Properties unique in nuclei far from β stability line

Nucleons with very small binding energies present in nuclei far from the β stability line produce a unique shell structure, which leads to the disappearance of traditional magic numbers or to the creation of new magic numbers and new deformation regions. We study the shell structure in terms of the variation of two important ingredients, the kinetic energy and the spin-orbit splitting, as a function of the orbital angular momentum ℓ, when binding energies of neutrons decrease towards zero. It is also shown that for low-lying threshold strength, a negative sign is possible for the polarization charge coming from the coupling of one-particle to isoscalar shape oscillations. Received: 1 May 2001 / Accepted: 4 December 2001     

Three-Dimensional Angular Momentum Projected Relativistic Point-Coupling Approach for Low-Lying Excited States in 24Mg

A tull three-dimensional angular momentum projection on top ot a triaxial relativistic mean-fieid calculation is implemented for the first time. The underlying Lagrangian is a point coupling model and pairing correlations are taken into account by a monopole force. This method is applied for the low-lying excited states in ^24Mg. Good agreement with the experimental data is found for the ground state properties. A minimum in the potential energy surface for the 22^＋ state, with β≈ 0.55,7 γ≈ 10^o, is used as the basis to investigate the rotational energy spectrum as well as the corresponding B（E2） transition probabilities as compared to the available data.     

Low-energy monopole and dipole response in nuclei at finite temperature

The multipole response of nuclei at temperatures T=0–2 MeV$T=0\text{–}2\text{MeV}$ is studied using a self-consistent finite-temperature RPA (random phase approximation) based on relativistic energy density functionals. Illustrative calculations are performed for the isoscalar monopole and isovector dipole modes and, in particular, the evolution of low-energy excitations with temperature is analyzed, including the modification of pygmy structures. Both for the monopole and dipole modes, in the temperature range T=1–2 MeV$T=1\text{–}2\text{MeV}$ additional transition strength appears at low energies due to thermal unblocking of single-particle orbitals close to the Fermi level. A concentration of dipole strength around 10 MeV excitation energy is predicted in ^60,62Ni. The principal effect of finite temperature on low-energy strength that is already present at zero temperature, e.g. in ⁶⁸Ni and ¹³²Sn, is the spreading of this structure to even lower energy and the appearance of states that correspond to thermally unblocked transitions.     

The multilevel pairing Hamiltonian versus the degenerate case

We study the pairing Hamiltonian in a set of non-degenerate levels. First, we review in the path integral framework the spontaneous breaking of the U(1) symmetry occurring in such a system for the degenerate situation. Then the behaviors with the coupling constant of the ground state energy in the multilevel and in the degenerate case are compared. Next we discuss, in the multilevel case, an exact strong coupling expansion for the ground state energy which introduces the moments of the single particle level distribution. The domain of validity of the expansion, which is known in the macroscopic limit, is explored for finite systems and its implications for the energy of the latter is discussed. Finally the seniority and Gaudin excitations of the pairing Hamiltonian are addressed and shown to display the same gap in leading order.     

Realistic nuclear Hamiltonian: Ab exitu approach

Fully-microscopic no-core shell model (NCSM) calculations of all stable s and p shell nuclei are used to determine a realistic NN   interaction, JISP16, describing not only the two-nucleon data but the binding energies and spectra of nuclei with A?16$A?16$ as well. The JISP16 interaction, providing rapid convergence of the NCSM calculations, is obtained in an ab exitu approach by phase-equivalent transformations of the JISP6 NN interaction.     

Evolution of the N = 20 and N = 28 shell closures in neutron-rich nuclei

The isospin dependence of shell closure phenomena is studied for light neutron-rich nuclei within a microscopic self-consistent approach using the Gogny force. Introducing configuration mixing, ³²Mg is found to be dynamically deformed, although the N = 20 spherical shell closure persists at the mean-field level for all N = 20 isotones. In contrast, the N = 28 spherical shell closure is found to disappear for N - Z≥ 10 whereas deformed shell closures are preserved and lead to shape coexistence in ⁴⁴ S. Configuration mixing shows that the ground state of this nucleus is triaxially deformed. The first 2⁺ excitation energy E_x = 1.46 MeV and the reduced transition probability B(E2;0⁺ _gs→ 2⁺ ₁)= 420 e ² fm ⁴ obtained with our approach are in good agreement with experimental data. Received: 26 July 2000 / Accepted: 30 August 2000     

Momentum distribution of relativistic nuclei with Hartree-Fock mesonic correlations

The impact of Hartree-Fock correlations on the nuclear momentum distribution is studied in a fully relativistic one-boson-exchange model. Hartree-Fock equations are exactly solved to first order in the coupling constants. The renormalization of the Dirac spinors in the medium is shown to affect the momentum distribution, as opposed to what happens in the non-relativistic case. The unitarity of the model is shown to be preserved by the present renormalization procedure. Received: 7 June 2002 / Accepted: 24 September 2002 / Published online: 10 December 2002 RID="a" ID="a"e-mail: juan@nucle.us.es Communicated by G. Orlandini     

