Thermal-neutron capture by 58Ni, 59Ni, and 60Ni -: art. no. 044318

We have studied the primary and secondary gamma rays (414 in Ni-59, 390 in Ni-60, and 240 in Ni-61) following thermal-neutron capture by the stable Ni-58, radioactive Ni-59, and stable Ni-60 isotopes. Most of these gamma rays have been incorporated into the corresponding level schemes consisting of 65 levels in Ni-59, 88 levels in Ni-60, and 40 levels in Ni-61. The measured neutron separation energies (S-n in keV) for Ni-59, Ni-60, and Ni-61 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 Ni-58, Ni-59, and Ni-60 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 Ni-59 and Ni-61) 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.