Structure of ${}^{83}$As, ${}^{85}$As and ${}^{87}$As: from semi-magicity to ${\hat{I}}^3$-softness

Abstract

The structure of ${}^{83}$As, ${}^{85}$As and ${}^{87}$As have been studied in fusion-fission reaction ${}^{238}$U+${}^9$Be. Fission fragments were identified in mass and atomic number using the VAMOS++ spectrometer and the coincident ${\hat{I}}^3$-rays were detected in the ${\hat{I}}^3$-ray tracking array AGATA. New transitions in ${}^{83}$As and ${}^{85}$As are reported and placed in the level schemes. A level scheme of the excited states in ${}^{87}$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, ${\hat{I}}^3$-soft regime in the open shell cases ${}^{85}$As and ${}^{87}$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 ${}^{79}$Cu, one proton above ${}^{78}$Ni.