The single closed-neutron-shell, one proton-hole nucleus 207Tl was populated in deep-inelastic collisions of a 208Pb beam with a 208Pb 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.
Wilson, E., Podolyák, Z., Grawe, H., Brown, B. A., Chiara, C. J., Zhu, S., … Walker, P. M. (2015). Core excitations across the neutron shell gap in 207Tl. Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 747, 88–92. https://doi.org/10.1016/j.physletb.2015.04.055