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Figure: Two predicted rotations of a nucleus that has ellipsoidal triaxial shape. Short, medium, and long principal axes are directed along the z, x, and y axes, respectively. Thick vertical arrow labelled R denotes the axis of rotation. Left: 105Pd shown in rotation, which has the short z axis of the triaxial shape aligning with the rotation axis R; this type of rotation is an analogue of quantum mechanical rotation of 105Pd in the lowest energy state for a given spin. Right: 105Pd shown with the short z axis of the triaxial shape tilted with respect to the rotation axis R and undergoing wobbling motion about the rotation axis. This rotation is an analogue of quantum mechanical rotation of 105Pd in energy states greater than the lowest one for a given spin.

A wobbling nuclear rotor: from the shape of 105Pd to nucleon-nucleon interactions 

The motivation – This study was designed to provide information on the shape, structure, and excitation modes of an isotope of palladium, 105Pd, a nucleus expected to have an ellipsoidal shape with three principal axes of different length, similar to the shape of a kiwi fruit. Atomic nuclei are aggregates of interacting protons and neutrons (collectively called nucleons) that organize themselves into an optimal shape, similar to the way that a liquid drop behaves. While shapes of spherical or axial symmetry are common among nuclei, asymmetric shapes, while often predicted, are harder to find. The triaxial shape in 105Pd has been identified through observation of the wobbling rotational motion, a motion in which the shape of the nucleus rotates around an axis that is tilted with respect to its principal axes. This motion is similar to that observed for a spinning top rotating around a nonvertical axis. The symmetries of quantum mechanics forbid wobbling in rotation of an axially symmetric shape; therefore, the identification of wobbling is direct evidence for the asymmetric triaxial shape of the 105Pd nucleus.

The discovery – Nuclear scientist Kris Starosta (Simon Fraser University) and research team members from Hungary, China, the UK, Spain, Poland, France, Japan, Bulgaria and the USA identified a series of quantized energy states forming two rotational bands above the set of the lowest-energy levels for a given spin. In addition, the team identified and characterized electro-magnetic transitions between observed states. The characterization of these transitions required detailed studies of intensity as a function of emission angle as well as a measurement of the direction of oscillation of the electric field for a number of observed transitions. The transitions turned out to have mainly electric quadrupole character. This was a key finding, because the expected signature of a wobbling rotation is to have a set of rotational bands connected by electric quadrupole transitions. Alternative explanations can be ruled out as they do not predict the sequence of energy states consistent with the observations or they predict transitions of magnetic monopole character that are inconsistent with the measurements reported in the paper.

Its significance – The identification of a triaxial shape in 105Pd provides conclusive evidence that interactions of protons and neutrons in this nucleus result in a shape that is not axially symmetric. The force binding nucleons into atomic nuclei is one of the four fundamental forces, yet it remains poorly understood. Understanding these forces is paramount for the development of nuclear models needed to predict the behaviour of nuclear systems under conditions that cannot be directly accessed for measurement, ranging from nuclear reactor cores to astrophysical sites for nucleosynthesis. Thus, the studies of 105Pd enable stringent testing of nuclear models and will contribute to a better understanding of effective nucleon-nucleon interactions in atomic nuclei.

Read the paper – “Experimental Evidence for Transverse Wobbling in Pd-105” by Timar, J; Chen, QB; Kruzsicz, B; Sohler, D; Kuti, I; Zhang, SQ; Meng, J; Joshi, P; Wadsworth, R; Starosta, K; Algora, A; Bednarczyk, P; Curien, D; Dombrádi, Z; Duchêne, G; Gizon, A; Gizon, J;, Jenkins, DG; Koike, T; Krasznahorkay, A; Molnár, J; Nyakó, BM; Paul, ES; Rainovski, G; Scheurer, JN; Simons, AJ; Vaman, C; Zolnai, L. Physical Review Letters 122(6):062501 (2019). DOI: 10.1103/PhysRevLett.122.062501.

Website article compiled by Jacqueline Watson with Theresa Kitos