Contrasting properties of particle-particle and hole-hole excitations in 206Tl and 210Bi nuclei

A complete-spectroscopy investigation of low-lying, low-spin states in the one-proton-hole and one neutron-hole nucleus Tl-206 has been performed by using thermal neutron capture and gamma-coincidence technique with the FIPPS Ge array at ILL Grenoble. The new experimental results, together with data for the one-proton-particle and one-neutron-particle nucleus Bi-210 (taken from a previous study done at ILL in the EXILL campaign), allowed for an extensive comparison with predictions of shell-model calculations performed with realistic interactions. No phenomenological adjustments were introduced in the calculations. In Bi-210, state energies, transition multipolarities and decay branchings agree well with theory for the three well separated multiplets of states which dominate the low-lying excitations. On the contrary, in Tl-206 significant discrepancies are observed: in the same energy region, six multiplets were identified, with a significant mixing among them being predicted, as a consequence of the smaller energy separation between the active orbitals. The discrepancies in Tl-206 are attributed to the larger uncertainties in the determination of the off-diagonal matrix elements of the realistic shell-model interaction with respect to the calculated diagonal matrix elements, the only ones playing a major role in the case of Bi-210. The work points to the need of more advanced approaches in the construction of the realistic interactions. (C) 2020 The Authors. Published by Elsevier B.V.