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 223Th has been investigated in measurements of γ radiation following the fusion-evaporation channel of the 208Pb(18O ,3n) 223Th reaction at 85 MeV beam energy. The level structure has been extended up to spin 49/2, and 33 new γ rays have been added using triple-γ 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 221Th having dominant K = 1/2 contribution. At the highest spin a backbending occurs around a rotational frequency of ¯hω = 0.23 MeV, very close to the one predicted in 222Th, where a sharp transition to a reflection-symmetric shape is expected.