Shape coexistence in nuclei appears to be unique in the realm of finite many-body quantum systems It differs from the various geometrical arrangements that sometimes occur in a molecule in that in a molecule the various arrangements are of the widely separated atomic nuclei In nuclei the various ''arrangements'' of nucleons involve (sets of) energy eigenstates with different electric quadrupole properties such as moments and transition rates, and different distributions of proton pairs and neutron pairs with respect to their Fermi energies Sometimes two such structures will ''invert'' as a function of the nucleon number, resulting in a sudden and dramatic change in ground-state properties in neighboring isotopes and isotones In the first part of this review the theoretical status of coexistence in nuclei is summarized Two approaches, namely, microscopic shell-model descriptions and mean-field descriptions, are emphasized The second part of this review presents systematic data, for both even- and odd-mass nuclei, selected to illustrate the various ways in which coexistence is observed in nuclei The last part of this review looks to future developments and the issue of the universality of coexistence in nuclei Surprises continue to be discovered With the major advances in reaching to extremes of proton-neutronmore » number, and the anticipated new ''rare isotope beam'' facilities, guidelines for search and discovery are discussed« less

Shape coexistence in atomic nuclei

In the present study we generalize the self-consistent Hartree-Fock-Bogoliubov (HFB) theory formulated in the coordinate space to the case which incorporates an arbitrary mixing between protons and neutrons in the particle-hole $(p\text{\ensuremath{-}}h)$ and particle-particle ($p\text{\ensuremath{-}}p$ or pairing) channels. We define the HFB density matrices, discuss their spin-isospin structure, and construct the most general energy-density functional that is quadratic in local densities. The consequences of the local gauge invariance are discussed and the particular case of the Skyrme energy-density functional is studied. By varying the total energy with respect to the density matrices the self-consistent one-body HFB Hamiltonian is obtained and the structure of the resulting mean fields is shown. The consequences of the time-reversal symmetry, charge invariance, and proton-neutron symmetry are summarized. The complete list of expressions required to calculate total energy is presented.

Local density approximation for proton-neutron pairing correlations: Formalism

The article reviews the general version of the Bohr collective model for the description of quadrupole collective states, including a detailed discussion of the model's kinematics. The quadrupole coordinates, momenta and angular momenta are defined and the structure of the isotropic tensor fields as functions of the tensor variables is investigated. After a comprehensive discussion of the quadrupole kinematics, the general form of the classical and quantum Bohr Hamiltonian is presented. The electric and magnetic multipole moment operators acting in the collective space are constructed and the collective sum rules are given. A discussion of the tensor structure of the collective wavefunctions and a review of various methods of solving the Bohr Hamiltonian eigenvalue equation are also presented. Next, the methods of derivation of the classical and quantum Bohr Hamiltonian from the microscopic many-body theory are recalled. Finally, the microscopic approach to the Bohr Hamiltonian is applied to interpret collective properties of 12 heavy even–even nuclei in the Hf–Hg region. Calculated energy levels and E2 transition probabilities are compared with experimental data.

https://iopscience.iop.org/article/10.1088/0954-3899/36/12/123101/pdf

Quadrupole collective states within the Bohr collective Hamiltonian

The interacting boson model (IBM) Hamiltonian is determined microscopically for general cases of low-lying 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 $B(\mathrm{E}2)$ ratios are presented for heavy neutron-rich exotic nuclei in experimentally unexplored regions such as the right-lower corner of $^{208}\mathrm{Pb}$ on the nuclear chart.

Formulating the interacting boson model by mean-field methods

Nuclear Data Sheets for A = 96

Collective quadrupole excitations in the 50 < Z, N < 82 nuclei with the general Bohr Hamiltonian

Experimental and Theoretical Investigations of Quadrupole Collective Degrees of Freedom in 104Ru

The generalized Bohr Hamiltonian is applied to a description of low-lying collective excitations in even-even isotopes of Te, Xe, Ba, Ce, Nd and Sm. The collective potential and inertial functions are determined by means of the Strutinsky method and the cranking model, respectively. A shell-dependent parametrization of the Nilsson potential is used. An approximate particle-number projection is performed in treatment of pairing correlations. The effect of coupling with the pairing vibrations is taken into account approximately when determining the inertial functions. The calculation does not contain any free parameter.

Collective quadrupole excitations in the 50<Z,N<82 nuclei with the generalized Bohr Hamiltonian

The low-lying quadrupole collective excitations of Ru and Pd isotopes

Quadrupole excitations of even-even Ru and Pd isotopes are described within microscopic approach based on the general collective Bohr model which includes the effect of coupling with the pairing vibrations. The excitation energies and E2 transition probabilities observed in 104-114Ru and 106-110Pd are reproduced in the frame of the calculation containing no free parameters.

K. Zajac

Papers

Collective quadrupole excitations in the 50 < Z, N < 82 nuclei with the general Bohr Hamiltonian

Experimental and Theoretical Investigations of Quadrupole Collective Degrees of Freedom in 104Ru

Collective quadrupole excitations in the 50<Z,N<82 nuclei with the generalized Bohr Hamiltonian

The low-lying quadrupole collective excitations of Ru and Pd isotopes

The low-lying quadrupole collective excitations of Ru and Pd isotopes