Abstract

Prompt $\gamma$ spectroscopy of the neutron-rich $96\mathrm{Kr}$, produced in transfer- and fusion-induced fission reactions, has been performed using the combination of the Advanced Gamma Tracking Array and the $\mathrm{VAMOS}++$ spectrometer. A second excited state, assigned to ${\mathrm{J\pi }}^{}=4^{+}$, is observed for the first time, and a previously reported level energy of the first $2^{+}$ excited state is confirmed. The measured energy ratio ${R4}_{/2}=E\left(4^{+}\right)/E\left(2^{+}\right)=2.12\left(1\right)$ indicates that this nucleus does not show a well-developed collectivity contrary to that seen in heavier $N=60$ isotones. This new measurement highlights an abrupt transition of the degree of collectivity as a function of the proton number at $Z=36$, of similar amplitude to that observed at $N=60$ at higher $Z$ values. A possible reason for this abrupt transition could be related to the insufficient proton excitations in the ${\mathrm{g9}}_{/2}$, ${\mathrm{d5}}_{/2}$, and ${\mathrm{s1}}_{/2}$ orbitals to generate strong quadrupole correlations or to the coexistence of competing different shapes. An unexpected continuous decrease of ${\mathrm{R4}}_{/2}$ as a function of the neutron number up to $N=60$ is also evidenced. This measurement establishes the Kr isotopic chain as the low-$Z$ boundary of the island of deformation for $N=60$ isotones. A comparison with available theoretical predictions using different beyond mean-field approaches shows that these models fail to reproduce the abrupt transitions at $N=60$ and $Z=36$.