Low-lying dipole response of ${}_{64}\mathrm{Ni}$

Two complementary real-photon scattering experiments were conducted on the proton-magic ${}_{64}\mathrm{Ni}$ nucleus to study the dipole response up to its neutron-separation energy of $S_n=9.7\mathrm{MeV}$. By combining both measurements, 87 $E1$ and 23 $M1$ transitions were identified above 4.3 MeV. The results of the observed $M1$ transitions were compared to shell-model calculations using two different model spaces. It was found that the inclusion of excitations across the $Z=28$ shell gap in the calculations has a large impact. Furthermore, average cross sections for decays to the ground state (elastic transitions) as well as to lower-lying excited states (inelastic decays) were determined. The corresponding $E1$ channel was compared to calculations within the relativistic equation of motion (REOM) framework. Whereas the calculations of highest possible complexity reproduce the fragmentation and overall behavior of the $E1$ average elastic cross section well, the predicted absolute cross sections are approximately twice as high as the experimental upper limits even though the latter also include an estimate of the inelastic-decay channel.