Seniority Structure in Neutron-Rich Nucleus Ag 128: Evidence for Robustness of N=82 Shell Closure in Silver Isotopes

The spectroscopic studies of very neutron-rich nucleus Ag128 have been performed for the first time at the Radioactive Isotope Beam Factory of RIKEN. A new seniority isomer with a half-life of 1.60(7) μs has been identified and is proposed to have a spin-parity of 16- with a maximally aligned configuration comprising three proton holes in the g9/2 orbital and one neutron hole in the h11/2 orbital. The new level structure in Ag128 is quite well described by shell model calculations without invoking excitations across the Z=50 and N=82 shell gaps, and presents a good case of seniority scheme in odd-odd nuclei in the south vicinity of the double-magic nucleus Sn132. With a classification of various components of the proton-neutron interaction, the inversion of lowest-lying 9- and 10- states between Ag128 and its neighboring isotone In130 is found to be dynamically ascribed to the seniority-nonconserving proton-neutron interaction components. The structure above 10- up to the 16- isomer in Ag128 shows remarkable similarities to seniority structures in the semimagic nuclei Pd128 and Cd130. These spectroscopic features in Ag128 indicate that the N=82 shell closure is still robust in silver isotopes.