New spectroscopy method enables direct comparisons of nuclear sizes across heavy elements

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.

An international collaborative research group centered around the National Institute for Fusion Science (NIFS), the University of Electro-Communications, the University of Clemson, and the University of Grenoble-Alpes has demonstrated a new method for evaluating nuclear radius using atomic spectroscopy at the University of Electro-Communications' experimental apparatus, Tokyo-EBIT.

The study is published in the journal Nature Communications.

To evaluate nuclear radii, the international collaborative research group focused on the effect in which the size of the nucleus causes a shift in the atom's energy levels (volume shift). Normally, this energy shift is extremely small because the nucleus is tiny relative to the bound electrons in the atom.

Therefore, the group adopted highly charged ions (HCIs) as the spectroscopic target to amplify this shift and accurately evaluate the nuclear radius. Since the bound electrons in HCIs are located near the nucleus, the volume shift is strongly reflected in the energy levels.

In the experiment, the researchers generated HCIs of lutetium (Lu) and ytterbium (Yb) using Tokyo-EBIT at the University of Electro-Communications. They alternately measured high-resolution extreme ultraviolet emission spectra and compared the emission line energies of Lu⁵⁹⁺, Lu⁶⁰⁺, Yb⁵⁸⁺ and Yb⁵⁹⁺.

Using the latest high-precision atomic structure calculations, the team converted the observed differences in emission line energies into differences in nuclear radii, which revealed that the nucleus of lutetium (atomic number 71) is slightly smaller than that of ytterbium (atomic number 70).

This result suggests that the expected odd-even staggering in the atomic-number dependence of nuclear radii in the rare-earth region holds for Yb-Lu, implying that the long-debated "reversal problem" regarding the lutetium nuclear radius has been resolved.

Significance of the research findings and future directions

The researchers demonstrated that the nuclear radii of heavy elements can be evaluated using extreme ultraviolet spectroscopy of HCIs. In particular, this method enables "direct comparisons between different elements"—which were difficult to achieve using conventional techniques such as laser spectroscopy of neutral atoms or singly charged ions—and provides a new approach to nuclear structure research in the heavy-element region.

Furthermore, the nuclear radii of rare-earth elements obtained in this study not only call into question the picture based on conventional experimental data but also suggest the existence of significant even-odd effects that cannot be fully explained by current nuclear structure theories, thereby highlighting new challenges in the study of heavy deformed nuclei.

Going forward, the researchers plan to expand research scope beyond the elements studied in this work—lutetium and ytterbium—to include neighboring rare-earth elements and radioactive nuclides to determine whether this phenomenon is general or specific to certain nuclides. Systematic measurements using HCI spectroscopy are expected to lead to new insights into nuclear structure.

Furthermore, this research applies HCI spectroscopy—a technique developed to advance our understanding of relativistic atomic physics and nuclear fusion science—to nuclear research.

Through this study, the team were able to further refine related HCI spectroscopy techniques, such as high-precision wavelength analysis, methods for injecting rare-earth elements into the experimental apparatus and high-precision atomic structure calculations.