Spectroscopic study of ⁶³Ni via cold neutron capture reaction: I. Nuclear structure of ⁶³Ni

The γ-radiation has been investigated for ⁶³Ni produced by cold neutron capture in an enriched ⁶²Ni sample. The 315 γ-rays produced were placed in a ⁶³Ni level scheme, in which 272 placements are new. We determined the excitation energies with 0.2-0.8 keV error and the branching ratios of 62 bound levels in ⁶³Ni, including 30 new levels. The Q-value of the ⁶²Ni(n, γ) ⁶³Ni reaction amounts to 6837.75 ± 0.18 keV. A large-scale shell-model calculation was performed that included the four single-particle orbits of 0f _7/2, 1p _3/2, 0f _5/2, and 1p _1/2, assuming an inert ⁴⁰Ca core for negative-parity states, and those of 1 _p3/2, 0f _5/2, 1p _1/2, and 0g _9/2, assuming a ⁵⁶Ni core for positive-parity states. The energies of the low-lying ⁶³Ni states were successfully reproduced. A mean-field statistical-model calculation was also performed by using a self-consistent interaction between the Hartree-Fock + Bardeen-Cooper-Schrieffer method and the statistical calculation. The calculation result is consistent with the experimental level density of ⁶³Ni.