Extreme ultraviolet spectra of highly charged ions put lutetium's nucleus below ytterbium's
NIFS-centered researchers measured four highly charged ions and report lutetium's nucleus is slightly smaller than ytterbium's. Earlier evaluations had odd-proton lutetium as the larger nucleus, an anomaly left unexplained for years.
The Scientist · Science desk
Key figures: lutetium, whose nucleus was found slightly smaller, has atomic number 71; ytterbium has atomic number 70. Ions Lu59+ and Yb58+ each have 12 bound electrons; Lu60+ and Yb59+ each have 11.
| Figure | Value | Note | Claim |
|---|---|---|---|
| Lutetium | 71 atomic number | 4 | |
| Ytterbium | 70 atomic number | 4 | |
| Lu59+ and Yb58+, each | 12 bound electrons | 10 | |
| Lu60+ and Yb59+, each | 11 bound electrons | 10 |
What happened
- Electron scattering, muon atomic spectroscopy and laser spectroscopy had all measured nuclear radii, but comparing different elements directly was not easy.
- The nuclear-size effect on atomic energy levels is normally extremely small, so the group used highly charged ions, whose bound electrons sit close to the nucleus, to amplify it.
- At the University of Electro-Communications' Tokyo-EBIT, the team alternately measured extreme ultraviolet emission spectra from lutetium and ytterbium ions.
Why it matters
- What's now possible capability Heavy-element radii become reachable from extreme ultraviolet lines of highly charged ions, a technique first developed for relativistic atomic physics and fusion science.
- What it limits constraint The radii are inferred, not read off: line-energy differences became radius differences only through atomic structure calculations, so the ordering is only as sound as those calculations.
- What doesn't add up contradiction The new radii cut against the picture from conventional data and also point to even-odd effects current nuclear structure theory cannot fully explain, so the anomaly goes and a theory gap stays.
The four ions pair up cleanly. Lutetium is atomic number 71 and ytterbium 70 [4], so Lu59+ and Yb58+ each hold 12 bound electrons, and Lu60+ and Yb59+ each hold 11 [10]. We'd expect matched electron counts to simplify the atomic structure calculation, since each pair then differs mainly in the nucleus.
The design choice that matters is the shared apparatus. Both elements were run on Tokyo-EBIT, with the spectra measured alternately [3]. In our view that guards against instrument drift and against offsets between laboratories. The old reversal came from evaluations based on past experiments [12]. This comparison comes from one instrument.
The thing this doesn't tell you is how large the gap is. The phys.org account of the paper calls lutetium "slightly smaller" [4] and does not include a radius difference or an uncertainty, so the margin over zero is out of view. The denominator is small too: two elements, two charge states each, four ions in all [11].
We think the direction is the finding to take seriously and the magnitude is the one to wait for. The paper, in Nature Communications [5], is hedged to match, according to that account: the result "suggests" the expected odd-even staggering holds, "implying" the "reversal problem" has been resolved [13].
The group says it will extend the work to neighboring rare-earth elements and radioactive nuclides, to determine whether the effect is general or specific to certain nuclides [8].
What to watch
- The paper's own radius difference and uncertainty for lutetium and ytterbium, which would show how far the 'slightly smaller' result sits from zero.
- An independent measurement of the lutetium-ytterbium ordering with a different technique or instrument.
- Nuclear structure calculations that reproduce the even-odd effect the group says current theories cannot fully explain.
Clarity's read
What the record supports and how the coverage leans. The claims behind it follow.
Reality
- Evidence58
- Adoption
- Not scored
- Hype gap+15
- Incentives
- Not scored
- Confidence55
Claim ledger
Ranked by verification strength, evidence, and original report placement.
- [1]
Electron scattering, muon atomic spectroscopy and laser spectroscopy of neutral atoms and singly charged ions have been used to measure nuclear radii, but it has not been easy to directly compare different elements.
- [2]
The volume shift caused by nuclear size is normally extremely small, so the group used highly charged ions, whose bound electrons sit near the nucleus, to amplify it.
- [3]
The researchers generated highly charged ions of lutetium and ytterbium at the University of Electro-Communications' Tokyo-EBIT, alternately measured high-resolution extreme ultraviolet emission spectra, and compared emission line energies of Lu59+, Lu60+, Yb58+ and Yb59+.
- [4]
Using the latest high-precision atomic structure calculations, the team converted observed differences in emission line energies into differences in nuclear radii, which showed that the lutetium nucleus (atomic number 71) is slightly smaller than that of ytterbium (atomic number 70).
- [5]
An international collaboration centered around NIFS, the University of Electro-Communications, Clemson and Grenoble-Alpes demonstrated the method, and the study is published in Nature Communications.
- [6]
The method enables direct comparisons between different elements, which were difficult with laser spectroscopy of neutral atoms or singly charged ions, and provides a new approach to nuclear structure research in the heavy-element region.
- [7]
The rare-earth nuclear radii obtained call into question the picture based on conventional experimental data and suggest significant even-odd effects that cannot be fully explained by current nuclear structure theories.
- [8]
The researchers plan to extend the work to neighboring rare-earth elements and radioactive nuclides to determine whether the phenomenon is general or specific to certain nuclides.
- [9]
The research applies highly charged ion spectroscopy, a technique developed to advance understanding of relativistic atomic physics and nuclear fusion science, to nuclear research.
- [10]
Lu59+ and Yb58+ each have 12 bound electrons, and Lu60+ and Yb59+ each have 11.
- [11]
The comparison covers two elements with two charge states each, four ions in all.
- [12]
Evaluations based on past experiments had shown an anomalous trend not seen in other rare-earth elements: the odd-proton lutetium nucleus was larger than the adjacent even-proton nucleus, a problem that remained unsolved for many years.
- [13]
The 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.
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Sources
1 independent publisher whose own reporting we read for this story.
- phys.orgNew spectroscopy method enables direct comparisons of nuclear sizes across heavy elements
1 article · October 15, 2026
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