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

Sammanfattning: 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.

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