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 to 14 , 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 1g9/2 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 1g9/2 proton alignment. The ground-state deformation of even-A Zn nuclei calculated by TRS shows the presence of N=40 closed shell.