Neutron-rich Pm ($Z=61$) isotopes were studied by delayed $\gamma$-ray spectroscopy at RIBF, RIKEN Nishina Center using the in-flight fission of a 345 MeV/nucleon ${}^{238}\mathrm{U}$ beam. A cluster-type Ge detector array, EURICA, was used to measure the delayed $\gamma$ rays from stopped ions. Isomeric $\gamma$ decays were observed in ${}^{159}\mathrm{Pm}$ and ${}^{161}\mathrm{Pm}$ with half-lives of 4.97(12) $\mu \mathrm{s}$ and 0.79(4) $\mu \mathrm{s}$, respectively. Level schemes for ${}^{159}\mathrm{Pm}$ and ${}^{161}\mathrm{Pm}$ were constructed in this study. The isomeric states of ${}^{159}\mathrm{Pm}$ and ${}^{161}\mathrm{Pm}$ could be interpreted as two quasiparticle excitations of neutrons with the configurations of $\nu (7/2[633]\otimes 5/2[523\left]\right)$ and $\nu (7/2[633]\otimes 1/2[521\left]\right)$, respectively. They are analogous to the isomers that have been observed systematically in other even-mass $N=98$ and $N=100$ isotones in this region. A projected shell model calculation was performed and it reproduced the order of three-quasiparticle states only if new Nilsson parameters with an $N$-dependent spin-orbit interaction were used. This work demonstrates that the strength of spin-orbit interactions in standard Nilsson parameters needs to be modified to study the properties of neutron-rich rare-earth nuclei around $A=165$, and provides new evidence supporting the existence of the deformed $N=98$ subshell gap in odd-mass nuclei for the first time.

• Received 18 August 2020
• Revised 12 July 2021
• Accepted 9 August 2021

DOI:https://doi.org/10.1103/PhysRevC.104.L021303