New Laser Spectroscopy Technique Reveals Nuclear Shape of Fermium and Neptunium

Researchers at the University of Gothenburg, working within an EU-funded collaboration that included HÜBNER Photonics and several European nuclear-physics facilities, have used a new laser-based spectroscopic method to probe the atomic nuclei of the radioactive actinides neptunium and fermium. The technique, described in a doctoral thesis titled “Optical Parametric Oscillators for Spectroscopy of Actinides”¹ and reported alongside related findings in Physical Review Letters,² relies on a pulsed laser built around an optical parametric oscillator (OPO) capable of generating wavelengths and colors with an intensity and precision that conventional laser systems cannot easily match.

By directing these laser pulses at target atoms and measuring the resulting hyperfine structure—minute shifts in the wavelengths absorbed—the team extracted detailed information on nuclear size and shape.² The measurements, gathered across several specialized European facilities equipped to handle short-lived radioactive isotopes, produced the first high-quality nuclear-shape description for fermium and neptunium, showing both nuclei to be elongated, or “rugby ball”-shaped, rather than spherical.

Because actinides such as fermium exist for only seconds and can be produced in only minute quantities, high-resolution laser spectroscopy offers one of the few practical windows into their nuclear structure. Beyond refining theoretical nuclear models and aiding the search for new isotopes and superheavy elements, the work has downstream relevance to nuclear waste reduction and to the production of radioisotopes used in cancer treatment.¹,³,⁴

Figure 1. Graphical illustration of the chart of nuclides and the measurement setup at the RISIKO mass separator, where laser spectroscopy was performed on short-lived actinide isotopes. Image courtesy of Sebastian Raeder.

Figure 2. Schematic representation of the projected charge density of the fermium-255 nucleus at the mean-field level, as calculated using nuclear theory; the elongated, “rugby ball”-like shape is evident in the density distribution. Dimensions are given in femtometers (fm). Image courtesy of Michael Bender.

References

1. Urquiza Gonzalez, M. V. Optical Parametric Oscillators for Spectroscopy of Actinides. Doctoral Thesis, University of Gothenburg, Gothenburg, Sweden, 2026. https://www.gu.se/en/event/mitzi-valeria-urquiza-gonzalez-optical-parametric-oscillators-for-spectroscopy-of-actinides (accessed Aug 26, 2026).
2. Urquiza-González, M.; Stemmler, M.; Albrecht, T. E.; et al. High-Resolution Laser Spectroscopy on the Hyperfine Structure of ²⁵⁵Fm (Z = 100). Phys. Rev. Lett. 2026, 136, 192501. DOI: 10.1103/2813-b49x.
3. University of Gothenburg. New Laser Method Gives Insight into Radioactive Atomic Nuclei; press release, 2026. https://www.gu.se/en/news/new-laser-method-gives-insight-into-radioactive-atomic-nuclei (accessed Aug 26, 2026).
4. Johannes Gutenberg University Mainz. Laser Spectroscopy Helps Reveal Hidden Nuclear Properties in Fermium; press release, 2026. https://press.uni-mainz.de/laser-spectroscopy-helps-reveal-hidden-nuclei-properties-in-fermium/ (accessed Aug 26, 2026).