Chiral rotation observed in128 Cs is studied using the newly developed microscopic triaxial projected shell model (TPSM) approach. The observed energy levels and the electromagnetic transition probabilities of the nearly degenerate chiral dipole bands in this isotope are well reproduced by the present model. This demonstrates the broad applicability of the TPSM approach, based on a schematic interaction and angular-momentum projection technique, to explain a variety of low-and high-spin phenomena in triaxial rotating nuclei.The classification of band structures from symmetry considerations has played a central role in our understanding of nuclear structure physics. Most of the rotational nuclei are axially symmetric with conserved angular-momentum projection along the symmetry axis. This symmetry has allowed to classify a multitude of rotational bands using Nilsson scheme and has been instrumental to unravel the intrinsic structures of deformed nuclei [1,2]. Although, most of the deformed nuclei obey axial symmetry at low-excitation energies and spin, there are also known regions of the periodic table, referred to as transitional nuclei, that violate axial symmetry and are described using the triaxial deformed mean-field. Further, some nuclei that are axial in the ground-state become triaxial at higher excitation energies and angular momenta. There are several empirical observations indicating that axial symmetry is broken in transitional regions. For instance, the moment of inertia of the transitional nuclei rises sharply with angular-momentum for
We expand the triaxial projected shell model basis to include triaxially deformed multi-quasiparticle states. This allows us to study the yrast and γ -vibrational bands up to high spins for both γ -soft and well-deformed nuclei. As a first application, a systematic study of the high-spin states in Er isotopes is performed. The calculated yrast and γ bands are compared with the known experimental data, and it is shown that the agreement between theory and experiment is quite satisfactory. The calculation leads to predictions for bands based on one-and two-γ phonons where current data are still sparse. It is observed that γ bands for neutron-deficient isotopes of 156 Er and 158 Er are close to the yrast band, and further these bands are predicted to be nearly degenerate for high-spin states.
The newly developed multi-quasiparticle triaxial projected shell-model approach is employed to study the high-spin band structures in neutron-deficient even-even Ce-and Nd-isotopes. It is observed that γ-bands are built on each intrinsic configuration of the triaxial mean-field deformation. Due to the fact that a triaxial configuration is a superposition of several K-states, the projection from these states results in several low-lying bands originating from the same intrinsic configuration. This generalizes the well-known concept of the surface γ-oscillation in deformed nuclei based on the ground-state to γ-bands built on multi-quasiparticle configurations. This new feature provides an alternative explanation on the observation of two I = 10 aligning states in 134 Ce and both exhibiting a neutron character.
Recent experimental data have demonstrated that 76 Ge may be a rare example of a nucleus exhibiting rigid γ-deformation in the low-spin regime. In the present work, the experimental analysis is supported by microscopic calculations using the multi-quasiparticle triaxial projected shell model (TPSM) approach. It is shown that to best describe the data of both yrast and γ-vibrational bands in 76 Ge, a rigid-triaxial deformation parameter γ ≈ 30 • is required. TPSM calculations are discussed in conjunction with the experimental observations and also with the published results from the spherical shell model. The occurrence of a γγ-band in 76 Ge is predicted with the bandhead at an excitation energy of ∼ 2.5 MeV. We have also performed TPSM study for the neighboring Ge-and Se-isotopes and the distinct γ-soft feature in these nuclei is shown to result from configuration mixing of the ground-state with multi-quasiparticle states.
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