One-proton-hole, one-neutron-hole excitations at low spins in Tl206

The low-spin structure of the Tl206 nucleus was studied in the thermal neutron capture reaction Tl205(n,γ)Tl206 at the Institut Laue-Langevin in Grenoble making use of the multidetector HPGe array Fission Product Prompt γ-ray Spectrometer and γγ-coincidence techniques. The information on discrete structures located below the neutron binding energy in Tl206 was extended: a total number of 99 γ rays (75 new) were observed and 21 excited states (8 new) were located. The analysis of the angular correlations of γ rays was used to extract information on transitions multipolarities, which helped with spin-parity assignments for the located levels. The obtained experimental results were compared to shell-model calculations involving one-proton-hole, one-neutron-hole excitations below the Pb208 core. The two-body nucleon-nucleon realistic interactions derived from CD-Bonn free nucleon-nucleon potential were used. Reasonable agreement is obtained for the excitation energies of the states which, according to calculations, have highly fragmented wave functions, particularly in the high-energy region where the density of levels increases. The observed discrepancies are interpreted as a consequence of the large uncertainties in the determination of the off-diagonal matrix elements of the realistic shell-model interaction, which are mainly responsible for the fragmentation of the wave functions.