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The yrast bands in Pu andPu have been studied up to spin 26+ by Coulomb excitation using 08Pb beams of 5.1 and 5.3 MeV/u. In the case of 4Pu a pronounced backbending has been observed for the first time in an actinide nucleus. Microscopic calculations are presented which indicate that the observed anomalies in the yrast sequences of both nuclei are due to alignment effects in the i &3g& proton shell.PACS numbers: 21.10.Be, 25.70.Cd, 27.90.+b Our understanding of the structure of heavy nuclei has been influenced decisively by the discovery of the backbending effect' and its subs'equent experimental and theoretical investigations. ' These irregularities occur in the yrast sequences of many deformed nuclei in particular in the lanthanide region. It was therefore rather surprising that in our investigation of the high-spin states of the equally strongly deformed actinide nuclei "'Th, "'"'"'U, and "'Cm by Coulomb excitation with "'Pb projectiles, no pronounced backbending effect could be observed. ' ' In these nuclei the ground-state band was investigated up to spin 30' corresponding to a rotational frequency of 270 keV. Thus the spin as well as the frequency at which the backbending occurs in the lanthanides had been exceeded considerably in these experiments without observing any strong irregularity.It is generally accepted that the backbending effect is caused by the rotational alignment of high angular momentum orbitals due to the Coriolis interaction. ' In the actinide region the i»» proton and the j»» neutron shells are both in the vicinity of the Fermi surface and can thus easily align. Indeed, the measured increase of the g factor of the yrast state in "'Th and "'U with increasing spin can be attributed to an increasing contribution of the magnetic moment of an aligned i»» proton pair to the magnetic moment produced by the rotating core. ' Prom the study of the energy spectra of the odd nuclei "'U and "'Np it can also be deduced that the i»"proton alignment plays an important role in the structure of high-spin states of the actinide nuclei. ' However, a backbending effect is only expected in case the interaction between the ground-state band (g band) and the two-quasiparticle aligned band (s band) is sufficiently weak. As the calculated strength of the interaction shows a periodicity in proton (neutron) number with two subsequent minima being about 6 (10) units apart, "" the nonoccurrence of a strong backbending in the actinide isotropes studied so far might thus result from the fortuitous circumstance that all nuclei investigated so far exhibit a strong interaction between the g band and the s band. Actually the series of the go Th g2U and "Cm isotope s spans a range of && =6, i.e. , at least one oscillation in the interaction strength; consequently, the 1983 The American Physical Society

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A ^-isomeric state in 182 0s with spin /= (20 ±2)H and half-life equal to 100 ns is found to decay in flagrant violation of the usual A^-selection rules. The observations-still preliminary-are on the other hand readily understood if it is assumed that the yrast configuration of 182 Os becomes triaxial above the backbend, and that the decay proceeds via the numerous collective levels of the triaxial rotor.

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