Physicists Measured the Shape of a Fermium Nucleus and Found the Reference Tables Were Wrong

An international team has measured the shape of a fermium-255 nucleus with high precision for the first time, and reported that it is strongly prolate — stretched along one axis, closer to a rugby ball than a sphere. The work, published in Physical Review Letters, also corrected values that have sat in standard nuclear data tables for years.

Fermium is element 100. It does not occur in nature in any usable quantity, it has to be made in a reactor or an accelerator, and the isotope in question decays quickly enough that experiments have to be built around producing atoms and interrogating them almost immediately. That combination is why a basic property like the shape of the nucleus has stayed poorly pinned down while far lighter elements have been characterized in exhaustive detail.

The measurement did not look at the nucleus directly. Instead the team used laser spectroscopy to examine the hyperfine structure of the atom's electron shell — the very small splittings in electron energy levels that arise because the electrons feel the shape and magnetism of the nucleus they orbit. "By probing the substructure of atomic energy levels in the electron shell, which is sensitive to nuclear properties, researchers were able to extract information about nuclear shape," the collaboration explained. Read the splittings precisely enough and the nucleus's geometry can be reconstructed from the outside in.

Two results came out of it. The first was the strong prolate deformation. The second was a magnetic dipole moment that disagrees with the previously published figure — meaning the tabulated value that other calculations have been quietly relying on was wrong. The corrected numbers line up with modern theoretical nuclear models, which is the outcome theorists were hoping for and not one that was guaranteed.

The collaboration was led by Johannes Gutenberg University Mainz, the Helmholtz Institute Mainz and the University of Gothenburg, drawing on 18 institutions in total, with laser hardware support from HÜBNER Photonics. First author Mitzi Urquiza-González headed the work.

The reason nuclear shape matters beyond bookkeeping is stability. Whether a heavy nucleus survives for microseconds or for years depends heavily on whether it resists spontaneous fission, and resistance to fission is governed in large part by deformation — a stretched nucleus is already partway toward splitting in two. Physicists have spent decades searching for an "island of stability," a predicted region among the superheavy elements where certain proton and neutron counts should yield isotopes far longer-lived than their neighbors. Getting there requires knowing precisely how nuclei deform as elements get heavier. Element 100, measured accurately, is a fixed point on the map that the models pointing toward that island have to reproduce.