Anomaly in the Photodisintegration ofNi58andNi60in the Giant-Dipole Resonance Region

The photoneutron cross sections for ${\mathrm{Ni}}^{58}$, ${\mathrm{Ni}}^{60}$, and natural nickel from threshold up to 25 MeV have been measured. The integrated ($\gamma ,n$) cross section for ${\mathrm{Ni}}^{60}$ is 2.6 times as large as the value for ${\mathrm{Ni}}^{58}$, and the two cross-section curves exhibit markedly different structure over the giant-dipole resonance region. The results suggest that it is important to include the shell effects explicitly in the theoretical treatment of the giant-dipole states in the medium nuclei such as nickel isotopes.