Physicists at the GSI/FAIR accelerator facility have gained insights into the structure of atomic nuclei of fermium, a synthetic chemical element of the actinoid series with atomic number 100. Using laser spectroscopy techniques, they traced the evolution of the nuclear charge radius and found a steady increase as neutrons were added to the nuclei.
Fermium isotopes studied by Warbinek et al. are highlighted in this chart. Image credit: S. Raeder.
“The heaviest nuclei known so far owe their existence to quantum-mechanical nuclear-shell effects,” said Dr. Sebastian Raeder, a researcher at the Helmholtz-Institut Mainz and the GSI Helmholtzzentrum für Schwerionenforschung, and his colleagues.
“These increase the stability of nuclei against spontaneous fission, enabling the formation of superheavy nuclei.”
“At specific numbers of protons (Z) or neutrons (N), so-called magic numbers, nucleonic shells show large energy gaps, resulting in increased nuclear stability.”
“This is analogous to the closed electron shells of noble gases resulting in their chemical inertness.”
“The heaviest known nucleus with a magic number for both protons (Z=82) and neutrons (N = 126) is lead-208, a spherical nucleus.”
“The location of the next spherical shell gap beyond lead-208 is yet unknown; nuclear models predict it most frequently at Z= 114, Z = 120 or Z = 126, and N =172 or N = 184.”
“This variation in the predictions is primarily, among other factors, owing to a large single-particle level density in the heaviest nuclei.”
Using a laser-based method, the authors investigated fermium atomic nuclei, which possess 100 protons (Z=100), and between 145 and 157 neutrons (N = 145-157).
Specifically, they studied the influence of quantum mechanical shell effects on the size of the atomic nuclei.
“This allowed shedding light on the structure of these nuclei in the range around the known shell effect at neutron number 152 from a new perspective,” Dr. Raeder said.
“At this neutron number, the signature of a neutron shell closure was previously observed in trends of the nuclear binding energy.”
“The strength of the shell effect was measured by high-precision mass measurements at GSI/FAIR in 2012.”
“As mass is equivalent to energy according to Einstein, these mass measurements gave hints about the extra binding energy the shell effect provides.”
“Atomic nuclei around neutron number 152 are an ideal testbench for deeper studies, as they happen to be shaped more like a rugby-ball, rather than spherical.”
“This deformation allows the many protons in their nuclei to be further apart than in a spherical nucleus.”
For the current measurements, the researchers examined fermium isotopes with lifetimes ranging from a few seconds to a hundred days, using different methods for producing the fermium isotopes and by methodological developments in the applied laser spectroscopy techniques.
The short-lived isotopes were produced at the GSI/FAIR accelerator facility, with only a few atoms per minute being available for the experiments in some cases.
The produced nuclei were stopped in argon gas and picked up electrons to form neutral atoms, which were then probed by laser light.
The neutron-rich, long-lived fermium isotopes (fermium-255, fermium-257) were produced in picogram amounts at Oak Ridge National Laboratory in Oak Ridge, USA, and at Institut Laue-Langevinat Grenoble, France.
Their results provide insight into the changes of the nuclear charge radius in fermium isotopes across the neutron number 152 and showed a steady, uniform increase.
“Our experimental results and their interpretation with modern theoretical methods show that in the fermium nuclei, nuclear shell effects have a reduced influence on the nuclear charge radii, in contrast to the strong influence on the binding energies of these nuclei,” said Dr. Jessica Warbinek, a researcher at CERN.
“The results confirm theoretical predictions that local shell effects, which are due to few individual neutrons and protons, lose influence when the nuclear mass increases.”
“Instead, effects dominate that are to be attributed to the full ensemble of all nucleons, with the nuclei rather seen as a charged liquid drop.”
The results were published in the journal Nature.
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J. Warbinek et al. 2024. Smooth trends in fermium charge radii and the impact of shell effects. Nature 634, 1075-1079; doi: 10.1038/s41586-024-08062-z