Characterization of octupole-type structures in Th 221

A detailed level scheme for $^{221}\mathrm{Th}$ has been established in an experiment using the $^{18}\mathrm{O}+{}^{207}\mathrm{Pb}$ reaction at 96 MeV. The evaporation residues from this fissile system were selected with the HERCULES detector array and residue-gated $\ensuremath{\gamma}$ rays were measured with Gammasphere. Three band structures of interlinked, alternating-parity levels are observed, two of which are non-yrast. In addition, several high-lying excitations are found. The yrast band is seen up to spin-parity $37/{2}^{\ensuremath{-}}$ and $39/{2}^{+}$, beyond which a high-spin feeding transition is observed. The non-yrast sequences are interpreted as parity-doublet structures, based on a configuration similar to that of the yrast band in $^{223}\mathrm{Th}$ $(K=5/2)$. The key properties of even-odd nuclei in this mass region $[B(E1)/B(E2)$ and $B(M1)/B(E2)$ ratios, spin alignments, parity splittings] are reviewed.