New K isomers in the neutron-rich N=100 isotones Sm 162, Eu 163, and Gd 164 NEW K ISOMERS in the NEUTRON-RICH N=100 ... R. YOKOYAMA et al.

Very neutron-rich $Z\ensuremath{\sim}60$ isotopes produced by in-flight fission of a 345 MeV/nucleon $^{238}\mathrm{U}$ beam at the RI Beam Factory, RIKEN Nishina Center, have been studied by delayed $\ensuremath{\gamma}$-ray spectroscopy. New isomers were discovered in the neutron-rich $N=100$ isotones $^{162}\mathrm{Sm}, ^{163}\mathrm{Eu}$, and $^{164}\mathrm{Gd}$. Half-lives, $\ensuremath{\gamma}$-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 ${4}^{\ensuremath{-}}$ isomers known in less neutron-rich $N=100$ isotones $^{168}\mathrm{Er}$ and $^{170}\mathrm{Yb}$. A deformed Hartree-Fock with angular momentum projection model suggests ${K}^{\ensuremath{\pi}}={4}^{\ensuremath{-}}$ two-quasiparticle states with $\ensuremath{\nu}7/2[633]\ensuremath{\bigotimes}\ensuremath{\nu}1/2[521]$ 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.