Structure of the N=126 nuclide 212Rn: Valence and core excited configurations

The level scheme of Rn-212 has recently been expanded and extended to spins of similar to 39h and excitation energies of about 13 MeV using the Hg-204(C-13,5n)Rn-212 reaction and gamma-ray spectroscopy. Time-correlated techniques were used to obtain channel selectivity and improved sensitivity. New gamma-ray branches from states associated with valence proton configurations as well as a number of new states below the 22(+) isomer have been identified. The excitation energy of the 22(+) core excited isomer itself has been established through the observation of several branches parallel to the main decay, implying a transition energy of 7.6 keV for the previously unobserved decay to the 20(2)(+) state. The level scheme above the 22(+) isomer includes two new isomers with tau=25(2) ns and tau=12(2) ns placed at 12,211 and 12,548 keV, respectively. These are attributed to configurations involving triple neutron core excitations coupled to the aligned valence protons. The results are compared to semiempirical shell-model calculations, which can account for many of the states observed, with considerable precision for the valence proton configurations but with significant energy discrepancies for some core excited configurations. Calculations within the deformed independent particle model (DIPM) have also been carried out for the main core excited configurations at high spin and compared with both experiment and the empirical shell-model approach. The possible sources of discrepancies in both approaches are discussed, and it is suggested that anomalously low excitation energies are predicted for specific configurations in the DIPM.