Longitudinal Wobbling Motion in Au187

Abstract

The rare phenomenon of nuclear wobbling motion has been investigated for the nucleus 187 Au. A longitudinal wobbling-bands pair has been identified and clearly distinguished from the associated signature-partner band on the basis of angular distribution measurements. Theoretical calculations in the framework of the Particle Rotor Model (PRM) are found to agree well with the experimental observations. This is the first experimental evidence for longitudinal wobbling bands where the expected signature partner band has also been identified, and establishes this exotic collective mode as a general phenomenon over the nuclear chart.Wobbling is a collective mode that may appear when the moments of inertia of all three principal axes of the nuclear density distribution are unequal. The mode is well known in classical mechanics and its occurrence is a clear signal for a triaxial nuclear shape. For a given angular momentum, uniform rotation about the axis with the largest moment of inertia corresponds to minimal energy. At a somewhat larger energy, this axis precesses (wobbles) about the space-fixed angular momentum axis. In a quantal system such as the nucleus (or a molecule), the mode manifests itself in the appearance of rotational bands that correspond to successive excitations of wobbling phonons, n ω , and alternating signature α = α 0 + n ω , which determines the spin sequence I = α + even number. Adjacent wobbling bands n ω+1 and n ω are connected by collectively-enhanced ∆I = 1 transitions of predominantly E2 character, which are generated by the wobbling motion of the entire charged body. This is in contrast with the signature-partner bands, which represent another type of excitation involving a partial dealignment of the odd particle with respect to its preferred axis; for those, the connecting ∆I = 1 transitions are of predominantly M1 character, with very little, if any, E2 admixtures.Although predicted quite sometime ago [1], this exotic nuclear motion has been observed only rarely in experiments so far, and the list of nuclei exhibiting it is quite short: 105 Pd [2], 135 Pr [3, 4] (and, possibly, 133 La [5]), 161 Lu [6], 163 Lu [7, 8], 165 Lu [9], 167 Lu [10] and 167 Ta [11]-a total of only 7(8) nuclei across the entire nuclear chart.All the aforementioned nuclei have an odd proton occu-pying a high-j orbital, which modifies the wobbling mode. Frauendorf and Dönau [12] classified this coupled mode as "longitudinal wobbling" (LW) and "transverse wobbling" (TW) when, respectively, the odd particle aligns its angular momentum along the medium axis (the axis with the largest moment of inertia), or along one of the perpendicular axes. The wobbling energy, E wobb [see Eq.(1) below], increases with increasing angular momentum for LW while it decreases for TW [12]. In all of the cases mentioned above (except, possibly, 133 La[5]), the wobbling bands have been identified as corresponding to TW because E wobb decreases with increasing angular momentum. We report on the observation of bands structures corresponding to...

Shape coexistence in Au187 studied by laser spectroscopy

“…This jump is due to the transition from weakly oblate (A > 186) to strongly prolate-deformed (A = 183-186) shapes [6][7][8][9]. The isotope 187 Au 108 lies in the immediate vicinity of this jump and exhibits multiple-coexisting structures [10,11]. The ground state (I π = 1/2 + ) is believed to be weakly oblate, whereas the I π = 9/2 − isomer is considered as a member of the 1/2 − [541]h 9/2 band at a moderate prolate deformation (see Ref.…”

Tilted precession bands in Nd135

“…[45,46] and therefore remains unclear. The experimental evidence for longitudinal wobbling is also reported in 187 Au [47] very recently, but the polarization measurement is not performed for the I = 1 interconnecting transitions and therefore the wobbling interpretation is not solid.…”

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

“…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].…”

“…It is noted that recent experiments suggested the wobbling bands in two gold nuclei as well, i.e., 187 Au [15] and 183 Au [16]. Their low-lying negative-parity states are empirically understood as the proton πh 9/2 orbital coupled with a prolate-deformed core.…”

“…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].…”