$_{36}^{96}$Kr$_{60}$–low-$Z$ boundary of the island of deformation at $N=60$

Prompt γ-ray spectroscopy of the neutron-rich 96Kr, produced in transfer- and fusion-induced fission reactions, has been performed using the combination of the Advanced Gamma Tracking Array and the VAMOSþþspectrometer. A second excited state, assigned to Jπ ¼ 4þ, is observed for the first time, and a previously reported level energy of the first 2þ excited state is confirmed. The measured energy ratio R4=2 ¼ Eð4þÞ=Eð2þÞ ¼ 2.12ð1Þ indicates that this nucleus does not show a well-developed collectivity contrary to that seen in heavier N ¼ 60 isotones. This new measurement highlights an abrupt transition of the degree of collectivity as a function of the proton number at Z ¼ 36, of similar amplitude to that observed at N ¼ 60 at higher Z values. A possible reason for this abrupt transition could be related to the insufficient proton excitations in the g9=2, d5=2, and s1=2 orbitals to generate strong quadrupole correlations or to the coexistence of competing different shapes. An unexpected continuous decrease of R4=2 as a function of the neutron number up to N ¼ 60 is also evidenced. This measurement establishes the Kr isotopic chain as the low-Z boundary of the island of deformation for N ¼ 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 ¼ 60 and Z ¼ 36.