Excitations of the magic N=50 neutron-core revealed in Ga81

Abstract

The high-spin states of the neutron-rich Ga81, with three valence protons outside a Ni78 core, were measured. The measurement involved prompt γ-ray spectroscopy of fission fragments isotopically identified using the combination of the variable mode spectrometer (VAMOS++) and the advanced gamma tracking array (AGATA). The new γ-ray transitions, observed in coincidence with Ga81 ions, and the corresponding level scheme do not confirm the high-spin levels reported earlier. The newly observed high-spin states in Ga81 are interpreted using the results of state-of-the-art large-scale shell model (LSSM) calculations. The lower excitation energy levels are understood as resulting from the recoupling of three valence protons to the closed doubly magic core, while the highest excitation energy levels correspond to excitations of the magic N=50 neutron core. These results support the doubly magic character of Ni78 and the persistence of the N=50 shell closure but also highlight the presence of strong proton-neutron correlations associated with the promotion of neutrons across the magic N=50 shell gap, only few nucleons away from Ni78.Received 11 January 2019Revised 30 April 2019DOI:https://doi.org/10.1103/PhysRevC.100.011301©2019 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasEnergy levels & level densitiesNuclear structure & decaysRare & new isotopesProperties59 ≤ A ≤ 89TechniquesGamma ray spectroscopyShell modelSpectrometers & spectroscopic techniquesNuclear Physics