Photoneutron cross sections for Ni isotopes: Toward understanding (n, gamma) cross sections relevant to weak s-process nucleosynthesis

Photoneutron cross sections were measured for Ni-58, Ni-60, Ni-61, and Ni-64 at energies between the one-neutron and two-neutron thresholds using quasimonochromatic gamma-ray beams produced in laser Compton scattering at the NewSUBARU synchrotron radiation facility. These photoneutron data are used to extract the gamma-ray strength function above the neutron threshold, complementing the information obtained by the Oslo method below the threshold. We discuss radiative neutron-capture cross sections and the Maxwellian-averaged cross sections for Ni isotopes including Ni-63, a branching point nucleus along the weak s-process path. The cross sections are calculated with the experimentally constrained gamma-ray strength functions from the Hartree-Fock-Bogolyubov plus quasiparticle-random-phase approximation based on the Gogny D1M interaction for both E1 and M1 components and supplemented with the M1 upbend.