Wobbling Motion in Nuclei: Transverse, Longitudinal and Chiral

Triaxial nuclear shapes are a very rare phenomena that are manifested experimentally via their unique signatures - chiral rotation and wobbling motion. The present work was aimed at studying these exotic rotational modes in two different regions of the nuclear chart viz. A ∼ 130 and 190.

Within the A ∼ 130 region, the primary focus of the present work was to study the 135Pr nucleus. This nucleus was the first case of wobbling observed outside of the well-studied A ∼ 160 region. Aided with a high-statistics measurement using the Gammasphere array at the Argonne National Laboratory, the present work was able to observe a second phonon (nω = 2) wobbling band in this nucleus. The nature of the wobbling bands was confirmed by the characteristic predominantly E2 nature of the nω+1 → nω linking transitions based on angular distribution measurements. These results sufficiently established the presence of wobbling motion in the A ∼ 130 region, independent of any particular spin or deformation.

In addition to the nω = 1 and nω = 2 wobbling bands, two chiral-partner bands with the configuration πh11/2 × νh11/2^2 have also been observed in 135Pr. Angular distribution analyses of the ∆I = 1 connecting transitions between the two chiral partners revealed a mixed M1/E2 nature. Based on these observations, 135Pr has been proposed to be the first observed case of a chiral wobbler wherein chiral rotation has been observed in coexistence with wobbling motion.

Beyond the A ∼ 130 region, the present work has also investigated the A ∼ 190 region to seek for evidence of nuclear wobbling motion. Experiments using the Gammasphere array were performed to populate levels in the 187 Au nucleus and a detailed analysis revealed two separate wobbling bands built on (πh9/2)1 and (πh1/2)−1 configurations. Based on angular distribution measurements, the ∆I = 1, E2 nature of the nω+1 → nω transitions was verified. The two structures so-observed were found to exhibit different types of wobbling: transverse and longitudinal. 187Au has hence been observed as a case of the first cleanly established longitudinal wobbler and of the possible coexistence of both forms of wobbling, a phenomenon never observed before.

Theoretical interpretations for the above work have been done within the framework of the Particle Rotor Model, the Quasiparticle Triaxial Rotor Model and the Triaxial Projected Shell Model and a good agreement with the experimental values has been obtained.