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

The experimental and theoretical evidence for intrinsic reflection-asymmetric shapes in nuclei is reviewed. The theoretical methods discussed cover a wide spectrum, from mean-field theory and its extensions to algebraic and cluster approaches. The experimental data for nuclear ground states and at low and high spin, cited as evidence for reflection asymmetry, are collected and categorized. The extensive data on electric dipole transition moments and their theoretical interpretation are surveyed, along with available data on electric octupole moments. The evidence for reflection-asymmetric molecular states in light nuclei is summarized. The application of reflection-asymmetric theories to descriptions of the fission barrier, bimodal fission, superdeformation, and hyperdeformations is reviewed, and some other perspectives in the wider context of nuclear physics are also given. [S0034-6861(96)00102-X]

Intrinsic reflection asymmetry in atomic nuclei

The concept of spontaneous symmetry breaking is applied to the rotating mean field of nuclei. The description is based on the tilted-axis cranking model, which takes into account that the rotational axis can take any orientation with respect to the deformed density distribution. The appearance of rotational bands in nuclei is analyzed, focusing on weakly deformed nuclei at high angular momentum. The quantization of the angular momentum of the valence nucleons leads to new phenomena. Magnetic rotation represents the quantized rotation of the anisotropic current distribution in a near spherical nucleus. The restricted amount of angular momentum of the valence particles causes band termination. The discrete symmetries of the mean-field Hamiltonian provide a classification scheme of rotational bands. New symmetries result from the combination of the spatial symmetries of the density distribution with the vector of the angular momentum. The author discusses in detail which symmetries appear for a reflection-symmetric density distribution and how they show up in the properties of the rotational bands. In particular, the consequences of rotation about a nonprincipal axis and of breaking the chiral symmetry are analyzed. Also discussed are which symmetries and band structures appear for non-reflection-symmetric mean fields. The consequences of breaking the symmetry with respect to gauge and isospin rotations are sketched. Some analogies outside nuclear physics are mentioned. The application of symmetry-restoring methods to states with large angular momentum is reviewed.

Spontaneous symmetry breaking in rotating nuclei

Investigation of nuclear structure by resonance fluorescence scattering

Hidden pseudospin and spin symmetries and their origins in atomic nuclei

Two-Phonon Octupole Excitation in 147 Gd

High-spin spectroscopy of the 142Eu, 143Eu and 144Eu nuclei

Connections between the superdeformed and normal-deformed states in $^{143}\mathrm{Eu}$ have been established by measuring energy sums of two consecutive \ensuremath{\gamma}-ray transitions in the linking cascade. Six decay paths have been observed connecting discrete states in the two potential wells. The absolute excitation energy and the most likely spin of the superdeformed band have been determined directly for the first time.

Linking transitions from the superdeformed band in 143Eu.

Proton Single-Particle States above Z=64

Prompt and delayed γ-rays have been observed from very neutron deficient 192–195Po nuclei by using the recoil-decay tagging (RDT) and recoil gating techniques The yrast levels up to the (10+) state in the 192Po were identified for the first time Comprehensive data for 194Po rendered it possible to extend the yrast line and to observe several positive and negative parity non-yrast states In the odd-mass isotopes 193Po and 195Po, favoured and unfavoured states on top of the 13/2+ state have been identified The results are discussed within the simple vibrator and rotor pictures as well as in the framework of coexisting spherical and deformed-intruder structures

M. Piiparinen

Papers

Two-Phonon Octupole Excitation in 147 Gd

High-spin spectroscopy of the 142Eu, 143Eu and 144Eu nuclei

Linking transitions from the superdeformed band in 143Eu.

Proton Single-Particle States above Z=64

Gamma-ray spectroscopy of 192–195 Po