Roles of nuclear weak rates on the evolution of degenerate cores in stars

Electron-capture and β-decay rates in stellar environments are evaluated with the use of new shell-model Hamiltonians for sd-shell and pf-shell nuclei as well as for nuclei belonging to the island of inversion. Important role of the nuclear weak rates on the final evolution of stellar degenerate cores is presented. The weak interaction rates for sd-shell nuclei are calculated to study nuclear Urca processes in O-Ne-Mg cores of stars with 8-10 M⊙ (solar mass) and their effects on the final fate of the stars. Nucleosynthesis of iron-group elements in Type Ia supernova explosions are studied with the weak rates for pf-shell nuclei. The problem of the neutron-rich iron-group isotope over-production compared to the solar abundances is shown to be nearly solved with the use of the new rates and explosion model of slow defraglation with delayed detonation. Evaluation of the weak rates is extended to the island of inversion and the region of neutron-rich nuclei near 78Ni, where two major shells contribute to their configurations

Roles of nuclear weak rates on the evolution of degenerate cores in stars

Electron-capture and β-decay rates in stellar environments are evaluated with the use of new shell-model Hamiltonians for sd-shell and pf-shell nuclei as well as for nuclei belonging to the island of inversion. Important role of the nuclear weak rates on the final evolution of stellar degenerate cores is presented. The weak interaction rates for sd-shell nuclei are calculated to study nuclear Urca processes in O-Ne-Mg cores of stars with 8-10 M⊙ (solar mass) and their effects on the final fate of the stars. Nucleosynthesis of iron-group elements in Type Ia supernova explosions are studied with the weak rates for pf-shell nuclei. The problem of the neutron-rich iron-group isotope over-production compared to the solar abundances is shown to be nearly solved with the use of the new rates and explosion model of slow defraglation with delayed detonation. Evaluation of the weak rates is extended to the island of inversion and the region of neutron-rich nuclei near 78Ni, where two major shells contribute to their configurations

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 Ni66 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 Ni66 as the lightest nuclear system with shape isomerism. By employing the two-neutron transfer reaction induced by an O18 beam on a Ni64 target, at the sub-Coulomb barrier energy of 39 MeV, all three lowest-excited 0+ states in Ni66 were populated and their ? decay was observed by ?-coincidence technique. The 0+ states lifetimes were assessed with the plunger method, yielding for the 02+, 03+, and 04+ decay to the 21+ 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 04+ to the spherical 21+ state, although overestimating its value. This result makes Ni66 a unique nuclear system, apart from U236,238, in which a retarded ? transition from a 0+ deformed state to a spherical configuration is observed, resembling a shape-isomerlike behavior. © 2017 American Physical Society