The rapid shape change in Zr isotopes near neutron number N=60 is identified to be caused by type II shell evolution associated with massive proton excitations to its 0g_{9/2} orbit, and is shown to be a quantum phase transition. Monte Carlo shell-model calculations are carried out for Zr isotopes of N=50-70 with many configurations spanned by eight proton orbits and eight neutron orbits. Energy levels and B(E2) values are obtained within a single framework in good agreement with experiment, depicting various shapes in going from N=50 to 70. The novel coexistence of prolate and triaxial shapes is suggested.

We show how shape transitions in the neutron-rich exotic Si and S isotopes occur in terms of shell-model calculations with a newly constructed Hamiltonian based on VMU interaction. We first compare the calculated spectroscopic-strength distributions for the proton 0d 5/2,3/2 and 1s 1/2 orbitals with results extracted from a 48 Ca(e,e'p) experiment to show the importance of the tensorforce component of the Hamiltonian. Detailed calculations for the excitation energies, B(E2) and two-neutron separation energies for the Si and S isotopes show excellent agreement with experimental data. The potential energy surface exhibits rapid shape transitions along the isotopic chains towards N =28 that are different for Si and S. We explain the results in terms of an intuitive picture involving a Jahn-Teller-type effect that is sensitive to the tensor-force-driven shell evolution. The closed sub-shell nucleus 42 Si is a particularly good example of how the tensor-force-driven Jahn-Teller mechanism leads to a strong oblate rather than spherical shape.

The shapes of neutron-rich exotic Ni isotopes are studied. Large-scale shell model calculations are performed by advanced Monte Carlo Shell Model (MCSM) for the pf -g 9/2 -d 5/2 model space. Experimental energy levels are reproduced well by a single fixed Hamiltonian. Intrinsic shapes are analyzed for MCSM eigenstates. Intriguing interplays among spherical, oblate, prolate and γunstable shapes are seen, including shape fluctuations, E(5)-like situation, the magicity of doublymagic 56,68,78 Ni, and the coexistence of spherical and strongly deformed shapes. Regarding the last point, strong deformation and change of shell structure can take place simultaneously, being driven by the combination of the tensor force and changes of major configurations within the same nucleus.

No abstract

We present a newly enhanced version of the Monte Carlo Shell Model method by incorporating the conjugate gradient method and energy-variance extrapolation. This new method enables us to perform large-scale shell-model calculations that the direct diagonalization method cannot reach. This new generation framework of the MCSM provides us with a powerful tool to perform most-advanced large-scale shell-model calculations on current massively parallel computers such as the K computer. We discuss the validity of this method in ab initio calculations of light nuclei, and propose a new method to describe the intrinsic wave function in terms of the shell-model picture. We also apply this new MCSM to the study of neutron-rich Cr and Ni isotopes using the conventional shell-model calculations with an inert 40 Ca core and discuss how the magicity of N = 28, 40, 50 remains or is broken. * )

No abstract

The interacting boson model (IBM) Hamiltonian is determined microscopically for general cases of lowlying quadrupole collectivity. Under the assumption that the multinucleon-induced surface deformations, which reflect nuclear forces and the Pauli principle, can be simulated by bosons, the interaction strengths of the IBM Hamiltonian are derived by mapping the potential energy surface of the mean-field model with Skyrme force onto the corresponding one of the IBM. These interaction strengths turn out to change gradually as functions of valence nucleon numbers. The energy eigenvalues and the wave functions are calculated with the exact treatment of the particle number and the angular momentum. We demonstrate how well the method works by taking Sm isotopes as an example, where a typical spherical-deformed shape-phase transition is reproduced successfully. We show that the physically relevant IBM interaction strengths can be determined unambiguously by the use of wavelet analysis. In addition, by the diagonalization of the boson Hamiltonian, quantum-mechanical correlation effects can be included in the eigenenergies, by which the basic properties of these nuclei are properly reproduced. The present method is applied to several other isotopic chains, Ba, Xe, Ru, Pd, W, and Os, in comparison to the experimental data. We point out the relevance of our results to the recently proposed critical-point symmetries. The predicted spectra and the B(E2) ratios are presented for heavy neutron-rich exotic nuclei in experimentally unexplored regions such as the right-lower corner of 208 Pb on the nuclear chart.

We present the first application of the newly developed EKK theory of the effective nucleonnucleon interaction to shell-model studies of exotic nuclei, including those where conventional approaches with fitted interactions encounter difficulties. This EKK theory enables us to derive the interaction suitable for several major shells (sd+pf in this work). By using such an effective interaction obtained from the Entem-Machleidt QCD-based χN 3 LO interaction and the Fujita-Miyazawa three-body force, the energies, E2 properties and spectroscopic factors of low-lying states of neutronrich Ne, Mg and Si isotopes are nicely described, as the first shell-model description of the "island of inversion" without fit of the interaction. The long-standing question as to how particle-hole excitations occur across the sd-pf magic gap is clarified with distinct differences from the conventional approaches. The shell evolution is shown to appear similarly to earlier studies. Introduction. -The nuclear shell model [1, 2] provides a unified and successful description of both stable and exotic nuclei, as a many-body framework which can be related directly to nuclear forces. Exotic nuclei are located far from the β-stability line on the Segrè chart, exhibiting very short life times, mainly due to an unbalanced ratio of proton (Z) and neutron (N ) numbers. Exotic nuclei differ remarkably in some other aspects from their stable counterparts, providing us with new insights in understanding atomic nuclei and nuclear forces [3][4][5]. As experimental data on exotic nuclei are, in general, less abundant compared to stable nuclei, theoretical calculations, interpretations and predictions play an ever increasing role.Shell-model (SM) calculations handle the nuclear forces in terms of two-body matrix elements (TBMEs). In the early days, TBMEs were empirically determined in order to reproduce certain observables. A well-known example is the effective interaction for p-shell nuclei by Cohen and Kurath [6]. A breakthrough towards more microscopically-derived TBMEs was achieved by Kuo and Brown for sd-shell nuclei [7]. Although basic features of the nucleon-nucleon (N N ) force for the SM calculation are included in these effective interactions, empirical adjustments of TBMEs were needed in order to reproduce various observables [8][9][10].These effective interactions were all derived for a Hilbert space represented by the degrees of freedom of one major (oscillator) shell. As we move towards ex-