Photoneutron cross sections ofNi58andNi60

Photoneutron cross sections of ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}$ and ${}_{}{}^{60}{}_{}{}^{}\mathrm{Ni}$ have been measured, using separated isotopes and nearly monoenergetic photons from in-flight annihilation of positrons. Both ($\gamma ,n$) and ($\gamma ,2n$) cross sections have been obtained; the ($\gamma ,3n$) cross section for both isotopes was found to be essentially negligible up to the highest energy measured, 33.5 MeV. The peak cross section of 27 mb for ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}$ occurs at 17.3 MeV, while for ${}_{}{}^{60}{}_{}{}^{}\mathrm{Ni}$ the peak cross section of 74 mb occurs at 16.3 MeV. The ($\gamma ,2n$) cross section of ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}$ is small, never exceeding 1.5 mb, whereas in the case of ${}_{}{}^{60}{}_{}{}^{}\mathrm{Ni}$ the ($\gamma ,2n$) cross section rises to a peak value of just over 10 mb at approximately 24 MeV, where it constitutes over 30% of the total photoneutron cross section. In both nuclei the ($\gamma ,2n$) cross section appears essentially to vanish above 33 MeV, whereas the single-neutron cross sections of both appear to remain nearly constant from about 25 MeV to the maximum measured energy. Considerable structure appears in the total cross section of both nuclei throughout the giant-resonance region. Structure is also evident in the ($\gamma ,2n$) cross sections, particularly in the case of ${}_{}{}^{60}{}_{}{}^{}\mathrm{Ni}$. Integrated total photoneutron cross sections up to 33.5 MeV were determined to be 286 MeV mb for ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}$ and 704 MeV mb for ${}_{}{}^{60}{}_{}{}^{}\mathrm{Ni}$. Our results are compared with those from previous photoneutron experiments, and, combined with measured photoproton cross sections, are used to test theoretical predictions based on shell-model calculations.