One of the major challenges in modern nuclear physics is understanding how nature produces the heaviest elements in the universe, such as lead or gold. Physicists think that many of these elements originate from the “rapid neutron-capture process,” which occurs during extreme astrophysical events like neutron star mergers and supernova explosions. Consequently, reproducing the exotic nuclei involved in these processes in the laboratory is crucial for studying nuclear structure and the cosmic origin of matter. This is achieved using accelerator and detector complexes that accelerate ion beams and induce interactions, allowing the processes and their products to be reproduced and observed in detail.
At INFN’s Legnaro National Laboratories, a beam of uranium-238 ions—the most naturally abundant uranium isotope—was accelerated to an energy of 6.2 MeV per nucleon for the first time using the PIAVE-ALPI superconducting accelerator complex. This achievement marks a significant technological milestone in the study of exotic nuclei and is the second success at the Legnaro Laboratories, following the production of the first exotic ion beam for the SPES project. The process utilized a very low beam intensity—approximately one nanoampere of particles—typical of low-energy nuclear physics experiments. Over a full month of experimental activity, only a few micrograms of uranium were accelerated; in these experiments, beam ions interacted individually with the nuclei of an extremely thin metal target. The campaign resulted from the synergistic use of several instruments—the AGATA gamma-ray detectors, the PRISMA magnetic spectrometer, and a dedicated detection system—combined in an exceptionally powerful experimental setup.
“Thanks to the accelerator and detector complex at the Legnaro Laboratories, it has been possible to identify and study in detail—from a spectroscopic perspective—both the neutron-rich fragments produced by the fission of uranium nuclei following collisions with the target, and the heavy ions produced via multi-nucleon transfer reactions,” explains Andrea Gottardo, scientific coordinator of the PIAVE-ALPI accelerators.
Researchers from various European institutions participated in the campaign, addressing fundamental questions coming from the structure of the neutron-rich nuclei involved in the rapid neutron-capture process to the formation and survival of the heavier reaction products generated in ion-ion collisions.
“Beyond its immediate scientific impact,” notes Enrico Fagotti, head of the accelerator division, “producing the first accelerated uranium ion beam represents a major strategic achievement for the Legnaro Laboratories; it demonstrates their capability to operate with complex heavy-ion beams while fully leveraging a world-class experimental infrastructure.”
This achievement establishes a direct link to the next chapter of research at the Legnaro Laboratories: many of the neutron-rich nuclei studied via uranium-induced reactions lie in the same regions of the nuclide chart that will soon be accessible through the exotic ion beams produced by the SPES project. The expertise gained in accelerator operation, beam management and delivery, and advanced spectroscopy—combined with the successful commissioning of these initial uranium-ion beam experiments—lays the scientific and technological groundwork for the SPES experimental program. The results obtained with stable ion beams are thus paving the way for fully exploiting the scientific opportunities offered by the SPES project and opening up new avenues for studying the most exotic atomic nuclei.