ABSTRACT
The low-spin structure of the semimagic ^{64}Ni nucleus has been considerably expanded: combining four experiments, several 0^{+} and 2^{+} excited states were identified below 4.5 MeV, and their properties established. The Monte Carlo shell model accounts for the results and unveils an unexpectedly complex landscape of coexisting shapes: a prolate 0^{+} excitation is located at a surprisingly high energy (3463 keV), with a collective 2^{+} state 286 keV above it, the first such observation in Ni isotopes. The evolution in excitation energy of the prolate minimum across the neutron N=40 subshell gap highlights the impact of the monopole interaction and its variation in strength with N.
ABSTRACT
A search for shape isomers in the ^{66}Ni 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 ^{66}Ni as the lightest nuclear system with shape isomerism. By employing the two-neutron transfer reaction induced by an ^{18}O beam on a ^{64}Ni target, at the sub-Coulomb barrier energy of 39 MeV, all three lowest-excited 0^{+} states in ^{66}Ni 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 ^{66}Ni a unique nuclear system, apart from ^{236,238}U, in which a retarded γ transition from a 0^{+} deformed state to a spherical configuration is observed, resembling a shape-isomerlike behavior.
ABSTRACT
The 02(+) state in 34Si has been populated at the GANIL-LISE3 facility through the ß decay of a newly discovered 1(+) isomer in 34Al of 26(1) ms half-life. The simultaneous detection of e(+)e(-) pairs allowed the determination of the excitation energy E(02(+))=2719(3) keV and the half-life T(1/2)=19.4(7) ns, from which an electric monopole strength of ρ(2)(E0)=13.0(0.9)×10(-3) was deduced. The 2(1)(+) state is observed to decay both to the 0(1)(+) ground state and to the newly observed 0(2)(+) state [via a 607(2) keV transition] with a ratio R(2(1)(+)â0(1)(+)/2(1)(+)â0(2)(+))=1380(717). Gathering all information, a weak mixing with the 0(1)(+) and a large deformation parameter of ß=0.29(4) are found for the 0(2)(+) state, in good agreement with shell model calculations using a new SDPF-U-MIX interaction allowing np-nh excitations across the N=20 shell gap.
