Neutron-deficient selenium isotopes are thought to undergo a rapid shape change from a prolate deformation near the line of beta stability towards oblate deformation around the line of $N=Z$. The point at which this shape change occurs is unknown, with inconsistent predictions from available theoretical models. A common feature in the models is the delicate nature of the point of transition, with the introduction of even a modest spin to the system sufficient to change the ordering of the prolate and oblate configurations. We present a measurement of the quadrupole moment of the first-excited state in radioactive ${}^{72}\mathrm{Se}$—a potential point of transition—by safe Coulomb excitation. This is the first low-energy Coulomb excitation to be performed with a rare-isotope beam at the reaccelerated beam facility at the National Superconducting Cyclotron Laboratory. By demonstrating a negative spectroscopic quadrupole moment for the first-excited $2^{+}$ state, it is found that any low-spin shape change in neutron-deficient selenium does not occur until ${}^{70}\mathrm{Se}$.

• Received 12 May 2018
• Revised 23 July 2018

DOI:https://doi.org/10.1103/PhysRevLett.121.082502