Reinterpretation of excited states in $^{212}$Po: Shell-model multiplets rather than $\alpha$-cluster states

A $\gamma$ -ray spectroscopic study of $^{212}$Po was performed at the Grand Accélérateur National d’Ions Lourds, using the inverse kinematics α-transfer reaction $^{12}$C($^{208}$Pb, $^{212}$Po) $^8$Be and the AGATA spectrometer. A careful analysis based on $\gamma \gamma$ 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 αcluster model calculations. A systematic comparison of the experimentally established excitation scheme of $^{212}$Po 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 $^{212}$Po is supported by experimental information with respect to the linear polarization of $\gamma$ rays, which suggests a magnetic character of the 432-keV $\gamma$ ray decaying from the state at an excitation energy of 1787 keV to the $6^+_1$ yrast state, and exclusive reaction cross sections.