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

Area of Science:

• Nuclear Physics
• Atomic and Molecular Physics

Background:

• Investigating the structure of very neutron-rich nuclei provides insights into nuclear forces and shell closures.
• The region south of the double-magic nucleus Tin-132 (Z=50, N=82) is crucial for understanding nuclear shell evolution.

Purpose of the Study:

• To perform the first spectroscopic study of the very neutron-rich nucleus Silver-128 (128Ag).
• To identify and characterize new nuclear states, particularly isomers, and their configurations.
• To test the validity of the seniority scheme in odd-odd nuclei near the N=82 shell closure.

Main Methods:

• Utilized radioactive isotope beams at the Radioactive Isotope Beam Factory (RIKEN).
• Performed spectroscopic studies to analyze the energy levels, spins, and parities of ^{128}Ag.
• Conducted shell model calculations to interpret the observed nuclear structure and interactions.

Main Results:

• Identified a new seniority isomer in ^{128}Ag with a half-life of 1.60(7) μs, proposed spin-parity 16⁻.
• The isomer's configuration involves three proton holes (g9/2) and one neutron hole (h11/2).
• Observed level structure is well-described by shell model calculations, supporting the seniority scheme near ^{132}Sn.
• Proton-neutron interactions explain the inversion of low-lying states between ^{128}Ag and ^{130}In.
• Spectroscopic features resemble seniority structures in neighboring semimagic nuclei (e.g., ^{128}Pd, ^{130}Cd).

Conclusions:

• The N=82 shell closure remains robust in silver isotopes, as evidenced by the spectroscopic features of ^{128}Ag.
• The study provides a clear case for the seniority scheme in odd-odd nuclei.
• Seniority-nonconserving proton-neutron interactions play a key role in the observed level inversions.