Laser spectroscopy reveals for the first time the precise nuclear shape of fermium-255, offering new clues in the search for superheavy elements

An international research team has, for the first time, precisely determined the nuclear structure of the actinide fermium-255, confirming that its nucleus exhibits a pronounced prolate deformation, similar to a rugby ball. The study, published in *Physical Review Letters*, was conducted in collaboration with 18 institutions, including Johannes Gutenberg University Mainz in Germany, the Helmholtz Institute Mainz, and the University of Gothenburg in Sweden. The findings not only correct several unreasonable nuclear property values in previous standard data tables but also provide crucial experimental validation for modern nuclear theory models.

Fermium is a synthetic heavy element that does not exist in nature. Fermium-255 contains 100 protons and 155 neutrons, making its experimental production extremely challenging. The research team employed a complex production route spanning multiple years and facilities. First, transuranium materials were subjected to several months of neutron irradiation at the High Flux Isotope Reactor at Oak Ridge National Laboratory in the United States, producing einsteinium-254. The material was then shipped to Mainz, Germany, for processing, and subsequently transferred to the Institut Laue-Langevin in France for further irradiation to produce einsteinium-255. With a half-life of approximately 40 days, einsteinium-255 continuously decays to produce fermium-255, providing a sample source for subsequent experiments.

At the RISIKO mass separator at Johannes Gutenberg University Mainz, researchers heated an extremely small fermium sample to approximately 1000 degrees Celsius to evaporate the atoms. They then used a custom-built titanium-sapphire laser system to irradiate the atoms and tune the frequency, achieving selective excitation and ionization detection. By resolving the hyperfine structures in two optical transitions, the team deduced the nuclear shape and magnetic properties from the minute splittings in the electronic energy levels. Because inner-shell electrons are located close to the nucleus, their energy-level shifts are influenced by the nuclear size, deformation, and magnetic moment, making high-precision laser spectroscopy a powerful tool for studying rare heavy nuclei.

The study reveals that the fermium-255 nucleus is not nearly spherical but exhibits a strongly elongated, prolate deformed structure. Combined with advanced atomic theory calculations from Jagiellonian University in Poland and related institutions in Mainz, the experimental data also yielded a new magnetic dipole moment result, which is inconsistent with the old value in previously used standard tables. Modern nuclear theory models developed at CEA Arpajon in France, IP2I Lyon, and the Technical University of Darmstadt in Germany successfully explain these new measurements, indicating that current theoretical descriptions of heavy-nucleus deformation and interactions are approaching a realistic picture.

The significance of this achievement lies in filling a gap in the study of nuclear properties of the heaviest elements. The shape of a heavy nucleus is closely related to its stability against spontaneous fission, which is the key factor limiting the existence of elements beyond uranium. Precision measurements of extremely scarce, short-lived heavy nuclei such as fermium-255 help improve nuclear fission models, enhance predictive capabilities for the lifetimes and stability of unknown superheavy elements, and provide a more reliable experimental basis for future exploration of the boundaries of the periodic table.