Researchers using the Tokyo-EBIT device at the University of Electro-Communications have resolved a long-debated anomaly in nuclear physics. By measuring extreme ultraviolet emission spectra of highly charged lutetium and ytterbium ions, scientists determined that the lutetium nucleus is slightly smaller than that of ytterbium.
Resolving the Lutetium Nuclear Radius Anomaly
The size of a nucleus, known as the nuclear charge radius, serves as an important physical quantity that reflects the arrangement of protons and neutrons within the nucleus. Investigating variations in this charge radius can broaden our insight into nuclear deformation and structure, especially within deformed nuclei like those found in rare earth elements.
Past approaches for determining nuclear radii have relied on laser spectroscopy of singly charged ions and neutral atoms, muon atomic spectroscopy, and electron scattering experiments, yet making direct comparisons across various elements has remained challenging. To overcome this hurdle, an international collaborative research group adopted highly charged ions as the spectroscopic target to amplify the effect in which the size of the nucleus causes a shift in the atom’s energy levels, known as a volume shift.
Tokyo-EBIT and Extreme Ultraviolet Spectroscopy
Energy levels exhibit a strong influence from the volume shift because bound electrons in highly charged ions reside very close to the nucleus. The researchers utilized the Tokyo-EBIT device located at the University of Electro-Communications. Developed in the 1990s, this electron beam ion trap provides a laboratory plasma in which a monoenergetic electron beam is compressed by a magnetic field to achieve high density, stripping away numerous electrons through frequent collisions.
Operating this apparatus allowed researchers to alternately measure high-resolution extreme ultraviolet emission spectra and compare the emission line energies of Lu59+, Lu60+, Yb58+, and Yb59+.
By translating the measured shifts in emission line energies into nuclear radius differences through state-of-the-art, high-precision atomic structure calculations, the team discovered that the lutetium nucleus (atomic number 71) is marginally smaller than the ytterbium nucleus (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.
Broader Implications for Heavy Deformed Nuclei
Beyond settling the lutetium debate, the findings introduce new questions for nuclear structure theory. In this study, the researchers demonstrated that the nuclear radii of heavy elements can be evaluated using extreme ultraviolet spectroscopy of highly charged ions. In addition, this investigation extends highly charged ion spectroscopy—a methodology originally nurtured to advance nuclear fusion science and relativistic atomic physics—into the realm of nuclear studies.
Improvements in associated highly charged ion spectroscopy methodologies—including advanced atomic structure calculations, techniques for introducing rare-earth elements into the experimental setup, and precise wavelength analysis—were successfully achieved by the team during this investigation. Moving ahead, they plan to aggressively advance spectroscopic studies of highly charged ions using these capabilities, with the goal of supporting diverse multidisciplinary domains such as astrophysics, fusion plasma diagnostics, and the creation of short-wavelength light sources.