Isomers in Pd128 and Pd126: Evidence for a Robust Shell Closure at the Neutron Magic Number 82 in Exotic Palladium Isotopes

The level structures of the very neutron-rich nuclei Pd128 and Pd126 have been investigated for the first time. In the r-process waiting-point nucleus Pd128, a new isomer with a half-life of 5.8(8) μs is proposed to have a spin and parity of 8⁺ and is associated with a maximally aligned configuration arising from the g_9/2 proton subshell with seniority υ=2. For Pd126, two new isomers have been identified with half-lives of 0.33(4) and 0.44(3) μs. The yrast 2⁺ energy is much higher in Pd128 than in Pd126, while the level sequence below the 8⁺ isomer in Pd128 is similar to that in the N=82 isotone Cd130. The electric quadrupole transition that depopulates the 8⁺ isomer in Pd128 is more hindered than the corresponding transition in Cd130, as expected in the seniority scheme for a semimagic, spherical nucleus. These experimental findings indicate that the shell closure at the neutron number N=82 is fairly robust in the neutron-rich Pd isotopes.