Laser spectroscopy uncovers hidden nuclear properties of elusive fermium

A tiny amount of one of the rarest substances on Earth has revealed a striking secret about the heaviest elements. In a study published in Physical Review Letters, an international team has measured the nuclear shape of fermium-255, an artificial isotope containing 100 protons and 155 neutrons. The experiment marks the first time researchers have resolved the hyperfine structure of this actinide with sufficient precision to determine its shape and magnetic properties. Their result shows that the nucleus is strongly prolate, meaning it is elongated like a rugby ball rather than compressed or perfectly spherical.

The finding offers a new view of matter at the extreme edge of the periodic table. Fermium does not occur naturally and is produced only in highly specialized nuclear facilities. Its nuclei are also intensely unstable, making them available in microscopic quantities and for limited periods. Yet the researchers were able to perform high-resolution laser spectroscopy using samples containing only tens of millions to approximately one billion atoms. Although that sounds like a large number, it represents an extraordinarily small quantity of material for nuclear experiments and demanded an elaborate production, purification, and measurement campaign spanning several countries.

Nuclear shape is not merely a visual description. In heavy nuclei, deformation can strongly influence how the nucleus stores energy, how it responds to electromagnetic forces, and how likely it is to split apart through spontaneous fission. This process occurs when the powerful repulsion among the many positively charged protons overwhelms the forces holding the nucleus together. Because fission places a fundamental limit on the survival of very heavy elements, accurate measurements of nuclear shape are essential for predicting whether undiscovered superheavy nuclei might persist long enough to be observed.

The new measurements were obtained by examining how the fermium nucleus interacts with the electrons surrounding it. In an atom, electrons occupy quantized energy levels, and transitions between those levels can be detected using laser light. The innermost electrons spend part of their time close to the nucleus, where they are sensitive to its size, charge distribution, and magnetic field. A deformed nucleus changes the electric field experienced by the electrons, while a nucleus with an unpaired neutron behaves like a tiny magnet. These effects produce hyperfine structure: small splittings in otherwise closely spaced atomic energy levels.

By tuning laser frequencies across two optical transitions, the team observed these subtle splittings in fermium-255 for the first time. The experiment took place at the RISIKO mass separator at Johannes Gutenberg University Mainz in Germany. Fermium samples were heated to about 1,000 degrees Celsius, causing atoms to evaporate. Carefully selected laser beams then excited the atoms and ionized them. The resulting ions could be identified and counted, allowing the researchers to reconstruct the hyperfine pattern even though the sample was extremely scarce.

The achievement depended on a production route that began years before the spectroscopy. Transuranium material was irradiated with neutrons for months at the High Flux Isotope Reactor at Oak Ridge National Laboratory in the United States, creating einsteinium-254. After experimental use and chemical processing, the material was transported to Germany and then to the Institute Laue-Langevin in France for additional neutron irradiation. This produced einsteinium-255, which was returned to Mainz. As einsteinium-255 decayed, with a half-life of about 40 days, it continuously generated fermium-255 for several weeks.

The precision of the result was made possible by custom-built titanium-sapphire laser systems and years of experience working with minute quantities of radioactive material. The collaboration included scientists and engineers from 18 institutions, led by researchers at Johannes Gutenberg University Mainz, the Helmholtz Institute Mainz, GSI/FAIR, and the University of Gothenburg in Sweden. HÜBNER Photonics GmbH supported the project through advanced laser expertise and by hosting Mitzi Urquiza-González during her doctoral research. Her work at the University of Gothenburg helped drive the experiment to completion.

The spectra were interpreted using advanced atomic calculations performed by researchers at Jagiellonian University in Kraków and the Helmholtz Institute Mainz. These calculations confirmed that fermium-255 has a strongly elongated nuclear shape and produced a magnetic dipole moment that differs from earlier tabulated values. The new result corrects inconsistencies in standard nuclear data, including values that led to physically unrealistic conclusions. It also agrees closely with modern nuclear models developed by teams in France and Germany, providing an important test of theories that describe the structure of the heaviest nuclei.

Fermium’s atomic energy levels were first observed at Mainz more than two decades ago, but the technology available at the time could not resolve their hyperfine structure. The new experiment therefore closes a long-standing gap in knowledge about an element near the upper boundary of experimentally accessible nuclear matter. By linking the measured atomic spectrum to the shape and magnetism of the nucleus, the study demonstrates how laser spectroscopy can turn a handful of atoms into a powerful probe of the nuclear landscape. The results will help improve models of fission and may guide the search for longer-lived superheavy elements at the limits of the periodic table.

News Publication Date: 15-May-2026

Web References: https://doi.org/10.1103/2813-b49x

References: Physical Review Letters, “High-Resolution Laser Spectroscopy on the Hyperfine Structure of 255Fm (𝑍=100),” DOI: 10.1103/2813-b49x.

Keywords

Fermium-255, nuclear physics, laser spectroscopy, hyperfine structure, nuclear deformation, actinides, superheavy elements, spontaneous fission, RISIKO mass separator, nuclear chart