Low-lying dipole response of ⁶⁴Ni

Two complementary real-photon scattering experiments were conducted on the proton-magic ⁶⁴Ni nucleus to study the dipole response up to its neutron-separation energy of S_n = 9.7 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.