparing the RPA calculations of atomic nuclei with those performed for SNM we establish a correspondence between the critical densities in the infinite system and the critical coupling constants for which the RPA calculations do not converge. We find a quantitative stability criterion to detect finite-size instabilities related to the spin $\mathbf{s}\cdot \Delta \mathbf{s}$ term of a functional. This criterion could be easily implemented into the standard fitting protocols to fix the coupling constants of the Skyrme functional

A mean field study of single-particle spectra evolution in Z=14 and N=28 chains

We study the mechanisms which reduce the proton 1d(3/2)-1d(5/2) spin-orbit splitting and the neutron 1f(7/2) subshell closure in 42Si. We use various self-consistent mean field models: non-relativistic Skyrme-Hartree-Fock and relativistic density-dependent Hartree-Fock. Special attention is devoted to the influence of a tensor component in the effective interaction. It is found that the tensor force indeed governs the reduction of the 1d proton spin-orbit splitting. On the other hand, the reduction of the neutron 1f(7/2) subshell closure is not clearly related to the tensor force.Comment: Submited in PRC. 6 pages, 4 figures, 2 table

Shell evolution and nuclear forces

We present a quantitative study of the role played by different components characterizing the nucleon-nucleon interaction in the evolution of the nuclear shell structure. It is based on the spin-tensor decomposition of an effective two-body shell-model interaction and the subsequent study of effective single-particle energy variations in a series of isotopes or isotones. The technique allows to separate unambiguously contributions of the central, vector and tensor components of the realistic effective interaction. We show that while the global variation of the single-particle energies is due to the central component of the effective interaction, the characteristic behavior of spin-orbit partners, noticed recently, is mainly due to its tensor part. Based on the analysis of a well-fitted realistic interaction in sdpf-shell model space, we analyze in detail the role played by the different terms in the formation and/or disappearance of N=16, N=20 and N=28 shell gaps in neutron-rich nuclei.Comment: 6 pages, 4 figure

Etats individuels et excitations collectives dans une approche de champ moyen avec des forces de Skyrme

Le sujet de la thèse est une description théorique et self-consistante de propriétés de noyaux atomiques instables, ainsi que l étude de l influence de la structure en couches sur les états de symétrie mixte (ESM). Le but général est de rendre compte de façon microscopique de la structure et des excitations de noyaux atomiques que l on peut produire à présent auprès des installations faisceaux radioactifs (GANIL, GSI). L étude des ESM vise à préciser la relation entre la structure microscopique et la symétrie de F-spin dans les noyaux vibrationnels. La base de notre description est le modèle de Skyrme-Hartree-Fock avec un terme tenseur dans l interaction. Les calculs sont faits pour les chaînes isotopiques Z=14 et Z=50, et les chaînes isotoniques N=28 et N=82. Nous montrons que la composante tenseur de la force est déterminante pour l évolution de la fermeture de couche dans la chaîne Z=14, tandis qu elle ne joue qu un faible rôle dans la fermeture de couche à travers la chaîne N=28. Nous avons ensuite accompli une étude systématique des excitations dipolaires dites résonances Pygmées dans le cadre du modèle QRPA. Le phénomène de peau de neutrons a aussi été étudié dans les noyaux riches en neutrons. Dans la dernière partie de la thèse nous avons étudié pour la première fois les ESM dans les noyaux autour de N=80 en utilisant le modèle QPM. Nous avons réussi à décrire dans ce cadre les résultats expérimentaux récents obtenus sur le spectre à basse énergie du noyau 138Ce.Subject of the thesis is the theoretical description of effects in neutron rich nuclei, within a self-consistent approach, as well as the investigation of the influence of the shell structure on the Mixed Symmetry States. The scientific goal is, within a microscopic method, to describe the structure and the excitations of atomic nuclei, which are now available at the radioactive ion facilities at GANIL and GSI. The topic of the Mixed Symmetry States investigation is to specify the mechanism by which the microscopic structure influences the F-spin symmetry in vibrational nuclei. In the development of the thesis, a SHF model has been used. An additional tensor term has been explicitly included. Calculations for the isotopic chains Z=14 and Z=50 as well as for the isotonic chains N=28, 82 have been performed. It was shown that the tensor term is indeed crucial for the evolution of the shell gap with the mass number for Z=14. On the other hand, it was proved that the tensor term has no influence on the shell gap for N=28. By the means of QRPA, a systematical study of slightly collective excitations the so called Pygmy resonance has been done. Theoretically was investigated the appearance of a structure in neutron rich nuclei known as neutron skin . In the framework of QPM it was done a complete survey of MSS for nuclei in the N=80 region. For a first time a microscopical description of such structures in this region has been done. In the framework of this model a description of the recent experimental data on low-lying excitations in 138Ce has been done.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Polarization corrections to single-particle energies studied within the energy-density-functional and quasiparticle random-phase approximation approaches

Background: Models based on using perturbative polarization corrections and mean-field blocking approximation give conflicting results for masses of odd nuclei. Purpose: We systematically investigate the polarization and mean-field models, implemented within self-consistent approaches that use identical interactions and model spaces, to find reasons for the conflicts between them. Methods: For density-dependent interactions and with pairing correlations included, we derive and study links between the mean-field and polarization results obtained for energies of odd nuclei. We also identify and discuss differences between the polarization-correction and full particle-vibration-coupling (PVC) models. Numerical calculations are performed for the mean-field ground-state properties of deformed odd nuclei and then compared to the polarization corrections determined using the approach that conserves spherical symmetry. Results: We have identified and numerically evaluated self-interaction (SI) energies that are at the origin of different results obtained within the mean-field and polarization-correction approaches. Conclusions: Mean-field energies of odd nuclei are polluted by the SI energies, and this makes them different from those obtained using polarization-correction methods. A comparison of both approaches allows for the identification and determination of the SI terms, which then can be calculated and removed from the mean-field results, giving the self-interaction-free energies. The simplest deformed mean-field approach that does not break parity symmetry is unable to reproduce full PVC effects

Spectroscopic properties of nuclear skyrme energy density functionals.

We address the question of how to improve the agreement between theoretical nuclear single-particle energies (SPEs) and observations. Empirically, in doubly magic nuclei, the SPEs can be deduced from spectroscopic properties of odd nuclei that have one more or one less neutron or proton. Theoretically, bare SPEs, before being confronted with observations, must be corrected for the effects of the particle vibration coupling (PVC). In the present work, we determine the PVC corrections in a fully self-consistent way. Then, we adjust the SPEs, with PVC corrections included, to empirical data. In this way, the agreement with observations, on average, improves; nevertheless, large discrepancies still remain. We conclude that the main source of disagreement is still in the underlying mean fields, and not in including or neglecting the PVC corrections