Approaching complete low-spin spectroscopy of Bi 210 with a cold-neutron capture reaction

The low-spin structure of the Bi210 nucleus was investigated in the neutron capture experiment Bi209(n,γ)Bi210 performed at ILL Grenoble at the PF1B cold-neutron facility. By using the EXILL multidetector array, consisting of 46 high-purity germanium crystals, and γγ-coincidence technique, 64 primary γ rays were observed (40 new) and a total number of 70 discrete states (33 new) were located below the neutron binding energy in Bi210. The analysis of the angular correlations of γ rays provided information about transitions multipolarities, which made it possible to confirm most of the previously known spin-parity assignments and helped establish new ones. The obtained experimental results were compared to shell-model calculations involving one-valence-proton, one-valence-neutron excitations outside the Pb208 core. It has been found that while up to the energy of ∼2 MeV each state observed in Bi210 has its calculated counterpart; at higher excitation energies some levels cannot be described by the valence particle couplings. These states may arise from couplings of valence particles to the 3- octupole phonon of the doubly magic Pb208 core and may serve as a testing ground for models which describe single particle-phonon excitations.