Excited states in the N ¼ 102 isotones 166 Gd and 164 Sm have been observed following isomeric decay for the first time at RIBF, RIKEN. The half-lives of the isomeric states have been measured to be 950(60) and 600(140) ns for 166 Gd and 164 Sm, respectively. Based on the decay patterns and potential energy surface calculations, including β 6 deformation, a spin and parity of 6 − has been assigned to the isomeric states in both nuclei. Collective observables are discussed in light of the systematics of the region, giving insight into nuclear shape evolution. The decrease in the ground-band energies of 166 Gd and 164 Sm (N ¼ 102) compared to 164 Gd and 162 Sm (N ¼ 100), respectively, presents evidence for the predicted deformed shell closure at N ¼ 100. In the exploration of the nuclear landscape, it is evident that the neutron-rich side of stability contains a vast unknown territory, where approximately half of all the bound nuclides remain to be identified. Furthermore, this is the domain of rapid-neutron-capture (r process) nucleosynthesis, which is poorly understood and yet is key to the creation of chemical elements from iron to uranium (Z ¼ 26-92) in stellar environments [1]. With the advent of the current generation of radioactive-beam facilities, it is now possible to address some of the open questions PRL 113, 262502 (2014) P H Y S I C A L
The level structures of the very neutron-rich nuclei 128 Pd and 126 Pd have been investigated for the first time. In the r-process waiting-point nucleus 128 Pd, a new isomer with a half-life of 5:8ð8Þ s is proposed to have a spin and parity of 8 þ and is associated with a maximally aligned configuration arising from the g 9=2 proton subshell with seniority ¼ 2. For 126 Pd, two new isomers have been identified with half-lives of 0.33(4) and 0:44ð3Þ s. The yrast 2 þ energy is much higher in 128 Pd than in 126 Pd, while the level sequence below the 8 þ isomer in 128 Pd is similar to that in the N ¼ 82 isotone 130 Cd. The electric quadrupole transition that depopulates the 8 þ isomer in 128 Pd is more hindered than the corresponding transition in 130 Cd, as expected in the seniority scheme for a semimagic, spherical nucleus. These experimental findings indicate that the shell closure at the neutron number N ¼ 82 is fairly robust in the neutron-rich Pd isotopes.
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