Coulomb Excitation of ⁸⁰Sr and the limits of the $N = Z = 40$ island of deformation

The region of N≈ Z≈ 40 has long been associated with strongly deformed nuclear configurations. The presence of this strong deformation was recently confirmed through lifetime measurements in N≈ Z Sr and Zr nuclei. Theoretically, however, these nuclei present a challenge due to the vast valence space required to incorporate all deformation driving interactions. Recent state-of-the-art predictions indicate a near axial prolate deformation for $N=Z$ and $N=Z+2$ nuclei between $N=Z=36$ and $N=Z=40$. In this work we investigate the shores of this island of deformation through a sub-barrier Coulomb excitation study of the $N=Z+4$ nucleus, {80}Sr. Extracting a spectroscopic quadrupole moment of Qₛ(2⁺₁) = 0.45+0.83-0.88~eb, we find that {80}Sr is inconsistent with significant axial prolate deformation. This indicates that the predicted region of strong prolate deformation around $N=Z=40$ is tightly constrained to the quartet of nuclei: {76,78}Sr and {78,80}Zr.