Characterization of octupole-type structures in Th 221

A detailed level scheme for Th-221 has been established in an experiment using the O-18 + Pb-207 reaction at 96 MeV. The evaporation residues from this fissile system were selected with the HERCULES detector array and residue-gated 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(-) 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 Th-223 (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.