Longitudinal wobbling in 133La

Abstract

Excited states of 133 La have been investigated to search for the wobbling excitation mode in the low-spin regime. Wobbling bands with nω = 0 and 1 are identified along with the interconnecting ∆I = 1, E2 transitions, which are regarded as one of the characteristic features of the wobbling motion. An increase in wobbling frequency with spin implies longitudinal wobbling for 133 La, in contrast with the case of transverse wobbling observed in 135 Pr. This is the first observation of a longitudinal wobbling band in nuclei. The experimental observations are accounted for by calculations using the quasiparticle-triaxial-rotor (QTR) model, which attribute the appearance of longitudinal wobbling to the early alignment of a π = + proton pair.

Questioning the wobbling interpretation of low-spin bands in $γ$-soft nuclei within the interacting boson-fermion model

“…The corresponding IBFM δ values for the B2 → Yrast transitions, plotted also in Fig. 3(d), are much smaller in magnitude than these experimental values, but are rather close to the QTR ones [12].…”

“…More recent experiments have shown new evidence for wobbling bands in odd-mass nuclei of several other mass regions, observed in the low-spin regime, e.g., 135 Pr [10,11], 133 La [12], 105 Pd [13], 127 Xe [14], 187 Au [15], and 183 Au [16], as well as two-quasiparticle wobbling bands at medium spins in 130 Ba [17] and 136 Nd [18]. The wobbling interpretation of these newly found bands was based on the prediction by the quasiparticle-plus-triaxialrotor (QTR) model, in which the moment of inertia of the triaxial core is assumed to be hydrodynamic and the odd quasiparticle aligned with principal axes, within the so-called frozen approximation [19].…”

“…There are two types of wobbling motion that can occur in odd-mass nuclei [19]: longitudinal wobbling (LW), where an odd particle is aligned parallel to the axis of largest moment of inertia, and transverse wobbling (TW), with the odd particle aligned perpendicular to that axis. This wobbling picture has been applied to interpret the observed low-spin yrare bands in the aforementioned nuclei, e.g., bands in 135 Pr [10,11], 105 Pd [13], and 183 Au [16] were attributed to TW bands, and those in 133 La [12], 127 Xe [14], and 187 Au [15] to LW bands. At higher spins, the triaxial deformation can become stable to such a degree that the wobbling motion is realized, for instance, in the two-quasiparticle TW bands in the even-even nuclei 130 Ba [17] and 136 Nd [18].…”

“…In 133 La, besides the 11/2 − 1 ground-state band, the experiment by Biswas et al [12] identified the first excited band based on the 13/2 − state at E x = 618 keV as LW band. The IBFM band B2, with bandhead 13/2 − 1 at E x = 518 keV, should be compared with the proposed LW band.…”

Tilted precession bands in Nd135

“…In addition, in the A ≈ 130 mass region, one-phonon and two-phonon transverse wobbling bands were reported in 135 Pr which are built on the π h 11/2 configuration with ε 2 ≈ 0.16 and γ ≈ 26 • deformation [33,41,42]. Transverse wobbling was firstly identified in the two-quasiparticle bands of the even-even nucleus 130 Ba [43], while longitudinal wobbling has been identified in 133 La [44]. However, the interpretation of the low-lying rotational bands in 135 Pr as onephonon transverse wobbling has been seriously questioned by Tanabe et al in Refs.…”