The origin of the spin-orbit splitting in some light nuclei using a modified interaction

The spin-orbit splittings in the spectra of nuclei with A = 5, 15, and 17 have been restudied, with a new set of matrix elements, within the framework of the shell model with core excitations. Investigated are the dependences of these splittings on core-excitation and on some of the different components of the nucleon- nucleon interaction. Our work confirms that the two-body spin-orbit interaction has a major effect whose magnitude depends on the size of the model space that is used. The effect of the tensor interaction is again seen to be considerably less important.     

Interplay of direct and spin-flip contributions in Gamow-Teller decay to odd-odd N = Z nuclei

Strengths of Gamow-Teller decays of T_z = ±1 nuclei to T_z = 0 odd-odd nuclei have been calculated by using spherical shell model and deformed Nilsson wave functions. The role and competition of the microscopic direct and spin-flip mechanisms generating Gamow-Teller transitions are analyzed. Analytical expressions derived for the B(GT) values give useful insight into the regularities of B(GT) values along the N = Z line. The crucial role of configuration mixing is discussed.     

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.     

Thomas-Fermi theory for atomic nuclei revisited

The recently developed semiclassical variational Wigner-Kirkwood (VWK) approach is applied to finite nuclei using external potentials and self-consistent mean fields derived from Skyrme interactions and from relativistic mean field theory. VWK consists of the Thomas-Fermi part plus a pure, perturbative ?² correction. In external potentials, VWK passes through the average of the quantal values of the accumulated level density and total energy as a function of the Fermi energy. However, there is a problem of overbinding when the energy per particle is displayed as a function of the particle number. The situation is analyzed comparing spherical and deformed harmonic oscillator potentials. In the self-consistent case, we show for Skyrme forces that VWK binding energies are very close to those obtained from extended Thomas-Fermi functionals of ?⁴ order, pointing to the rapid convergence of the VWK theory. This satisfying result, however, does not cure the overbinding problem, i.e., the semiclassical energies show more binding than they should. This feature is more pronounced in the case of Skyrme forces than with the relativistic mean field approach. However, even in the latter case the shell correction energy for e.g., ²⁰⁸Pb turns out to be only ∼−6 MeV what is about a factor two or three off the generally accepted value. As an ad hoc remedy, increasing the kinetic energy by 2.5%, leads to shell correction energies well acceptable throughout the periodic table. The general importance of the present studies for other finite Fermi systems, self-bound or in external potentials, is pointed out.     

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     

Helium-cluster decay widths of molecular states in beryllium and carbon isotopes

⁴He (i.e., α particle) and ⁶He emissions from possible molecular states in beryllium and carbon isotopes have been investigated using a mean-field-type cluster potential. Calculations can reasonably describe the α-decay widths of studied states in beryllium and carbon isotopes, and also ²⁰Ne, compared with experiments. For the nucleus ¹⁰Be, we discussed α-decay widths with different shapes or different decay modes, in order to understand the very different decay widths of two excited states. The widths of ⁶He decays from ¹²Be and α decays from ^13,14C are predicted, which could be useful for future experiments.     

Widths of fission and cluster decays of nuclei with regard to the channel coupling

Generalization of the R-matrix theory of nuclear reactions with the use of the method of projection operators has been performed to describe the fission and cluster decays of atomic nuclei with the emission of two or three fragments with successive quantum-mechanical consideration of the fission and decay channel coupling.     

The love-hate relationship between the shell model and cluster models

We adopt a personal approach here reviewing several calculations over the years in which we have experienced confrontations between cluster models and the shell model. In previous cluster conferences, we have noted that cluster models go hand in hand with Skyrme-Hartree-Fock calculations in describing states which cannot easily, if at all, be handled by the shell model. These are the highly deformed (many particle-many hole) intruder states, linear chain states, etc. In the present work, we will consider several topics: the quadrupole moment of ⁶Li; the nonexistence of low-lying intruders in ⁸Be; and then jumping to the f _7/2 shell, we discuss the two-faceted nature of the nuclei, which sometimes display shell-model properties and other times cluster properties.     

Supersymmetry for nuclear cluster systems

A supersymmetry scheme is proposed for nuclear cluster systems. The bosonic sector of the superalgebra describes the relative motion of the clusters, while its fermionic sector is associated with their internal structure. An example of core + α configurations is discussed in which the core is a p-shell nucleus and the underlying superalgebra is U(4|12). The α-cluster states of the nuclei ²⁰Ne and ¹⁹F are analyzed and correlations between their spectra, electric quadrupole transitions, and one-nucleon transfer reactions are interpreted in terms of U(4|12) supersymmetry. Received: 14 September 2001 / Accepted: 26 October 2001     

Second-forbidden beta-decay and the effect of (V+A)- andS-interaction admixtures:36Cl

The shapefactor, the logft-value, the electron longitudinal polarization and the distribution of electrons emitted from oriented nuclei of³⁶Cl have been calculated by applying two different nuclear models. Firstly a pure 1d_3/2→1d_3/2 transition has been considered. Secondly a more refined shell model of thes-d shell configuration space has been taken into account. It is shown that there is no great influence of the shell model version to the observables, that the observables depend strongly on the relativistic nuclear matrix elements, that the so-called “two parameter equation” for the shapefactor cannot be used in the case of³⁶Cl and that there is no larger sensitivity of (V+A) admixtures to the electron longitudinal polarization than normally found in allowed decays. Comparisons with the existing experimental data are made.     

