Multifaceted Quadruplet of Low-Lying Spin-Zero States in 66Ni: Emergence of Shape Isomerism in Light Nuclei

A search for shape isomers in the Ni-66 nucleus was performed, following old suggestions of various mean-field models and recent ones, based on state-of-the-art Monte Carlo shell model ( MCSM), all considering Ni-66 as the lightest nuclear system with shape isomerism. By employing the two-neutron transfer reaction induced by an O-18 beam on a Ni-64 target, at the sub-Coulomb barrier energy of 39 MeV, all three lowest-excited 0(+) states in 66Ni were populated and their gamma decay was observed by gamma-coincidence technique. The 0(+) states lifetimes were assessed with the plunger method, yielding for the 0(2)(+), 0(3)(+), and 0(4)(+) decay to the 2(1)(+) state the B(E2) values of 4.3, 0.1, and 0.2 Weisskopf units (W.u.), respectively. MCSM calculations correctly predict the existence of all three excited 0(+) states, pointing to the oblate, spherical, and prolate nature of the consecutive excitations. In addition, they account for the hindrance of the E2 decay from the prolate 0(4)(+) to the spherical 2(1)(+) state, although overestimating its value. This result makes 66Ni a unique nuclear system, apart from (236);U-238, in which a retarded. transition from a 0(+) deformed state to a spherical configuration is observed, resembling a shape-isomerlike behavior.