We have studied the primary and secondary $\gamma$ rays (414 in ${}^{59}\mathrm{Ni}$, 390 in ${}^{60}\mathrm{Ni}$, and 240 in ${}^{61}\mathrm{Ni}$) following thermal-neutron capture by the stable ${}^{58}\mathrm{Ni}$, radioactive ${}^{59}\mathrm{Ni}$, and stable ${}^{60}\mathrm{Ni}$ isotopes. Most of these $\gamma$ rays have been incorporated into the corresponding level schemes consisting of 65 levels in ${}^{59}\mathrm{Ni}$, 88 levels in ${}^{60}\mathrm{Ni}$, and 40 levels in ${}^{61}\mathrm{Ni}$. The measured neutron separation energies ($S_n$ in keV) for ${}^{59}\mathrm{Ni}$, ${}^{60}\mathrm{Ni}$, and ${}^{61}\mathrm{Ni}$ are, respectively, $8999.28\pm 0.05$, $\mathrm{11\hspace{0.17em}387.73}\pm 0.05$, and $7820.11\pm 0.05$. The measured thermal-neutron capture cross sections (in barns) for ${}^{58}\mathrm{Ni}$, ${}^{59}\mathrm{Ni}$, and ${}^{60}\mathrm{Ni}$ are, respectively, $4.13\pm 0.05$, $73.7\pm 1.8$, and $2.34\pm 0.05$. In all three cases, primary electric-dipole $\left(E1\right)$ transitions account for the bulk of the total capture cross section. We have calculated these $E1$ partial cross sections (in ${}^{59}\mathrm{Ni}$ and ${}^{61}\mathrm{Ni}$) using direct-capture theory and models of compound-nuclear capture. The agreement between theory and experiment is good. The experimental level schemes have been compared with the results from a large-basis shell-model calculation. The agreement was also found to be quite good.

• Received 5 March 2003

DOI:https://doi.org/10.1103/PhysRevC.70.044318