Shape coexistence in the neutron-deficient $^{188}$Hg investigated via lifetime measurements

Abstract

Background: Shape coexistence in the Z ≈ 82 region has been established in mercury, lead and polonium isotopes. For even-even mercury isotopes with 100 ≤ N ≤ 106 multiple fingerprints of this phenomenon are observed, which seems to be no longer present for N ≥ 110. According to a number of theoretical calculations, shape coexistence is predicted in the 188 Hg isotope. Purpose: The aim of this work was to measure lifetimes of excited states in 188 Hg to infer their collective properties, such as the deformation. Extending the investigation to higher-spin states, which are expected to be less affected by band-mixing effects, can provide additional information on the coexisting structures. Methods: The 188 Hg nucleus was populated using two different fusion-evaporation reactions with two targets, 158 Gd and 160 Gd, and a beam of 34 S provided by the Tandem-ALPI accelerator complex at the Laboratori Nazionali di Legnaro. The channels of interest were selected using the information from the Neutron Wall array, while the γ rays were detected using the GALILEO γ-ray spectrometer. Lifetimes of excited states were determined using the Recoil Distance Doppler-Shift method, employing the dedicated GALILEO plunger device. Results: Lifetimes of the states up to spin 16 were measured and the corresponding reduced transition probabilities were calculated. Assuming two-band mixing and adopting, as done commonly, the rotational model, the mixing strengths and the deformation parameters of the unperturbed structures were obtained from the experimental results. In order to shed light on the nature of the observed configurations in the 188 Hg nucleus, the extracted transition strengths were compared with those resulting from state-of-the-art beyond-mean-field calculations using the symmetry-conserving configuration-mixing approach, limited to axial shapes, and the 5-dimensional collective Hamiltonian, including the triaxial degree of freedom. Conclusions: The first lifetime measurement for states with spin ≥ 6 suggested the presence of an almost spherical structure above the 12 + 1 isomer and allowed elucidating the structure of the intruder band. The comparison of the extracted B(E2) strengths with the two-band mixing model allowed the determination of the ground-state band deformation. Both beyond-mean-field calculations predict coexistence of a weakly-deformed band with a strongly prolate-deformed one, characterized by elongation parameters similar to those obtained experimentally, but the calculated relative position of the bands and their mixing strongly differ.