Recent experimental data have revealed large mirror energy differences (MED) between high-spin states in the mirror nuclei ${}^{67}\mathrm{Se}$ and ${}^{67}\mathrm{As}$, the heaviest pair where MED have been determined so far. The MED are generally attributed to the isospin symmetry breaking caused by the Coulomb force and by the isospin-nonconserving part of the nucleon-nucleon residual interaction. The different contributions of the various terms have been extensively studied in the $\mathit{fp}$ shell. By employing large-scale shell-model calculations, we show that the inclusion of the $g_{9/2}$ orbit causes interference between the electromagnetic spin-orbit and the Coulomb monopole radial terms at high spin. The large MED are attributed to the aligned proton pair excitations from the $p_{3/2}$ and $f_{5/2}$ orbits to the $g_{9/2}$ orbit. The relation of the MED to deformation is discussed.

• Received 21 July 2010

DOI:https://doi.org/10.1103/PhysRevC.82.061301