The nucleus 163Lu has been populated through the reaction 139La(29Si,5n) with a beam energy of 157 MeV. Three triaxial, strongly deformed (TSD) bands have been observed with very similar rotational properties. The first excited TSD band has earlier been assigned as a one-phonon wobbling excitation built on the lowest-lying (yrast) TSD band. The large B(E2)(out)/B(E2)(in) value obtainable for one of four observed transitions between the second and first excited TSD bands is in good agreement with particle-rotor calculations for a two-phonon wobbling excitation.

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The collective wobbling mode, the strongest signature for the rotation of a triaxial nucleus, has previously been seen only in a few Lu isotopes in spite of extensive searches in nearby isotopes. A sequence of transitions in the N = 94 167 Ta nucleus exhibiting features similar to those attributed to the wobbling bands in the Lu nuclei has now been found. This band feeds into the πi 13/2 band at a relative energy similar to that seen in the established wobbling bands and its dynamic moment of inertia and alignment properties are nearly identical to the i 13/2 structure over a significant frequency range. Given these characteristics, it is likely that the wobbling mode has been observed for the first time in a nucleus other than Lu, making this collective motion a more general phenomenon. PACS number(s): 21.10. Re, 23.20.Lv, 27.70.+q Our understanding of the wobbling mode in nuclei (and the associated stable triaxial deformation) has evolved quickly over the past decade. Bohr and Mottelson [1] first proposed that the rotation of a stable triaxially deformed nucleus would result in the presence of wobbling excitations. These excitations occur because the rotational angular momentum is not aligned with any of the body-fixed axes; rather it precesses and wobbles around one of these axes in a manner similar to that of an asymmetric top.In 1995, Schnack-Petersen et al.[2] first suggested that rotational bands based on proton i 13/2 excitations in 163,165 Lu are associated with a triaxial strongly deformed (TSD) potential well. The large deformation is mainly due to the occupation of the intruder i 13/2 orbital, and the triaxial deformation (γ = 20 • ) results from an N = 94 shell gap that develops with enhanced quadrupole deformation ( 2 ≈ 0.37). No direct experimental evidence for triaxiality was observed until the wobbling mode was confirmed in 163 Lu by Ødegård et al.[3]. This seminal work established the existence of a band feeding into the πi 13/2 structure where the two sequences have nearly identical moments of inertia and alignments over a large frequency range. The similarities of the moments of * Present address: inertia and alignments are a predicted feature for a wobbling band as the intrinsic structure for both bands should be the same; the only difference between the two is the degree to which the rotational angular momentum vector lies off axis. The collective wobbling behavior can thus be described within a phonon model, where the energy of each band is equal to E =¯h 2 2J I (I + 1) +hω w (n w + 1/2), wherehω w = hω rot (J x − J y )(J x − J z )/(J y J z ) [1]. The n w = 0 phonon number is assigned to the energetically lowest band in the family, as its angular momentum vector lies closest to a body axis, and in the case of the Lu isotopes, this is associated with the πi 13/2 band. Wobbling excitations with n w = 1, 2, 3, etc. then follow, each lying successively higher in energy as the rotational angular momentum vector progressively lies farther from the body axis with increasing n w . Indeed, Jense...

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