The semiempirical molecular orbital calculation on [NiCl6]4− cluster

The parameter-free SCCC-MO calculation including interatomic — Coulomb and crystal field effects was performed for the ground and first excited state of [NiCl₆]^4– cluster. The two metal-ligand distances corresponding to LiCl and NaCl crystals have been explicitly considered. It was found that the used method gives the correct order of molecular energy levels but the calculated self-consistent charges strongly depend on the chosen form of population analysis. Further it was shown that all one-electron eigenvectors of the system undergo some changes during excitation and for that reason the contributions to 10Dq from all irreducible representations should be considered. Influence of the rest of NaCl-type crystals upon all calculated values was found to be negligible.     

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     

In-beam and decay spectroscopy of transfermium nuclei

In recent years the body of experimental data on nuclei with masses A?250 has increased dramatically. Nuclei that had been out of reach for experimental studies have now become available for study through a variety of approaches, both with in-beam spectroscopic methods and through spectroscopy following the decay of isomeric states or alpha decays at the focal plane of powerful separators. This article aims to collect the currently available experimental data on nuclei between Cm (Z=96) and Db (Z=105). The review of this data builds on the evaluations in the literature and focusses on those datasets obtained most recently.     

Molecular structure of nuclei

Cluster structures of nuclei are discussed, with emphasis on nuclear clustering in unstable nuclei. The subjects we discuss are alpha condensed states, clustering in Be and B isotopes, and clustering in ³²Mg and ³⁰Ne. The subject of alpha cluster condensation comes from the clustering nature of dilute nuclear matter. We discuss that recent heavy-ion central collision experiments give us nice evidence of the clustering in dilute nuclear matter. We then present a new prediction of the existence of the “alpha cluster condensed states” in the self-conjugate 4n nuclei around the breakup threshold energy into n alpha-particles. As for the clustering in neutron-rich Be, we discuss the comparison between the antisymmetrized molecular-dynamics results and the recent experimental data, which shows that the clustering feature manifests itself very clearly in neutron-rich Be isotopes both in the ground and excited states. Clustering in Be isotopes near neutron dripline is intimately related to the breaking of the neutron magic number N = 8. We report our recent study about the possible relationship between the clustering and the breaking of the neutron magic number N = 20 in ³²Mg and ³⁰Ne. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: horiuchi@ruby.scphys.kyoto-u.ac.jp     

Role of the cluster deformations in explaining the exotic decay half-lives

We investigate the influence of nuclear deformations of the cluster and daughter nuclei on the exotic cluster decay half-lives of $\ensuremath 221\leq A \leq 242$ for the favored cluster decay of the radioactive nuclei by using the semiclassical WKB method and the Bohr-Sommerfeld quantization condition. The results have also been presented for the spherical nuclei case in order to show clearly the effects of the deformations on the exotic decay half-lives. The half-lives become close to the experimental data when both the deformation of daughter and cluster nuclei are taken into account in the calculations. Furthermore, considering cluster deformations together with the orientation angles of daughter and cluster also provides positive contributions to the results.     

Unbound excited states in C

The neutron-rich carbon isotopes ^19,17C have been investigated via proton inelastic scattering on a liquid hydrogen target at 70 MeV/nucleon. The invariant mass method in inverse kinematics was employed to reconstruct the energy spectrum, in which fast neutrons and charged fragments were detected in coincidence using a neutron hodoscope and a dipole magnet system. A peak has been observed with an excitation energy of 1.46(10) MeV in ¹⁹C, while three peaks with energies of 2.20(3), 3.05(3), and 6.13(9) MeV have been observed in ¹⁷C. Deduced cross sections are compared with microscopic DWBA calculations based on p-sd   shell model wave functions and modern nucleon–nucleus optical potentials. Jπ$J^{\pi }$ assignments are made for the four observed states as well as the ground states of both nuclei.     

An application of the multichannel cluster model theory to the elastic proton-α scattering

In the framework of the cluster model reaction theory a calculation for the elastic α-p scattering below 30 MeV was performed. The nucleon-nucleon potential, which contains spin orbit and tensor forces, has been succesfully used in reaction calculations for other light nuclei. The calculated α-p cross section and polarization values are in quantitative agreement with experimental data.     

High spin states in48Cr

The self-consistent finite Fermi-system theory with three different parameter sets is used to calculate decay energies and Gamow-Teller strength distributions of neutronrich short-lived nuclides near shell closures Z=50, N=82. Among the parameters, the role of a strong surface neutron-proton attraction and an isovector spin-orbit force is discussed. The obtainedβ-decay half-lives are compared with large-scale calculation results and with available experimental data. Some predictions for the ground state properties of other unstable closed-shell nuclei are presented.