For the first time, physicists have clearly observed a rare and complex motion of atomic nuclei called longitudinal wobbling.

An atomic nucleus is composed of protons and neutrons, collectively called nucleons. In an excited state, some nuclei with an odd number of nucleons exhibit a complex wobble — like that of a spinning top — because of their imbalanced geometry. Previous observations detected wobbling around only either the longest or shortest axis of nuclei having fewer than 170 nucleons.

Nirupama Sensharma at the University of Notre Dame in Indiana and her colleagues bombarded a ytterbium target with fluorine ions to produce the form, or isotope, of gold called gold-187. Analysis of gamma rays that the nuclei produced as they decayed indicated that the nuclei were wobbling. This makes 187Au the heaviest known wobbling isotope.

The team detected the 187Au nucleus wobbling about an axis that was neither its longest nor its shortest, but was of an intermediate length — the first clear observation of such behaviour.