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 γ 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 γ degree of freedom, while pointing to a more deformed 0+2 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.
- multiple Coulomb excitation
- N = 40
- sub-shell closure