Three-Quasiparticle Isomers In Odd-Even (159)'Pm-161: Calling For Modified Spin-Orbit Interaction For The Neutron-Rich Region

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 U-238 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 Pm-159 and Pm-161 with half-lives of 4.97(12) mu s and 0.79(4) mu s, respectively. Level schemes for 159 Pm and Pm-161 were constructed in this study. The isomeric states of Pm-159 and Pm-161 could be interpreted as two quasiparticle excitations of neutrons with the configurations of nu(7/2[633] circle times 5/2[523]) and nu(7/2[633] circle times 1/2[521]), 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.