Fermium-255 nucleus has a rugby-ball shape, laser data show

The team used specialized titanium laser systems to excite and ionize the atoms, with ionization removing an electron. They then resolved the hyperfine structure of two optical transitions, meaning two changes between electronic energy states involving light. That resolution separated effects that earlier equipment could not distinguish.

Scientists at Johannes Gutenberg University Mainz first observed atomic energy levels in fermium more than 20 years ago. The technology available then could not resolve the fine details needed for accurate nuclear structure data, leaving some parameters incomplete or inconsistent.

The surrounding electron cloud served as a sensitive, indirect probe of the nucleus.

How did the team obtain enough fermium-255?

Fermium is an artificially produced radioactive element, and the chain needed to obtain enough of it took months. The researchers first exposed transuranium material—elements heavier than uranium—to neutron beams for months at the High Flux Isotope Reactor at Oak Ridge National Laboratory in the United States. That process produced einsteinium-254.

After initial experiments in the United States, the material went to Mainz, Germany, for processing and then to the Institute Laue-Langevin in France for additional neutron irradiation. The second irradiation produced einsteinium-255, which decays into fermium-255; the material then returned to Mainz for spectroscopy.

Einsteinium-255 has a half-life of about 40 days, meaning half its atoms decay during that period. That interval allowed it to supply fermium-255 over several weeks. Repeated chemical separations yielded individual samples containing tens of millions to about one billion fermium atoms.

At JGU’s RISIKO separator, researchers heated samples to approximately 1,000 degrees Celsius, or 1,832 degrees Fahrenheit, to release atoms for laser analysis. The titanium lasers then excited and ionized those atoms, enabling researchers to resolve two optical transitions from the tiny quantities available.

The experiment depended on a months-long journey through three countries to gather enough atoms.

What do the corrected data change?

Researchers combined the experimental spectra with advanced atomic calculations from teams at Jagiellonian University in Kraków, Poland, and the Helmholtz Institute Mainz. The joint analysis supported the strongly prolate shape and the revised magnetic dipole moment while correcting earlier values that had not matched physical expectations.

The measurements agree closely with modern nuclear theories developed by research groups in France and Germany. More dependable values for deformation and magnetism can strengthen fission models and improve predictions for the stability of superheavy elements, meaning exceptionally heavy nuclei that scientists have not yet discovered.

For nuclear modelers, fermium-255 now supplies measured deformation and magnetic inputs rather than inconsistent earlier estimates. The practical gain is a firmer benchmark for calculations that test fission and possible heavier elements.

Because the method infers nuclear properties instead of imaging the nucleus, and because the available atom counts were limited, the result still depends on the quality of the spectra and atomic calculations. The study also concerns one nucleus; extending the same constraints to still heavier elements remains a modeling task.

Mitzi Urquiza-González, Matou Stemmler, Thomas E. Albrecht and colleagues reported the work in “High-Resolution Laser Spectroscopy on the Hyperfine Structure of Fm255 (Z=100).” The paper appeared in Physical Review Letters on 15 May 2026, with DOI 10.1103/2813-b49x.

Better measurements of one extreme nucleus give nuclear scientists a firmer benchmark for predicting others.