The structure of ${}^{83}\mathrm{As}$, ${}^{85}\mathrm{As}$, and ${}^{87}\mathrm{As}$ have been studied in fusion-fission reaction ${}^{238}\mathrm{U}+{}^9\mathrm{Be}$. Fission fragments were identified in mass and atomic number using the VAMOS$++$ spectrometer and the coincident $\gamma$ rays were detected in the $\gamma$-ray tracking array AGATA. New transitions in ${}^{83}\mathrm{As}$ and ${}^{85}\mathrm{As}$ are reported and placed in the level schemes. A level scheme of the excited states in ${}^{87}\mathrm{As}$ is proposed for the first time. The data are interpreted in frame of large-scale shell-model calculations, SU3 symmetries, and beyond mean-field frameworks. A spherical regime at magic number $N=50$ is predicted and the location of the proton $g_{9/2}$ orbital is proposed for the first time. Development of collectivity in a prolate deformed, $\gamma$-soft regime in the open shell cases ${}^{85}\mathrm{As}$ and ${}^{87}\mathrm{As}$, most neutron-rich isotopes beyond $N=50$, is concluded. Data and theoretical calculations give confidence to a relatively high extrapolated excitation energy about 4 MeV of the $9/2^{+}$ state in ${}^{79}\mathrm{Cu}$, one proton above ${}^{78}\mathrm{Ni}$.

• Received 18 March 2022
• Accepted 27 June 2022

DOI:https://doi.org/10.1103/PhysRevC.106.014320