Characterization of octupole-type structures inTh221

Abstract

A detailed level scheme for 221 Th has been established in an experiment using the 18 O + 207 Pb reaction at 96 MeV. The evaporation residues from this fissile system were selected with the HERCULES detector array and residue-gated γ 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 223 Th (K = 5/2). The key properties of even-odd nuclei in this mass region [B(E1)/B(E2) and B(M 1)/B(E2) ratios, spin alignments, parity splittings] are reviewed.

Backbending in the pear-shaped Th90223 nucleus: Evidence of a high-spin octupole to quadrupole shape transition in the actinides

“…Concerning the 223 Th and 225 Th yrast configurations, it has been established that they are based on K > 1/2 configurations. High-lying octupole bands, which exhibit the features of an almost degenerate parity doublet, have been found in 221 Th and interpreted as based on the same K = 5/2 configuration as the yrast band in 223 Th [13]. The new band found in this work, composed of the sequences 3a and 3b in the level scheme of Fig.…”

“…For 221 Th, the 7/2 + level is accepted to be the ground state. Calculations predict a strong K = 1/2 component for the state, which leads to the lack of a parity doublet in the level scheme [13,30]. It is also the case for 227 Th which has a K = 1/2 ground state configuration.…”

“…4. Reviol et al [13] have extended the study to odd-N nuclei. They concluded that the rotation-induced condensation of octupole phonons carries over to the odd-N nuclei even if the presence of an additional neutron in the core seems to reduce the fluctuations in the octupole degree of freedom.…”

“…This is what is observed in nuclei such as 219,223,225 Th [8,27,28] and also 221 Ra [29], where K 3/2: two quasidegenerate bands appear. Let us notice that the case of odd-mass nuclei with K = 1/2 such as 221 Th is particular: one of the bands is shifted up due to Coriolis effects and no parity doubling appears, the level scheme being similar to that of even-even isotopes [ [8,13], and references therein].…”

“…One can notice that a similar reaction was used in Ref. [8] to produce 223 Th nuclei, while mainly 221 Th nuclei were obtained by using instead either an 16 O beam [12] or a 207 Pb target [13].…”