Single-particle and collective excitations in the N=28 isotones Fe-54 and Mn-53

The fusion-evaporation reaction S-32+Si-28 at 125 MeV beam energy was used to populate high-spin states in the semimagic N = 28 nuclei Mn-53 and Fe-54. With a combination of the Gammasphere spectrometer and ancillary devices including the Microball CsI(Tl) array, extensive high-spin level schemes are derived. They exhibit rotational-like collective structures and competing single-particle excitations. The experimental results are compared with predictions from shell-model calculations, for which the inclusion of isopin-symmetry-breaking terms is found to improve the description. An interpretation of the high-spin states is put forward using cranked Nilsson-Strutinsky calculations, indicative of contributions from collective excitations beyond some 8-MeV excitation energy and highlighting the importance of the g(9/2) intruder orbital in this energy range.