Search for shape-coexisting 0$^+$ states in $^{66}$Ni from lifetime measurements

The lifetime of the 0$_3^+$ state in $^{66}$Ni, two neutrons below the $N=40$ subshell gap, has been measured. The transition $B(E2;0_3^+ \rightarrow 2_1^+)$ is one of the most hindered E2 transitions in the Ni isotopic chain and it implies that, unlike $^{68}$Ni, there is a spherical structure at low excitation energy. We have performed extensive shell-model calculations that correctly predict this result, obtaining a spherical 0$^+$ state at the correct energy and with an extremely low $B(E2;0_3^+ \rightarrow 2_1^+)$ value.