Search

Shape coexistence in doubly even nuclei is reviewed. Two main theoretical approaches are presented. The first is essentially the shell model with the excitation of pairs of protons and/or neutrons across closed shells or subshells together with a residual proton-neutron interaction. The second is the deformed mean-field approach. The first is broadly defined so that it includes various truncation schemes to the shell model including generalized seniority and the interacting boson model. The presentation of the theory has two main aims: to provide a framework into which the majority of theoretical studies of shape coexistence can be placed and to provide a framework within which a unified view can be discussed. Selected experimental data are shown from 16 O to 238 U. Our criteria for selection emphasize detailed spectroscopic evidence (“fingerprints”) for coexisting shapes.

A general method is developed to treat density-dependent effective nucleon-nucleon interactions in shell-model calculations. A trial function is introduced, that can generate any excitation mode of the nucleus. The method is based on the evaluation of the expectation value of the full Hamiltonian with respect to this generating Slater determinant wave function. Rearrangement effects, due to the density-dependence of the interaction are systematically included. The proposed formalism is applied in first instance to simplified excitation modes, such as two-quasiparticle and particle-hole configurations. A detailed study on the importance of the various types of rearrngement terms is carried out. A more specific application of the general formalism is presented in carrying out an extended shell-model calculation on 116Sn, including, apart from the neutron 2qp-excitations, proton 1p-1h excitations and coupled excitations of the type (1p-1h) ⊗ (2qp).

Shape coexistence in odd-mass nuclei near closed shells (±1 and ±3 nucleons) is reviewed. Two main approaches are presented. The more historical one starts from the explicit introduction of a deformation degree of freedom and allows particular Nilsson orbitals to occur very low in excitation energy. A second approach starts from spherical particle-hole (p-h) excitations across a closed shell and allows interactions with quadrupole vibrations of the underlying core nucleus. The equivalence between the two approaches is discussed in some detail. Attention is given also to other approaches to the description of coexistence. A detailed review of the experimental evidence for coexistence in odd-mass nuclei near the Z = 28, 50, 82 and N = 28, 50, 82 closed shells is made. In addition, the fingerprints of coexisting states, i.e. the most pronounced nuclear structure properties characterizing such states, are presented. These fingerprints provide a necessary (though not sufficient) set of conditions for identifying coexistence throughout the nuclear mass table. At the end, we briefly describe the connection of the present study to other areas of nuclear structure such as related configurations in doubly-even nuclei, many-particle many-hole excitations, deep-lying hole states, and the possibility of incorporating these degrees of freedom into an Interacting Boson Approximation (IBA) framework. A major conclusion of this review is that there is a clear connection between shape coexistence and shell-model intruder states. A variety of topics is suggested for future experimental and theoretical investigation.

Microscopic nuclear structure information that can be reached by proton scattering through isobaric analogue resonances (IAR) is discussed, mainly within the framework of weak-coupling. The concept of isospin for unbound states is examined. A critical evaluation of the methods for extracting nuclear structure information from the experimental results (such as excitation functions, angular distributions, etc.) is given. The mass regions that are studied in detail are the Pb-region and the N = 82 neutron single-closed shell nuclei. Attention is given to the comparison between weak-coupling calculations and experimental results supporting this concept in many nuclei. Level schemes as well as proton partial decay widths and angular distributions have been calculated and compared with the existing data concerning the proton decay of IAR. The concept of generalized neutron particle-hole (GNPH) state is introduced and its occurence extensively discussed within the Pb-region and N = 82 nuclei.