Pseudo-spin as a relativistic symmetry

The existence of broken pseudo-spin symmetry in the Pb nucleus has been studied in the relativistic mean field approach using realistic Lagrangian parameters. Its relationship to spin orbit splitting and the vanishingly small surface delta character of the mean spin orbit potential are investigated. In the ²⁰⁸Pb nucleus the broken pseudo-spin doublets are found to exist above the neutron (proton) Fermi surfaces. Received: 16 April 1998 / Revised version: 26 June 1998     

On the difference between proton and neutron spin-orbit splittings in nuclei

The latest experimental data on nuclei at ¹³²Sn permit us for the first time to determine the spin-orbit splittings of neutrons and protons in identical orbits in this neutron-rich doubly magic region and compare the case to that of ²⁰⁸Pb. Using the new results, which are now consistent for the two neutron-rich doubly magic regions, a theoretical analysis defines the isotopic dependence of the mean-field spin-orbit potential and leads to a simple explicit expression for the difference between the spin-orbit splittings of neutrons and protons. The isotopic dependence is explained in the framework of different theoretical approaches. Received: 13 February 2002 / Accepted: 20 February 2002     

Decoupling and coherence in E1 and E2 moments in drip line nuclei

I discuss first the effect of decoupling of extended wave functions and the coherence in the low-energy E1 strength in drip line nuclei ¹²Be and ¹³O, which are studied by large-scale shell model calculations including 3 ?ω configuration space. The calculated results are compared to recent experimental data of Coulomb excitations. The quenching of the core polarization charges in drip line nuclei is also discussed in relation to recent observations of quadrupole moments in B-isotopes. Received: 1 May 2001 / Accepted: 4 December 2001     

Spectroscopy of nuclei far from stability at GANIL: Recent experiments

The structure of neutron-rich light nuclei around N = 20, 28 has been investigated at GANIL by means of in-beam gamma spectroscopy using fragmentation reactions of ³⁶S and ⁴⁸Ca beams on a Be target. Gamma decay of relatively high-lying excited states have been measured for the first time in nuclei around ³²Mg and ⁴⁴S. Level schemes are proposed and discussed for a large number of these neutron-rich nuclei around N = 20 and N = 28. Received: 1 May 2001 / Accepted: 4 December 2001     

Masses of atomic nuclei far from stability

It is shown that a new way of applying a set of nuclear-mass relations among neighbouring nuclei, known as the Garvey–Kelson (GK) relations, holds exceptionally well for all currently measured masses. It is then demonstrated that these relationships are not adequately fulfilled for the best-known procedures to predict unknown masses. This suggests that these models may be optimized by constraining them to satisfy the (generalized) GK equations.     

Generalized hybrid derivative coupling model for finite nuclei

The generalized hybrid derivative coupling model has been applied to explore various ground state properties of different nuclei. In this work we have confined our calculation only to the model characterized by the hybridization parameter α = 1/4 which gives better results than the other models of the same class, as we have seen earlier, for nuclear matter calculations. The binding energy, single-particle energy spectra, density and charge radii of different doubly closed nuclei like ¹⁶O, ⁴⁰Ca, ⁴⁸Ca, ⁹⁰Zr, ¹³²Sn, ²⁰⁸Pb have been studied. The success of this model, in describing the doubly closed nuclei, motivates us to extend this calculation further in the case of open shell nuclei after incorporating the pairing interaction and using a BCS transformation. We have calculated the binding energy for such nuclei. We have also studied the isotopic shift for different Pb isotopes with respect to ²⁰⁸Pb. We have compared our results with the other standard theoretical results as well as with the experimental values. Received: 18 August 2000 / Accepted: 13 April 2001     

Superheavy nuclei in deformed mean--field calculations

The ground–state properties of superheavy nuclei are investigated within various parametrisations of relativistic and nonrelativistic nuclear mean–field models. The heaviest known even–even nuclei starting with Z = 98 are used as a benchmark to estimate the predictive power of the models and forces. From that starting point, deformed doubly magic nuclei are searched in the region 100 ≤ Z ≤ 130 and 142 ≤ N ≤ 190. Received: 6 April 1998 / Revised version: 16 June 1998     

Unrestricted symmetry-projected Hartree-Fock-Bogoliubov calculations for SD-shell nuclei

Abstact: The solution of the Hartree–Fock–Bogoliubov problem with restoration of the broken symmetries before the variation has been generalized for the use of totally unrestricted quasi–particle determinants. With this method doubly–even, doubly–odd and odd nuclei can be treated on the same footing. Comparison with the results of complete shell–model diagonalizations shows that already one–determinant representations yield a very good approximation to the exact solutions even in the middle of the 1s0d shell. The problem is especially suited for numerical implementation on parallel computers. First tests show a linear dependence of the inverse CPU time with the number of processors used. Received: 26 February 1998 / Revised version: 8 May 1998     

Neutron matter with a model interaction

An infinite system of neutrons interacting by a model pair potential is considered. We investigate a case when this potential is sufficiently strong attractive, so that its scattering length a tends to infinity, a . It appeared, that if the structure of the potential is simple enough, including no finite parameters, reliable evidences can be presented that such a system is completely unstable at any finite density. The incompressibility as a function of the density is negative, reaching zero value when the density tends to zero. If the potential contains a sufficiently strong repulsive core then the system possesses an equilibrium density. The main features of a theory describing such systems are considered. Received: 17 December 1999 / Accepted: 23 December 1999