RESOLVING THE LEDGER
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Add as preferred source 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 because of their extremely limited availability from artificial production. Their study requires dedicated production routes and 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.
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), a branch of GSI/FAIR, Germany, and the University of Gothenburg, Sweden, the home institution of the paper's first author, Mitzi Urquiza-González. HÜBNER Photonics GmbH contributed to the project by hosting and supporting Urquiza-González during her Ph.D. and through its expertise in advanced laser technology, which was essential to the successful experiment.
Atoms consist of a very small, positively charged nucleus surrounded by a cloud of electrons. The innermost 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 shells 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 provide information about details of the nucleus.
The first atomic energy levels of fermium were observed more than 20 years ago at JGU. However, because of technical limitations at the time, the hyperfine structure could not be resolved, leading to incomplete and partially inconsistent nuclear data.
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 for 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 several tens of millions to 1 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°C (1,800°F), 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.
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 them. 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 knowledge of nuclear properties in the heaviest elements and demonstrates that precision measurements are possible even with extremely small quantities of material.
Mitzi Urquiza-González et al, High-Resolution Laser Spectroscopy on the Hyperfine Structure of 255 Fm (𝑍=100), Physical Review Letters (2026). DOI: 10.1103/2813-b49x
Journal information: Physical Review Letters
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