High K structure of 180W and 181W.

High-spin states in the well deformed nuclei 180W and 181W have been populated using 136Xe(48Ca,4n) and 136Xe(48Ca,3n) reactions at 190MeV. The TESSAS gamma-ray spectrometer array at the Daresbury Laboratory was employed. In 180W, the yrast band is identified up to spin 26+(tentatively 28+). Excited rotational bands above the Kpi=14- isomer (T1/2 =2mus, 3263keV) were observed, based on intrinsic states up to Kpi=(24+). In 181W, the 9/2+[624] ground-state-band is observed to spin 57/2. Several rotational sequences and intrinsic levels were also observed, half-lives ranging up to 200ns, and K-values up to 57/2. In addition, using the CAESAR array at the A.N.U., a Fermi-aligned v(i13/2)2 rotational band (Ka8) was observed in 180W following the 176Yb(9Be,5n) reaction at 55MeV. Its structure corresponds to a t-band in the tilted cranking representation. It crosses the ground-state-band at spin 16, giving rise to backbending in the yrast sequence. Following the 176Yb(9Be,4n)181W reaction at 40MeV, the 7/2-[514] band in 181W has been extended up to spin 33/2 (37/2), with backbending at spin about 21/2 and the structures above the Kpi=21/2+ isomer have been confirmed. In 180W and 181W, very high seniority intrinsic states have been found (Ka24 in 180W and Ka57/2 in 181W), which decay partly to the I=24 and I=53/2 members of the ground-state-bands, respectively. There is complete breakdown of the K-selection rule. It is suggested that the hexadecapole deformation and t-bands may play an important role.