New $K$ isomers in the neutron-rich $N$=100 isotones $^{162}$Sm, $^{136}$Eu , and $^{164}$Gd
Résumé
Very neutron-rich Z∼60 isotopes produced by in-flight fission of a 345 MeV/nucleon U238 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 Sm162, Eu163, and Gd164. 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-rich N=100 isotones Er168 and Yb170. A deformed Hartree-Fock with angular momentum projection model suggests Kπ=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.