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JOINT PRESS RELEASE OF JOHANNES GUTENBERG UNIVERSITY MAINZ, GSI/FAIR, AND HÜBNER PHOTONICS GMBH (GERMANY) TOGETHER WITH THE UNIVERSITY OF GOTHENBURG (SWEDEN)
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First high-precision measurements of the fermium-255 nucleus validate modern nuclear models and advance the search for superheavy elements
For the first time, researchers have determined the shape of the actinide nucleus of fermium‑255 and measured its structure with high precision and resolution. This breakthrough, published in Physical Review Letters, supports modern theoretical models and opens new possibilities for understanding the behavior of the heaviest atomic nuclei.
Studying the shapes of atomic nuclei provides essential insights into their internal structure. In very heavy nuclides, nuclear shape is closely linked to their stability against spontaneous fission and is therefore a key factor in the search for longer‑lived superheavy elements. Spontaneous fission arises from the strong repulsion between the many protons in heavy nuclei and ultimately limits the existence of elements beyond uranium (element 92).
Investigations of these nuclei are highly challenging due to their extremely limited availability from artificial production. Their study requires dedicated production routes as well as highly sensitive experimental techniques. In the present work, intricate production pathways spanning several years and multiple facilities yielded samples containing only a few billion atoms. This was nevertheless sufficient to perform advanced laser spectroscopy on fermium‑255, which contains 100 protons and 155 neutrons in its nucleus. By probing the substructure of atomic energy levels in the electron shell, which is sensitive to nuclear properties, researchers were able to extract information about the nuclear shape.
The experiments were carried out by an international collaboration of scientists and engineers from 18 institutions, led by Johannes Gutenberg University Mainz (JGU), the Helmholtz Institute Mainz (HIM), which is a branch of GSI/FAIR, Germany, and the University of Gothenburg, Sweden, which is also the home institution of the paper's first author, Mitzi Urquiza-González. HÜBNER Photonics GmbH contributed to the project by hosting and supporting Mitzi Urquiza-González during her PhD and through its expertise in advanced laser technology, which was essential for the successful realization of the experiment.
By combining the experimental data on the hyperfine structure of the optical spectrum with advanced atomic theory calculations, the researchers determined that the fermium‑255 nucleus is strongly prolate, resembling a rugby ball. The new measurements correct unphysical values in standard tabulations based on earlier reports and are well described by state-of-the-art nuclear models.
Structure of the atomic nucleus and interaction with the electron shell
Atoms consist of a very small, positively charged nucleus surrounded by a cloud of electrons. The innermost of these electrons move very close to the nucleus and are therefore sensitive to its size, shape, and magnetic properties. By studying these effects with high‑precision laser spectroscopy, scientists can learn about the structure of the nucleus. This is done by shining laser light onto the atoms and carefully tuning its color (frequency) to detect tiny changes in the electrons' energy levels between quantum states.
Most atomic nuclei are not perfectly round, but deformed, with many resembling a rugby ball. This has important consequences for their interaction with the electric field generated by the surrounding electrons. A second electromagnetic effect occurs in nuclei that contain an odd number of neutrons: they act as tiny magnets. Overall, the interplay of such nuclei with their electron shell leads to a tiny splitting of electronic transitions into several closely spaced levels. This is referred to as the hyperfine interaction. These splittings can be measured with high accuracy using laser spectroscopy techniques, which thus inform about details of the nucleus.
The first atomic energy levels of fermium were observed more than 20 years ago at JGU. However, due to technical limitations at the time, the hyperfine structure could not be resolved, leading to incomplete and partly inconsistent nuclear data.
From isotope production to laser spectroscopy
Fermium does not occur naturally and must be produced artificially, making experimental investigations particularly demanding. The production of fermium-255 began with months-long neutron irradiations of transuranium material at the High Flux Isotope Reactor at Oak Ridge National Laboratory (USA), producing einsteinium-254. After initial use in experiments in the USA, the material was transported to Mainz (Germany) for initial processing before it was sent to the Institute Laue-Langevin in France to undergo further neutron irradiation to produce einsteinium‑255. This isotope, which decays to fermium-255, was finally returned to Mainz.
With its half-life of 40 days, einsteinium-255 acts as a continuous source of fermium-255 over several weeks. Regular chemical separations at JGU allowed the preparation of multiple samples containing between several tens of millions and up to one billion atoms for the spectroscopy experiments.
The highly sensitive laser spectroscopy measurements were performed at the RISIKO separator at JGU. In the experiment, the fermium samples were heated to approximately 1,000 degrees Celsius, causing atoms to evaporate. These atoms were then irradiated with laser light, and resonant excitation led to ionization, enabling selective detection. The team successfully resolved the hyperfine structure of two optical transitions, made possible by custom-built Ti:sapphire laser systems and extensive expertise in handling extremely small sample quantities.
Theoretical calculations in agreement with experimental findings
To interpret the experimental spectra, dedicated atomic theory calculations were carried out at Jagiellonian University in Kraków, Poland, and at HIM in Mainz. These calculations confirmed the strongly deformed nuclear shape and yielded a magnetic dipole moment that disagrees with previously tabulated values and corrects these. The results are in excellent agreement with modern nuclear theory predictions developed at CEA Arpajon (France), IP2I Lyon (France), and the Technical University of Darmstadt (Germany).
The experiment fills an important gap in our knowledge of nuclear properties in the heaviest elements and demonstrates that precision measurements are possible even with extremely small quantities of material.
By providing accurate information on nuclear shape and magnetic properties in such heavy systems, the results directly improve models of nuclear fission, helping to predict the stability of yet unknown superheavy elements and guiding future discoveries at the limits of the periodic table.
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