Core Excitations Across The Neutron Shell Gap In Tl-207

The single closed-neutron-shell, one proton-hole nucleus Tl-207 was populated in deep-inelastic collisions of a Pb-208 beam with a Pb-208 target. The yrast and near-yrast level scheme has been established up to high excitation energy, comprising an octupole phonon state and a large number of core excited states. Based on shell-model calculations, all observed single core excitations were established to arise from the breaking of the N = 126 neutron core. While the shell-model calculations correctly predict the ordering of these states, their energies are compressed at high spins. It is concluded that this compression is an intrinsic feature of shell-model calculations using two-body matrix elements developed for the description of two-body states, and that multiple core excitations need to be considered in order to accurately calculate the energy spacings of the predominantly three-quasiparticle states. (C) 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license.