Coexisting shape- and high- K isomers in the shape transitional nucleus 188 Pt

Abstract

A high-spin study of the shape transitional nucleus 188 Pt reveals the unusual coexistence of both shapeand K -isomeric states. Reduced B(E2) transition probabilities for decays from these states inferred from the data clearly establish their hindered character. In addition to other excited structures, a rotational band built upon the K isomer is identified, and its configuration has been assigned through an analysis of alignments and branching ratios. The shape evolution with spin in this nucleus has been inferred from both experimental observables and cranking calculations. The yrast positive parity structure appears to evolve from a near-prolate deformed shape through triaxial at intermediate excitation, and eventually to oblate at the highest spins.Shape coexistence is an important theme in nuclear structure research, and has been reported in many nuclei in the chart of nuclides [1]. The most extensive manifestation of this nuclear phenomenon known anywhere on the nuclear mass surface is found in the neutron-deficient isotopes at and near Z = 82 [1]. The shape coexistence and/or shape evolution in the Pt nuclei with Z = 78 has generated a lot of research interest in recent years. It has been observed that with increasing proton and neutron numbers, beyond Z = 72 and N = 106, the nuclear shape becomes more prone to non-axial fluctuation, induced by multi-quasiparticle excitation. Indeed, in the self-consistent Relativistic Hartree-Bogoliubov (RHB) calculations, the gradual transition from the prolate deformed 186 Pt, through the region of triaxially deformed [188][189][190][191][192][193][194][195][196][197][198] Pt, to the slightly oblate 200 Pt, and finally the spherical 202-204 Pt isotopes has been revealed [2]. Again, in the self-consistent Hartree-FockBogoliubov (HFB) calculations using non-relativistic Skyrme and Gogny interactions, a sudden prolate to oblate shape change was observed in the Pt chain of isotopes around A = 188 [2]. A good agreement was found when the latter calculation was compared * Corresponding author. E-mail address: somm@barc.gov.in (S. Mukhopadhyay).with a recent study on the evolution of the total energy surface and the nuclear shape in the isotopic chain Pt in the framework of the interacting boson model, including configuration mixing (IBM-CM) [3]. All these different theoretical approaches point towards the common fact that the lightest Pt isotopes are slightly deformed and prolate. With increasing neutron number, those attain more strongly deformed shape, while at the same time the γ deformation increases, leading to a triaxial nuclear shape. Eventually, the triaxial shape becomes oblate with the addition of more neutrons.The presence of isomeric states is also well established in the A ≈ 180 mass region both in experiment and theory. The occurrence of these metastable states can be interpreted in terms of both protons and neutrons in high-Ω orbitals. In axially symmetric nuclei, transitions involving large changes in the projection of angular momentum on the symmetry axis, K...

High-spin structure in the transitional nucleus Xe131 : Competitive neutron and proton alignment in the vicinity of the N=82 shell closure

“…Beside transitions from 130,131 Cs, contaminant peaks are caused by the ground-state band of 188 Pt [68] stemming from a dominant fusion-evaporation reaction in the 181 Ta backing of the target. All three known γ rays in the positive-parity band with energies of 189, 389, and 992 keV are mutually coincident in the HORUS experiment and were arranged according to their intensity balance as proposed by Kerek et al [31].…”

Study of shape coexistence in the 180−190Hg isotopes by SO(6) representation of eigenstates

“…Since making a 1p-1h excitation across the closed shell at Z=50 takes about 4.5 MeV (the proton shell gap), the unperturbed energy for 2p-2h excitations comes up to about 9 MeV. Even though pairing amongst the particles and holes will lower the energy in an important way to 4-5 MeV, 5 this is still far from the observed excitation energy of 1.7 MeV. Some essential element is missing when starting from the spherical intrinsic symmetry of the shell model.…”

“…The reduced electric monopole and quadrupole transition probabilities are considered as the observables which as well as the quadrupole moment ratios within the low-lying state bands prepare more information about the nuclear structure. The most general one-body multipole transition operator has the form [5,7]: operator may be found by setting l=0 in above equation as [5,7]:…”

“…In the Interacting Boson Model (IBM) , which describes the nuclear structure of eveneven nuclei within the U(6) symmetry, possessing U (5), SU (3) and O(6) dynamical symmetry limits, shape phase transitions were studied 25 years ago with using the classical limit of the model. These descriptions point out that there is a first order shape phase transition between U (5) and SU (3) limits, namely between spherical and deformed limits which Hg isotopes are expected to lie in this transitional region. The analytic description of nuclear structure at the critical point of phase transitions has attracted extensive interest in the recent decades.…”

Isomers and oblate collectivity at high spin in neutron-rich Pt isotopes

“…Proton-rich Pt isotopes have been studied rather extensively e.g. [4,8,9], a e-mail: sujit.tandel@cbs.ac.in and found to exhibit well-developed rotational bands both near the ground states and at high spin. With increase in neutron number from the lightest stable isotope 190 Pt (N=112) to the heaviest 198 Pt (N = 120), the decrease in ground state deformation and collectivity is evidenced by the increase in 2 + 1 excitation energy in even-A nuclei accompanied by a corresponding reduction in B(E2; 2 + →0 + ) values.…”