Lasers reveal a rugby-ball-shaped nucleus inside fermium-255

Fermium-255 exists only in tiny artificial samples, yet its nucleus has now yielded a remarkably detailed portrait. High-resolution laser measurements show that the heavy nucleus is strongly elongated and carries magnetic properties that differ sharply from older estimates.

The isotope contains 100 protons and 155 neutrons. Researchers determined that its nucleus has a stable prolate shape, meaning it resembles a rugby ball rather than a sphere.

The results provide the first high-resolution measurement of fermium-255’s hyperfine structure. That fine splitting of atomic energy levels carries information about the nucleus hidden inside the electron cloud.

An international collaboration involving 18 institutions conducted the work. Scientists from Johannes Gutenberg University Mainz, the Helmholtz Institute Mainz, and the University of Gothenburg led the effort, with Mitzi Urquiza-González as first author.

Atoms consist of a compact, positively charged nucleus surrounded by electrons. Electrons closest to the nucleus can respond to its size, shape, and magnetic properties.

Those effects slightly change the electrons’ allowed energy levels.

Laser spectroscopy can measure these shifts by tuning light precisely to the frequencies needed to move electrons between quantum states. In nuclei with an odd number of neutrons, magnetic effects cause additional splitting known as hyperfine structure.

Fermium-255 has a nuclear spin of 7/2. Its atomic states therefore divide into multiple closely spaced components that earlier experiments could not resolve clearly.

The first atomic energy levels of fermium were observed more than 20 years ago at Johannes Gutenberg University. Limited resolution, however, left important nuclear properties uncertain and produced inconsistent values.

The new work resolved much of that structure.

For one optical transition, the researchers observed 15 of 22 expected hyperfine components. For another, they detected 13 of 21. That level of detail allowed much more precise measurements of the nuclear interactions.

The largest change involved fermium-255’s magnetic dipole moment, a measure of its nuclear magnetism.

Earlier low-resolution work had produced a value of minus 3.46 nuclear magnetons. That estimate fell well outside the pattern seen in neighboring heavy nuclei and entered what researchers regarded as an unphysical region.

The new analysis produced a value of about minus 0.75 nuclear magnetons. A more detailed calculation gave minus 0.743, with an uncertainty of 0.049 nuclear magnetons.

That result falls within expected physical limits and aligns far more closely with modern models of heavy nuclei.

Researchers also determined an electric quadrupole moment of about plus 5.84 electron-barns. The large positive value indicates a strongly prolate nucleus.

The calculated deformation parameter was 0.280.

The nuclear moments closely resemble those measured in californium-253. That similarity suggests the unpaired neutron occupies the same type of nuclear orbital in both isotopes.

Obtaining enough fermium for the experiment presented a major challenge.

Fermium does not occur naturally and must be produced through a complicated sequence of nuclear reactions and radioactive decays.

The process began with einsteinium-254 supplied for neutron irradiation. Material was exposed to neutrons at the Institut Laue-Langevin in Grenoble, France, producing einsteinium-255.

That isotope has a half-life of about 40 days and decays into fermium-255. It therefore served as a continuing source of fermium for several weeks.

Scientists repeatedly separated newly produced fermium chemically at Johannes Gutenberg University.

Individual spectroscopy samples contained only about 100 million to 1 billion fermium-255 atoms. Although that sounds large, it represents an extraordinarily small amount of material.

The samples were placed in the RISIKO mass separator and heated to about 1,000 degrees Celsius. Heating released neutral fermium atoms that researchers could expose to laser light.

Custom titanium-sapphire laser systems supplied the carefully tuned light needed for the measurements.

When the laser frequency matched an atomic transition, fermium atoms absorbed energy and became excited. Further laser interaction ionized them, allowing the team to identify and count the resulting ions selectively.

The experimental arrangement also reduced Doppler broadening, which can blur spectral lines because atoms move at different speeds.

That improvement was essential for separating the tightly packed hyperfine components that earlier experiments had blended together.

After ionization, fermium ions were accelerated and separated according to their mass-to-charge ratio before individual ions were detected.

The team measured two optical transitions near wavelengths of 398 nanometers.

Turning those atomic measurements into nuclear properties required theoretical calculations. Fermium has no stable reference isotope that could provide the needed experimental calibration.

Scientists at Jagiellonian University in Kraków and the Helmholtz Institute Mainz carried out independent atomic calculations. Four approaches showed strong agreement.

The experimentally derived nuclear moments matched predictions from modern nuclear theories.

One model predicted a magnetic moment of minus 1.0 nuclear magnetons and a quadrupole moment of plus 6.2 electron-barns.

A Hartree-Fock-Bogoliubov calculation came even closer, predicting minus 0.78 and plus 5.78, respectively. Both values differed from the measurements by less than 5 percent.

That agreement matters because the heaviest nuclei are difficult to test experimentally.

Their large number of positively charged protons creates intense electrical repulsion inside the nucleus. In very heavy elements, that competition affects deformation and resistance to spontaneous fission.

Nuclear shape therefore becomes an important part of understanding why some heavy nuclei survive longer than others.

These resources explore earlier fermium measurements, related actinide nuclei and the challenges of testing theories near the periodic table’s limits.

Smooth trends in fermium charge radii and the impact of shell effects: Measurements across fermium isotopes examine changes in nuclear size and the influence of shell structure. (Nature, 2024)

Nuclear moments and isotope shifts of the actinide isotopes 249–253Cf probed by laser spectroscopy: Spectroscopy of californium supplies related nuclear measurements for comparisons with fermium. (Physical Review C, 2023)

Recent progress in laser spectroscopy of the actinides: This review explains experimental techniques and their use in studying heavy radioactive atoms and nuclei. (Progress in Particle and Nuclear Physics, 2021)

First Observation of Atomic Levels for the Element Fermium (Z=100): The foundational experiment identified fermium atomic transitions that enabled later spectroscopy. (Physical Review Letters, 2003)

The quest for superheavy elements and the limit of the periodic table: A review examines production challenges, nuclear stability and theoretical questions surrounding the heaviest elements. (Nature Reviews Physics, 2024)

Research findings are available online in the journal Physical Review Letters.

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