Nuclear Structure and Exotic Decays; Doubly-Magic 56Ni and Semi-Magic 58Ni

The nuclear structure of two nuclei, 56Ni and 58Ni, has been studied using fusion-evaporation
reaction experiments. The experiments were performed at the Lawrence
Berkeley and Argonne National Laboratories in the U.S.A.

The number of known spherical and deformed states in 56Ni has been
significantly extended. One case of prompt proton emission from a
well-deformed excited state into the ground state of 55Co was established. The spherical excited states were interpreted within the
spherical shell model, revealing a good agreement between theory and
experiment, with one notable exception for the yrast and yrare 8+ states. The
deformed states were interpreted using Cranked Nilsson-Strutinsky
calculations, again with very accurate results.

A similar investigation was performed for 58Ni, resulting in one of the
most extensive level schemes known for nuclei. High-spin states including
several rotational bands were found, and a significant number of
spherical and normally deformed states were also discovered. The rotational
bands were interpreted through Cranked Nilsson-Strutinsky calculations,
which allowed the configurations of the bands to be determined. The
normally deformed states of 58Ni were investigated with spherical
shell-model calculations, using two different interactions. The
results are in general satisfactory. The results indicate, however,
that further improvements of the theoretical calculations demands an
increase of the number of
subshells, allowed in the effective interactions.
A total of 12 prompt proton decays and two prompt alpha decays were
discovered in 58Ni. This is the
highest number of known particle decays of this type for any nucleus,
to date. New aspects of the proton decays have been observed, for
instance, two cases of in-band decays and two levels which decay with
competing proton, alpha, and gamma branches.