β(-) decay study of the Mn-66-Fe-66-Co-6(6)-Ni-66 decay chain

Abstract

Background: Shell evolution can impact the structure of the nuclei and lead to effects such as shape coexistence. The nuclei around Ni-68 represent an excellent study case, however, spectroscopic information of the neutron-rich, Z < 28 nuclei is limited. Purpose: The goal is to measure gamma-ray transitions in Fe-66, Co-66, and Ni-66 populated in the beta-decay of Mn-66 to determine absolute beta feedings and relative gamma-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 Fe-65,Fe-66, Co-66, and Ni-66 were studied in the beta(-) decay of Mn-66 produced at ISOLDE, CERN. The beam was purified by means of laser resonance ionization and mass separation. The f and gamma 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 beta-decay half-lives of the nuclei. Results: The relative small beta-decay ground state feeding of Fe-66 obtained in this work is at variant to the earlier studies. Spin and parity 1(+) was assigned to the Co-66 ground state based on the strong ground-state feeding in the decay of Fe-66 as well as in the decay of Co-66. Experimental log(f t) values, gamma-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 Mn-66 to Ni-66 chain were proposed. Conclusions: The beta-decay chain starting Mn-66 toward Ni-66, crossing N = 40, evolves from deformed nuclei to sphericity. The beta-decay population of a selected number of 0(+) and 2(+) states in Ni-66, which is understood within shape coexistence framework of Monte Carlo shell model calculations, reveals the crucial role of the neutron 0g(9/2) shell and proton excitations across the Z = 28 gap.