CERN Accelerating science

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 
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 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
(
𝐸
2
;
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.