The recent experimental observation of isospin symmetry breaking (ISB) in the ground states of the $T=3/2$ mirror pair ${}^{73}\mathrm{Sr}\text{−}{}^{73}\mathrm{Br}$ is theoretically studied using large-scale shell-model calculations. The large valence space and the successful PFSDG-U effective interaction used for the nuclear part of the problem capture possible structural changes and provide a robust basis to treat the ISB effects of both electromagnetic and nonelectromagnetic origin. The calculated shifts and mirror-energy differences are consistent with the inversion of the $I^{\pi }=1/2^{-},5/2^{-}$ states between ${}^{73}\mathrm{Sr}$ and ${}^{73}\mathrm{Br}$ and suggest that the role played by the Coulomb interaction is dominant. An isospin breaking contribution of nuclear origin is estimated to be $\approx 25\mathrm{keV}$.

• Received 3 July 2020
• Accepted 8 September 2020

DOI:https://doi.org/10.1103/PhysRevC.102.031302