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

A search for shape isomers in the 66Ni 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 66Ni as the lightest nuclear system with shape isomerism. By employing the two-neutron transfer reaction induced by an 18O beam on a 64Ni target, at the sub-Coulomb barrier energy of 39 MeV, all three lowest-excited 0+ states in 66Ni were populated and their γ decay was observed by γ-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 236,238U, in which a retarded γ transition from a 0+ deformed state to a spherical configuration is observed, resembling a shape-isomerlike behavior.status: publishe