Axial and triaxial degrees of freedom in 72Zn

The unstable N = 42 nucleus 72Zn has been studied using multiple safe Coulomb excitation in inverse kinematics. The experiment was performed at the REX-ISOLDE facility at CERN making first use of the silicon detector array C-REX in combination with the gamma-ray spectrometer Miniball. The high angular coverage of C-REX allowed to determine the reduced transition strengths for the decay of the yrast 0+1 , 2+1 and 4+1 as well as of the 0+2 and 2+2 states in 72Zn. The quadrupole moments of the 2+1 , 4+1 and 2+2 states were extracted. Using model independent quadrupole invariants, the ground state of 72Zn was found to have an average deformation in the gamma degree of freedom close to maximum triaxiality. In comparison to experimental data in zinc isotopes with N < 40, the collectivity of the 4+1 state in neutron-rich 72Zn is significantly larger, indicating a collective yrast band based on the ground state of 72Zn. In contrast, a low experimental B(E2; 0+2 -> 2+1 ) strength was determined, indicating a different structure for the 0+2 state. Shell-model calculations propose a 0+2 state featuring a larger fraction of the (spherical) N = 40 closed-shell configuration in its wave function than for the 0+1 ground state. The results were also compared with beyond mean field calculations which corroborate the large deformation in the gamma degree of freedom, while pointing to a more deformed 0+2 state. These experimental and theoretical findings establish the importance of the gamma degree of freedom in the ground state of 72Zn, located between the 68,70Ni nuclei that have spherical ground states, and 76Ge, which has a rigid triaxial shape. (c) 2023 Published by Elsevier B.V. This is an open access article under the CC BY license (http:// creativecommons .org /licenses /by /4 .0/). Funded by SCOAP3.

Automated summary

The unstable nucleus 72Zn with N=42 was studied using multiple safe Coulomb excitation in inverse kinematics. The ground state of 72Zn was found to have an average deformation in the gamma degree of freedom close to maximum triaxiality. The collectivity of the 4+1 state in neutron-rich 72Zn is significantly larger compared to zinc isotopes with N < 40.