Researchers have determined the shape of the actinide nucleus of fermium-255 for the first time, using high-resolution laser spectroscopy to measure its structure. Published in Physical Review Letters, the breakthrough supports modern theoretical models and provides insights into the nuclear behavior and spontaneous fission limits of the heaviest atomic nuclei.
Production Pathways and Isotope Sourcing
Investigating heavy nuclei remains exceptionally challenging due to limited availability from artificial production. According to Phys.org, intricate production pathways spanning multiple facilities and several years yielded samples containing only a few billion atoms, which proved sufficient for advanced laser spectroscopy on fermium-255.
The process began with months-long neutron irradiations of transuranium material at the High Flux Isotope Reactor at Oak Ridge National Laboratory in the USA, producing einsteinium-254. Following initial use in American experiments, the material traveled to Mainz, Germany, for initial processing before heading to the Institute Laue-Langevin in France. Further neutron irradiation at that facility produced einsteinium-255, an isotope with a half-life of 40 days that decays to fermium-255 and acts as a continuous source over several weeks.
Regular chemical separations at Johannes Gutenberg University Mainz enabled the preparation of multiple samples containing between several tens of millions and 1 billion atoms for the experiments.
High-Resolution Laser Spectroscopy at RISIKO
The highly sensitive measurements took place at the RISIKO separator at Johannes Gutenberg University Mainz. Researchers heated the fermium samples to approximately 1,000 degrees Celsius, or 1,800 degrees Fahrenheit, causing the atoms to evaporate. Irradiating these evaporated atoms with laser light triggered resonant excitation and subsequent ionization, enabling selective detection.
Custom-built titanium-sapphire laser systems and specialized handling techniques allowed the team to successfully resolve the hyperfine structure of two optical transitions. Phys.org notes that while the first atomic energy levels of fermium were observed more than 20 years ago at the university, technical limitations at that time prevented resolution of the hyperfine structure, resulting in incomplete nuclear data.
Nuclear Deformation and Shell Effects
Most atomic nuclei are not perfectly round; many resemble a rugby ball. This deformation influences how nuclei interact with electric fields generated by surrounding electrons. Furthermore, nuclei containing an odd number of neutrons act as tiny magnets due to a second electromagnetic effect. The interplay between these nuclei and their electron shells splits electronic transitions into closely spaced levels, known as the hyperfine interaction.
An international research team investigating fermium isotopes across the GSI/FAIR accelerator facility and Johannes Gutenberg University Mainz tracked the evolution of the nuclear charge radius, observing a steady increase as neutrons were added. This steady rise indicates that localized nuclear shell effects have a reduced influence on the nuclear charge radius in these heavy nuclei.
Theoretical Validation and Future Implications
To interpret the experimental spectra, dedicated atomic theory calculations were performed at Jagiellonian University in Kraków, Poland, and at HIM in Mainz, yielding findings that align closely with the experimental data.
Understanding nuclear shapes provides essential insights into internal structure and stability against spontaneous fission. Because spontaneous fission arises from strong proton repulsion, it limits the existence of elements beyond uranium and remains a central factor in the ongoing search for longer-lived superheavy elements.
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