Backbending in the pear-shaped $^{223}_{90}$Th nucleus: Evidence of a high-spin octupole to quadrupole shape transition in the actinides

Relatively neutron-rich thorium isotopes lie at the heart of a nuclear region of nuclei exhibiting octupole correlation effects. The detailed level structure of Th-223 has been investigated in measurements of gamma radiation following the fusion-evaporation channel of the Pb-208(O-18, 3n) Th-223 reaction at 85 MeV beam energy. The level structure has been extended up to spin 49/2, and 33 new gamma rays have been added using triple-gamma coincidence data. The spins and parities of the newly observed states have been confirmed by angular distribution ratios. In addition to the two known yrast bands based on a K = 5/2 configuration, a non-yrast band has been established up to spin 35/2. We interpret this new structure as based on the same configuration as the yrast band in Th-221 having dominant K = 1/2 contribution. At the highest spin a backbending occurs around a rotational frequency of h omega = 0.23 MeV, very close to the one predicted in Th-222, where a sharp transition to a reflection-symmetric shape is expected.