High-resolution spectroscopy of neutron-rich Br isotopes and signatures for a prolate-to-oblate shape transition at N=56

The first systematic experimental study of the neutron-rich Br isotopes with two complementary state-of-the-art techniques is presented. These isotopes were populated in the fission process at two different facilities, GANIL and ILL. New spectroscopic information was obtained for odd-even ${}^{87–93}\mathrm{Br}$ isotopes and the experimental results were compared with state-of-the-art large scale shell model (LSSM) and discrete nonorthogonal (DNO) shell model calculations. As a result of such theoretical approaches, a transition from prolate (${}^{87,89}\mathrm{Br}$) to oblate (${}^{91,93}\mathrm{Br}$) shapes is obtained from the subtle balance between proton and neutron quadrupole deformations, as a clear signature of a pseudo-SU3 quadrupole regime.