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 ²¹³Tl and 874(5) keV in ²¹⁵Tl. The measured half-lives of 1.34(5) ms in ²¹³Tl and 3.0(3) ms in ²¹⁵Tl 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 𝐴+1 Pb 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 208Pb.