Alternating parity bands and octupole effects in 221Th and 223Th

Level structures of the two neutron deficient odd-A nuclei 221Th and 223Th have been investigated by means of the reactions 208Pb(16O,3n)221Th and 208Pb(18O,3n)223Th. In-beam conversion electron and γ-ray spectroscopy in coincidence with the recoiling evaporation residues (R) has been carried out, including γγR-, e-γR- and e-e-R-coincidence measurements, as well as the determination of γ-ray angular distributions and γ-ray multiplicities using the NaI crystal ball. Level schemes could be established up to 13 ℎ above the ground state in 221Th and up to 33/2+ in 223Th. In 221Th one band of alternating parity states is observed, whereas in 223Th two such bands with equal intensity are seen. The level schemes are interpreted in terms of an octupole deformed core with an unpaired nucleon. From the E1/E2 branching ratio the electric dipole moments can be deduced; for 221Th one finds vertical strokeanti D0vertical stroke = 0.33±0.06 e.fm and for 223Thvertical strokeanti D0vertical stroke = 0.44±0.09 e.fm, the largest value observed so far in this mass region. The alignments of the different bands in 223Th with simplex quantum number s=±i are approximately equal and amount to = 3.7±0.3 for the positive-parity bands, a value which is in accord with cranking calculations which assume octupole deformation. (orig.)