A. Petrovici

Shape coexistence phenomena in medium mass nuclei

Abstract We review the theoretical interpretation of experimental results on low and high spin states in some even mass A=60−80 nuclei within the complex versions of the Excited Vampir and Fed Vampir variational approaches. The coexistence of oblate and prolate deformed configurations and their mixing at low spin are found to be responsible for the irregular sequences of states and the isomeric decays identified at low excitation energy in even-even nuclei. This behaviour is determined essentially by the isoscalar neutron-proton interaction. At higher spins the coexistence of states with different deformations causes complicated multiple band structures and strong fragmentation of the B(E2) strengths. It also influences the occurence of almost identical bands at normal deformation in some even mass nuclei. In some odd-odd nuclei the shape coexistence furthermore suggests a possible mechanism to populate high spin isomeric states. For the ground state properties of N=Z nuclei the competition between neutron-proton and like-nucleon isovector pairing plays an important role. With increasing angular momentum the neutron-proton alignment dominates the like-nucleon one in the ground bands of odd-odd N=Z nuclei while in the doubly even N=Z nuclei both types of alignment are of comparable importance.

Nucleon pair structure of realistic many body wave functions

The scalar part of the two-body density is calculated for the20Ne ground band and for the yrast 2+ state in68Ge as obtained by solving the Hartree-Fock-Bogoliubov problem with symmetry-projection before the variation. The nucleon pair structure of these states is analysed and the results obtained with different effective Hamiltonians are compared.

Calculation of charge and transition charge densities in some even mass Ge isotopes from microscopic nuclear structure wave functions

Microscopic nuclear structure wave functions obtained within the EXCITED FED VAMPIR approach are used to calculate the charge densities for the ground states as well as the transition charge densities for the transitions from the ground to the first and second 2+ states in some doubly even Ge isotopes. The results are compared with the data extracted from elastic and inelastic electron scattering experiments. Except for the heavier isotopes, where the calculations fail to yield enough collectivity, rather nice agreement is obtained. This result strongly supports the theoretically predicted shape coexisting structures in the low spin states of the nuclei in this mass region.

NEGATIVE PARITY STATES IN 68Ge: EXPERIMENT AND THEORY

High spin negative parity states have been studied in 68Ge via in-beam γ-ray spectroscopy at HHIRF. Several close-lying 7−, 9−, and 11− states are observed with a 13− to tentatively 17− cascade feeding them. Numerous E1 crossing transitions between the negative and positive parity levels are observed, along with possible M1 connecting transitions between the low lying negative parity bands. Previous microscopic calculations of the shape coexisting positive parity levels have been extended to the negative parity levels. The calculations used the EXCITED VAMPIR model based on complex HFB transformations with neutron-proton correlations and parity mixing in the mean field. The complex decay pattern and multiple high spin negative parity bands can be explained by a variable strength of mixing of the different deformed configurations which are bunched in small excitation intervals.

Structure of negative-parity states in68Ge

A microscopic description of negative parity states in the nucleus68Ge is presented. The investigation is based on the EXCITED VAMPIR model in which via a chain of variational calculations first an optimal configuration space ofi symmetry-projected Hartree-Fock-Bogoliubov determinants for thei lowest states of a given symmetry is constructed and subsequently the residual interaction is diagonalised. Though some considerable quantitative discrepancies are obtained, qualitatively the theoretical results agree rather nicely with the complex pattern of negative parity states in this nucleus which has been recently found experimentally. Underlying structures and electromagnetic properties are discussed.

Self-consistent description of shape coexistence in the A≃100 Zr nuclei

The shape transition, shape coexistence, and shape mixing characterizing the netron-rich Zr nuclei in the A100 region are consistently described at low and high spin states in the frame of the complex Excited Vampir variational approach using a realistic effective interaction obtained from Bonn A potential in a large model space. The influence of the shape mixing on the structure and dynamics of the investigated Zr isotopes is discussed and comparison with the available data is presented.

Exotic structure and decay of medium mass nuclei near the drip lines

Nuclei at or near the N=Z line are of particular interest as micro-laboratory for fundamental effects. The simulation of many astrophysical objects requires the knowledge of properties and decay rates of nuclei near the drip lines. The structure and dynamics of proton-rich A70 nuclei and neutron-rich A100 nuclei are influenced by shape coexistence and mixing. A realistic description of shape coexistence phenomena requires beyond-mean-field approaches. Results concerning the self-consistent description of exotic phenomena including anomalies in mirror energy differences of proton-rich nuclei, triple shape coexistence in neutron-rich N=58 Sr and Zr isotopes as well as Gamow-Teller β-decay strength distributions, half-lives, and β- delayed neutron emission probabilities of neutron-rich nuclei in the A100 mass region obtained in the frame of the complex Excited Vampir model using realistic effective interactions in large model spaces are presented.

Self-consistent description of coexistence phenomena in medium mass nuclei

Shape coexistence and mixing, isospin mixing, the competition between neutron‐proton and like‐nucleon pairing correlations have been identified as the main characteristic features of nuclei near the N = Z line in the A≃70 mass region. The self‐consistent treatment of exotic phenomena dominated by their interplay represents a challenge for the nuclear many‐body models. The realistic description of tiny effects in this mass region aiming to test the fundamental interactions and symmetries as well as the required theoretical predictions concerning the nuclear properties relevant for astrophysical scenarios are still open problems of the low‐energy nuclear physics today.