Emerging collectivity in neutron-hole transitions near doubly magic ²⁰⁸Pb

Excited-state lifetimes were measured by direct fast-timing methods in three N=125 isotones — ²⁰⁹Po, ²¹¹Rn, and ²¹³Ra — near doubly magic ²⁰⁸Pb. These nuclei have a single neutron hole and successively add pairs of protons relative to ²⁰⁸Pb. The first-excited state to ground-state transition, 5/2₁⁻→1/2₁⁻, has almost identical energy in each isotone and can be associated with the single neutron-hole transition νf_5/2⁻¹→νp_1/2⁻¹. The extent to which the protons act as spectators is assessed based on the measured transition rates, which show a systematic increase along the isotone chain, and by comparisons with large-basis shell-model calculations. The shell model accounts for some of the increased transition strength but consistently underestimates the experimental values. It also fails to explain the near-constant transition energies. These results suggest emerging collectivity beyond the shell-model valence space and show that the near-constant transition energies are not a consequence of a pure neutron-hole transition, but rather the outcome of complex nucleon-nucleon correlations that increase quadrupole collectivity.