Coulomb Energy Difference as a Probe of Isospin-Symmetry Breaking in the Upper f p-Shell Nuclei

The anomaly in Coulomb energy differences (CEDs) between the isospin T = 1 states in the odd-odd N = Z nucleus Br-70 and the analogue states in its even-even partner Se-70 has remained a puzzle. This is a direct manifestation of isospin-symmetry breaking in effective nuclear interactions. Here, we perform large-scale shell-model calculations for nuclei with A = 66 to 78 using the new filter diagonalization method based on the Sakurai-Sugiura algorithm. The calculations reproduce well the experimental CED. The observed negative CED for A = 70 are accounted for by the cross-shell neutron excitations from the f p shell to the g(9/2) intruder orbit with the enhanced electromagnetic spin-orbit contribution at this special nucleon number.