Atomic spectroscopy used to resolve one nuclear size anomaly and reveal another

Researchers have used a new approach to measure differences in the sizes of atomic nuclei, resolving a longstanding discrepancy while uncovering a new finding that current nuclear theories cannot fully explain.

The size of an atomic nucleus, measured by its nuclear charge radius, provides information about how protons and neutrons are arranged within it and is used to understand nuclear structure and deformation.

Electron scattering experiments, muon atomic spectroscopy, and laser spectroscopy of neutral atoms and singly charged ions have been used to measure nuclear radii, but measuring differences in nuclear size between elements has remained challenging.

Previous measurements found the nuclear radii of lutetium to be larger than that of ytterbium, an anomalous finding among rare-earth elements where the typical trend is a slight zigzag pattern between nuclei with an even number of protons (such as ytterbium) and those with an odd number (such as lutetium).

Aiming to resolve this puzzle, researchers used extreme-ultraviolet spectroscopy to study atoms from which many electrons had been removed, a method that allows direct comparisons between different elements. The results were published in Nature Communications.

Using an electron beam ion trap at the University of Electro-Communications in Tokyo, the group precisely measured the light emitted by the highly charged ions. This method amplified an effect where the size of the nucleus causes a shift in the atom's energy levels.

The team generated highly charged ions of lutetium and ytterbium, specifically comparing the emission line energies of Lu⁵⁹⁺, Lu⁶⁰⁺, Yb⁵⁸⁺, and Yb⁵⁹⁺. The results revealed that the nucleus of lutetium is slightly smaller than that of ytterbium, indicating that these elements in fact do follow the expected trend.

The variation in the nuclear sizes was larger than expected, suggesting that the trend may be more pronounced than previously thought, a result that cannot be fully explained by current nuclear structure theories.

The team plans to apply the technique to additional nuclei to determine if they follow this trend. According to the National Institute of Fusion Science in Gifu, Japan, the team believes this work will "contribute to a wide range of interdisciplinary research fields, including fusion plasma diagnostics, astrophysics, and the development of short-wavelength light sources."