$\beta^-$ decay study of the $^{66}$Mn $-$ $^{66}$Fe $-$ $^{66}$Co $-$ $^{66}$Ni decay chain

Background: Shell evolution can impact the structure of the nuclei and lead to effects such as shape coexistence. The nuclei around Ni68 represent an excellent study case, however, spectroscopic information of the neutron-rich, Z<28 nuclei is limited. Purpose: The goal is to measure γ-ray transitions in Fe66,Co66, and Ni66 populated in the β− decay of Mn66 to determine absolute β feedings and relative γ-decay probabilities and to compare the results with Monte Carlo shell model calculations in order to study the influence of the relevant single neutron and proton orbital occupancies around Z=28 and N=40. Method: The low-energy structures of Fe65,66,Co66, and Ni66 were studied in the β− decay of Mn66 produced at ISOLDE, CERN. The beam was purified by means of laser resonance ionization and mass separation. The β and γ events detected by three plastic scintillators and two MiniBall cluster germanium detectors, respectively, were correlated in time to build the low-energy excitation schemes and to determine the β-decay half-lives of the nuclei. Results: The relative small β-decay ground state feeding of Fe66 obtained in this work is at variant to the earlier studies. Spin and parity 1+ was assigned to the Co66 ground state based on the strong ground-state feeding in the decay of Fe66 as well as in the decay of Co66. Experimental log(ft) values, γ-ray de-excitation patterns, and energies of excited states were compared to Monte Carlo shell model calculations. Based on this comparison, spin and parity assignments for the selected number of low-lying states in the Mn66 to Ni66 chain were proposed. Conclusions: The β-decay chain starting Mn66 toward Ni66, crossing N=40, evolves from deformed nuclei to sphericity. The β-decay population of a selected number of 0+ and 2+ states in Ni66, which is understood within shape coexistence framework of Monte Carlo shell model calculations, reveals the crucial role of the neutron 0g9/2 shell and proton excitations across the Z=28 gap.