First Evidence of Shape Coexistence in the Ni-78 Region: Intruder 0(2)(+) State in Ge-80
Abstract
The N = 48 Ge-80 nucleus is studied by means of beta-delayed electron-conversion spectroscopy at ALTO. The radioactive Ga-80 beam is produced through the isotope separation on line photofission technique and collected on a movable tape for the measurement of gamma and e(-) emission following beta decay. An electric monopole E0 transition, which points to a 639(1) keV intruder 0(2)(+) state, is observed for the first time. This new state is lower than the 2(1)(+) level in Ge-80, and provides evidence of shape coexistence close to one of the most neutron-rich doubly magic nuclei discovered so far, Ni-78. This result is compared with theoretical estimates, helping to explain the role of monopole and quadrupole forces in the weakening of the N = 50 gap at Z = 32. The evolution of intruder 0(2)(+) states towards Ni-78 is discussed.