Abstract
Lifetime measurements with the recoil distance Doppler-shift technique have been performed to determine yrast E2 transition strengths in 178Pt. The experimental data are related to those on neighboring Pt isotopes, especially recent data on 180Pt, and compared to calculations within the interacting boson model and a Hartree-Fock Bogoliubov approach. These models predict prolate deformed ground states in Pt isotopes close to neutron midshell consistent with the experimental findings. Further, evidence was found that the prolate intruder structure observed in neutron-deficient Hg isotopes that is minimum in energy in 182Hg becomes the ground state configuration in 178Pt and neighboring 180Pt with nearly identical transition quadrupole moments. The new data on 178Pt are further discussed in the context of the systematics along the Pt isotopic chain with respect to a possible sharp shape transition towards a weakly deformed or a quasi-vibrational ground state whereas the prolate structure increases in energy in 174,176Pt and becomes an intruder configuration similar to, for example, 180,182Hg.
Highlights
Neutron deficient nuclei in the A = 180 mass region close to the Z = 82 main shell closure are the subject of numerous experimental and theoretical studies
Evidence was found that the prolate intruder structure observed in neutron-deficient Hg isotopes that is minimum in energy in 182Hg becomes the ground state configuration in 178Pt and neighboring 180Pt with nearly identical transition quadrupole moments
In Hg isotopes close to neutron midshell, i.e., around N = 104, clear evidence was found for a weakly oblate ground state configuration with quite constant energy systematics and a prolate intruder band with a parabolic trend of the level energies with respect to the neutron number where minimum energies of the latter are reached for 182Hg [4]
Summary
The prolate ground state configuration of 180Pt is evident from theoretical approaches, e.g., with the Interacting Boson Model (IBM) Hamiltonian derived from the Gogny energy density functional [10] and with the IBM with configuration mixing (IBM-CM) and a self-consistent meanfield Hartree-Fock-Bogoliubov (HFB) approach, starting from the Gogny-D1S interaction [11] Even though these calculations explain the experimental observations very well, there are deviating results for the lighter even-even Pt isotopes from 178Pt down to 172Pt as pointed out in [11]: The IBM-CM calculation predicts a smooth transition to a weakly prolate deformation for decreasing neutron number towards 172Pt. In contrast, the self-consistent meanfield HFB approach hints at a transition from a prolate structure in 178Pt via a coexistence of prolate deformation and a γ-soft structure towards spherical shape in 172,174Pt. Before this work experimental data were too sparse to clarify this situation. The authors of this publication give a (tentative) life-
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