Reinterpretation of excited states in Po 212: Shell-model multiplets rather than α -cluster states

19 pags., 14 figs., 3 tabs.
A γ-ray spectroscopic study of Po212 was performed at the Grand Accélérateur National d'Ions Lourds, using the inverse kinematics α-transfer reaction C12(Pb208,Po212)Be8 and the AGATA spectrometer. A careful analysis based on γγ coincidence relations allowed us to establish 14 new excited states in the energy range between 1.9 and 3.3 MeV. None of these states, however, can be considered as candidates for the levels with spins and parities of 1- and 2- and excitation energies below 2.1 MeV, which have been predicted by recent α-cluster model calculations. A systematic comparison of the experimentally established excitation scheme of Po212 with shell-model calculations was performed. This comparison suggests that the six states with excitation energies (spins and parities) of 1744 (4-), 1751 (8-), 1787 (6-), 1946 (4-), 1986 (8-), and 2016 (6-) keV, which previously were interpreted as α-cluster states, may in fact be of positive parity and belong to low-lying shell-model multiplets. This reinterpretation of the structure of Po212 is supported by experimental information with respect to the linear polarization of γ rays, which suggests a magnetic character of the 432-keV γ ray decaying from the state at an excitation energy of 1787 keV to the 61+ yrast state, and exclusive reaction cross sections.
This project has received funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No. 654002 and FEDER funds. We furthermore acknowledge support from the Spanish Ministerio de Ciencia, Inovación y Universidades under contracts FPA2014-57196-C5, FPA2017-84756-C4, and SEV-2014-0398, the Generalitat Valenciana under Grant No. PROMETEO/2019/005, the Swedish Research Council (Vetenskapsrådet, VR 2016-3969), and the German BMBF under Contracts No. 05P18PKFN9 and No. 05P19PKFNA.