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

Abstract

Shape coexistence in the $Z\approx 82$ region has been established in mercury, lead and polonium isotopes. Even-even mercury isotopes with $100\le N\le 106$ present multiple fingerprints of this phenomenon, which seems to be no longer present for $N\ge 110$. According to a number of theoretical calculations, shape coexistence is predicted in the ${}^{188}$Hg isotope. 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 accelerators complex at the Laboratori Nazionali di Legnaro. The channels of interest were selected using the information from the Neutron Wall array, while the ${\hat{I}}^3$ rays were detected using the GALILEO ${\hat{I}}^3$-ray array. The lifetimes of the excited states were determined using the Recoil Distance Doppler-Shift method, employing the dedicated GALILEO plunger device. Using the two-bands mixing and rotational models, the deformation of the pure configurations was obtained from the experimental results. The extracted transition strengths were compared with those calculated with the state-of-the-art symmetry-conserving configuration-mixing (SCCM) and five-dimentional collective Hamiltonian (5DCH) approaches in order to shed light on the nature of the observed structures in the ${}^{188}$Hg nucleus. An oblate, a normal- and a super-deformed prolate bands were predicted and their underlying shell structure was also discussed.

v1: 13 pages, 10 figures, comparison between IBM-CM and SCCM calculations; v2: 16 pages, 13 figures, discussion on the mixing amplitudes from the experimental B(E2) values, comparison between SCCM and 5DCH calculations