First Exploration of Monopole-Driven Shell Evolution above the N=126 Shell Closure: New Millisecond Isomers in Tl 213 and Tl 215

Abstract

Isomer spectroscopy of heavy neutron-rich nuclei beyond the N=126 closed shell has been performed for the first time at the Radioactive Isotope Beam Factory of the RIKEN Nishina Center. New millisecond isomers have been identified at low excitation energies, 985.3(19) keV in Tl213 and 874(5) keV in Tl215. The measured half-lives of 1.34(5) ms in Tl213 and 3.0(3) ms in Tl215 suggest spins and parities 11/2- with the single proton-hole configuration πh11/2 as leading component. They are populated via E1 transitions by the decay of higher-lying isomeric states with proposed spin and parity 17/2+, interpreted as arising from a single πs1/2 proton hole coupled to the 8+ seniority isomer in the PbA+1 cores. The lowering of the 11/2- states is ascribed to an increase of the πh11/2 proton effective single-particle energy as the second νg9/2 orbital is filled by neutrons, owing to a significant reduction of the proton-neutron monopole interaction between the πh11/2 and νg9/2 orbitals. The new ms isomers provide the first experimental observation of shell evolution in the almost unexplored N>126 nuclear region below doubly magic Pb208. © 2024 American Physical Society.