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

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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.
Artículo firmado por 22 autores. M.S. would like to acknowledge E. Pompe and R. Stasinski from the University of Oxford and W. Gins from KU Leuven for their support with statistical analysis. We acknowledge the support of the ISOLDE Collaboration and technical teams. This project has received funding from the European Unions Seventh Framework Programme for Research and Technological Development under Grant Agreement No 262010. The MCSM calculations were performed on K computer at RIKEN AICS (hp160211, hp170230, hp180179). This work was supported in part by MEXT as "Priority Issue on PostK Computer" (Elucidation of the Fundamental Laws and Evolution of the Universe) and JICFuS. This work has been funded by FWO-Vlaanderen (Belgium), by GOA/2015/010 (BOF KU Leuven), and by the Interuniversity Attraction Poles Programme initiated by the Belgian Science Policy Office (BriX network P7/12). This material is based upon work supported by the US Department of Energy, Office of Science, Office of Nuclear Physics, under Award No. DE-FG02-94-ER40834, by Spanish MINECO via Project No. FPA201565035-P, by the Slovak grant agency VEGA (Contract No. 2/0129/17), and by the Slovak Research and Development Agency (Contract No. APVV-15-0225).
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