New K isomers in the neutron-rich N = 100 isotones ~(162)Sm, ~(163)Eu, and ~(164)Gd

摘要

Very neutron-rich Z ～ 60 isotopes produced by in-flight fission of a 345 MeV/nucleon ~(238)U beam at the RI Beam Factory, RIKEN Nishina Center, have been studied by delayed γ -ray spectroscopy. New isomers were discovered in the neutron-rich N = 100 isotones ~(162)Sm, ~(163)Eu, and ~(164)Gd. Half-lives, γ -ray energies, and relative intensities of these isomers were obtained. Level schemes were proposed for these nuclei and the first 2~+ and 4~+ states were assigned for the even-even nuclei. The first 2~+ and 4~+ state energies decrease as the proton numbers get smaller. The energies and the half-lives of the new isomers are very similar to those of 4~- isomers known in less neutron-richN = 100 isotones ~(168)Er and ~(170)Yb. A deformed Hartree-Fock with angular momentum projection model suggests K~π = 4~- two-quasiparticle states with ν7/2[633] ? ν1/2[521] configurations with similar excitation energy. The results suggest that neutron-rich N = 100 nuclei are well deformed and the deformation gets larger as Z decreases to 62. The onset of K isomers with the same configuration at almost the same energy in N = 100 isotones indicates that the neutron single-particle structures of neutron-rich isotones down to Z = 62 do not change significantly from those of the Z = 70 stable nuclei. Systematics of the excitation energies of new isomers can be explained without the predicted N = 100 shell gap.