The nucleus of fermium-255 is not round. An experiment using lasers shows that it has a distinctly elongated shape, like a stretched balloon.
Fermium is a radioactive element whose nucleus contains 100 protons. Its isotope fermium-255 is particularly difficult to study. Its half-life is approximately 20 hours. This means that after this period, half of the nuclei present in a sample have decayed.
AI-generated illustrative image based on a description of fermium-255
The researchers did not directly observe the shape of the nucleus. They studied how fermium atoms absorb light. Two very close wavelengths, around 398 nm in the violet range, were used.
The nucleus has a very slight influence on the electrons surrounding it. These interactions produce tiny differences in the atom's light spectrum. By measuring them precisely, researchers can obtain information about the properties of the nucleus.
The experiment was carried out using the RISIKO mass separator at Johannes Gutenberg University Mainz. A high-resolution ion source made it possible to distinguish these very small variations, known as hyperfine structure.
The measurements were then compared with calculations describing the electrons in fermium. The researchers were thus able to determine the magnetic moment of the nucleus and its electric quadrupole moment. This second quantity can be used in particular to assess how far the nucleus deviates from a spherical shape.
The result shows that the nucleus of fermium-255 is strongly and persistently elongated along one axis. Physicists describe this shape as “prolate.” It corresponds to what was expected for several neighboring heavy nuclei.
The researchers finally compared their measurements with several nuclear physics models. These models generally reproduce the observed properties well. Fermium-255 can therefore serve as a reference for testing these models and preparing future measurements of even heavier elements.