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

Excited-state lifetimes were measured by direct fast-timing methods in three N = 125 isotones - Po-209, Rn-211, and Ra-213 - near doubly magic Pb-208. These nuclei have a single neutron hole and successively add pairs of protons relative to Pb-208. The first-excited state to ground-state transition, 5/2(1)(-) -> 1/2(1)(-), has almost identical energy in each isotone and can be associated with the single neutron-hole transition nu f(5/2)(-1) -> nu p(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. (C) 2021 The Author(s). Published by Elsevier B.V.