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 γ-ray spectrometer Miniball. The high angular coverage of C-REX allowed to determine the reduced transition strengths for the decay of the yrast 0₁+, 2₁+ and 4₁+ as well as of the 0₂+ and 2₂+ states in ⁷²Zn. The quadrupole moments of the 2₁+, 4₁+ and 2₂+ states were extracted. Using model independent quadrupole invariants, the ground state of ⁷²Zn was found to have an average deformation in the γ degree of freedom close to maximum triaxiality. In comparison to experimental data in zinc isotopes with N<40, the collectivity of the 4₁+ 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;0₂+→2₁+) strength was determined, indicating a different structure for the 0₂+ state. Shell-model calculations propose a 0₂+ state featuring a larger fraction of the (spherical) N=40 closed-shell configuration in its wave function than for the 0₁+ ground state. The results were also compared with beyond mean field calculations which corroborate the large deformation in the γ degree of freedom, while pointing to a more deformed 02+ state. These experimental and theoretical findings establish the importance of the γ 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.