Thermal-neutron capture by⁵⁸Ni, ⁵⁹Ni, and ⁶⁰Ni

We have studied the primary and secondary γ rays (414 in ⁵⁹Ni, 390 in ⁶⁰Ni, and 240 in ⁶¹Ni) following thermal-neutron capture by the stable ⁵⁸Ni, radioactive ⁵⁹Ni, and stable ⁶⁰Ni isotopes. Most of these γ rays have been incorporated into the corresponding level schemes consisting of 65 levels in ⁵⁹Ni, 88 levels in ⁶⁰Ni, and 40 levels in ⁶¹Ni. The measured neutron separation energies (S_n in keV) for ⁵⁹Ni, ⁶⁰Ni, and ⁶¹Ni are, respectively, 8999.28±0.05, 11 387.73±0.05, and 7820.11 ±0.05. The measured thermal-neutron capture cross sections (in barns) for ⁵⁸Ni, ⁵⁹Ni, and ⁶⁰Ni are, respectively, 4.13±0.05, 73.7±1.8, and 2.34±0.05. In all three cases, primary electric-dipole (E1) transitions account for the bulk of the total capture cross section. We have calculated these E1 partial cross sections (in ⁵⁹Ni and ⁶¹Ni) 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.