HIGH-SPIN STATES IN IR-181 AND BACKBENDING PHENOMENA IN THE OS-PT REGION

The Tm-169(O-16,4n)181Ir reaction has been employed to investigate the high spin states of "'Ir using in-beam gamma-spectroscopy. A well-developed system of levels built on the h 9/2 subshell was identified up to a maximum spin of (41/2-). Two rotational bands built on the isomeric states with tau-1/2 = 0.33-mu-s (E(x) = 289.2 keV) and 0.13-mu-s (E(x) = 366.2 keV), respectively, were observed. The deduced g(K) values of 1.19 +/- 0.11 and 1.50 +/- 0.12 indicate Nilsson assignments of 9/2-[514] and 5/2+[402], respectively, for the bandheads of these bands. A high spin (I greater-than-or-equal-to 19/2) isomer with tau-1/2 = 22 ns was found at an excitation energy above 1.96 MeV. The experimental results are discussed in terms of rotational models including Coriolis coupling and providing for a stable triaxial shape of the 181Ir nucleus.