The first results of the AGATA experiment at the National Laboratories of Legnaro (LNL) shed new light on the "fusion hindrance" phenomenon at very low energies. This effect, well known for medium-mass nuclei, is still debated for light nuclei, which are of great interest for astrophysics.
These results have just been published in the journal Physics Letters B. Understanding how two atomic nuclei fuse when their energy becomes extremely low is a major challenge for nuclear physics. At these energies, the probability of fusion decreases drastically, sometimes much more than predicted by theoretical models. This fusion hindrance phenomenon is well known for medium-mass nuclei, but remains debated for lighter systems, which nevertheless play a key role in astrophysics. Conducted in the fall of 2023 at LNL, the experiment focused on the fusion of the ¹²C + ²⁸Si system, measured down to energies well below the Coulomb barrier, where reactions occur through quantum tunneling effect.
This work, coordinated by Prof. Giovanna Montagnoli and Dr. Alberto Stefanini, was carried out as part of an international collaboration that includes researchers from the University of Padova and LNL, using the AGATA gamma spectrometer coupled with DSSD segmented silicon detectors, and an electrostatic beam deflector. By detecting gamma rays and charged particles in coincidence, the researchers were able to measure extremely low fusion probabilities, down to a few tens of nanobarns, significantly reducing background noise. These high-precision data allow testing of theoretical models in an energy range that is still largely unexplored. The results clearly show that at very low energies, currently used models overestimate the fusion probability: a marked reduction is observed, consistent with the idea of a gradual cancellation of coupling effects between nuclei in fusion dynamics. This behavior is part of a systematic trend recently observed for various nuclear systems. This work therefore provides essential experimental data to refine nuclear fusion models at very low energies and contributes significantly to a better understanding of fusion reactions between light nuclei that occur in astrophysical environments where temperatures are high but collision energies are very low.
