The size of a nucleus (nuclear charge radius) is an important physical quantity that reflects the arrangement of protons and neutrons within the nucleus. Particularly in deformed nuclei, such as those of rare-earth elements, examining changes in the charge radius can deepen our understanding of nuclear structure and nuclear deformation.

Until now, electron scattering experiments, muon atomic spectroscopy and laser spectroscopy of neutral atoms and singly charged ions have been used to measure nuclear radii; however, it has not been easy to directly compare different elements.

For lutetium (Lu) and ytterbium (Yb), evaluations based on past experiments had presented an anomalous trend not observed in other rare-earth elements: The nucleus of lutetium, which has an odd number of protons, is larger than that of the adjacent even-proton nucleus. This had remained an unsolved problem for many years.