Investigating core excitations in the 131 Sn one-valence-hole nucleus ∗

The structure of the Sn-131 nucleus was studied at the Institut Laue-Langevin by prompt-delayed gamma-ray spectroscopy across the 0.3 mu s, 23/2(-) isomeric state at 4670 keV, following cold-neutron-induced fission of U-235 and Pu-241 targets. New gamma-ray transitions are observed and the results are compared with theoretical calculations performed with the Hybrid Configuration Mixing Model. The latter suggests that the majority of the states can be well-described in terms of couplings between single-hole degrees of freedom and Sn-132 core excitations, pointing to a robust neutron shell closure at N = 82.