High-spin States of ⁶⁸Zn

For the nuclei in the A≈70 mass region, due to the large subshell gaps around Fermi surfaces in the single-particle spectra at prolate and/or oblate deformation, they exhibit rich and varied shape-related phenomena, such as rapid shape transition, shape coexistence, and triaxiality, and have been the focus of theoretical and experimental investigations in recent years. High-spin studies of these nuclei can give us a straightforward interpretation in terms of the interplay between the single-particle and collective degrees of freedom. Here, it is reported that an experimental investigation on the high-spin properties of ⁶⁸Zn via the ⁷Li(⁶⁴Ni, p2n)⁶⁸Zn fusion-evaporation reaction at a beam energy of 34 MeV, which was performed in the HI-13 tandem facility of the China Institute of Atomic Energy by Peking University Experimental Nuclear Physics Group. The ⁶⁴Ni target with a thickness of 0.99 mg/cm² on 14.9 mg/cm² Pb backing was used. The in-beam gamma-rays were detected by an array consisting of 6 segmented clover detectors and 24 high-purity germanium (HPGe) with bismuth germinate (BGO) anti-Compton suppressors. The digital data acquisition system GDDAQ based on XIA Pixie16 developed by our group was used for the detector array. Approximately 1.8×10⁹ γ-γ coincident events were detected. A γ-γ symmetric matrix and a γ-γ-γ cube were built from the coincidence events. The level scheme analysis was performed by using the RADWARE package. To obtain the directional correlations of γ-ray deexciting oriented states (DCO) intensity ratios to determine the multipolarities of γ-ray transitions, the detectors at 90.0° with respect to the beam direction were sorted against the detectors at 30.0° to produce a two-dimensional angular correlation matrix. The yrast band of ⁶⁸Zn is expanded from 8 \hslash to 14\hslash , allowing the observation of the first band crossing. The characteristics of E-GOS curve for the yrast sequence in ⁶⁸Zn indicate that this nucleus may undergo an evolution from quasivibrational to quasirotational structure with increasing angular momentum. Based on a systematic summary of the available experimental alignments for the even-A ^66-70Zn isotopes, the structural evolutions induced by the increase in angular momentum, as well as by the change in neutron numbers, in these even-A Zn isotopes were discussed in comparison with the cranked Woods-Saxon-Strutinsky calculations by means of total-Routhian-surface (TRS) methods. The alignment of a pair of 1g_9/2 neutrons is suggested to be responsible for the first band crossing observed in ⁶⁸Zn, meanwhile, the second band crossing is predicted to occur at higher rotational frequency and is associated with the 1g_9/2 proton alignment. The ground-state deformation of even-A Zn nuclei calculated by TRS shows the presence of N=40 closed shell.