Nuclear fusion occurs when two atomic nuclei collide and overcome their Coulomb barrier, touch and merge to form a new composite nucleus. As the nuclei approach, their diffuse matter distributions increasingly overlap, allowing protons and neutrons to be exchanged. Fusion models treat these as small effects, assuming the identity of the two nuclei are essentially unchanged until they touch. While successful at describing the enhancement of fusion cross-sections seen at below-barrier energies, these models give fusion yields that are systematically higher than experiment above and far below the barrier. We have made measurements that show that multiple transfers – occurring before the barrier radius – play a crucial role in the fusion of heavy nuclei, as used to synthesise superheavy elements. In the reflected flux in reactions of 40Ca+208Pb, we observe an ‘explosion’ of mass transfer reactions that begin well outside the barrier, revealing a massive increase in complexity compared to what was previously understood. These transfers are associated with large changes in the potential and binding energies of the colliding nuclei have compensatory high excitation energies, the result of favouring smooth trajectories. This result challenges our current framework of fusion and the provides an explanation for systemic failures of current model predictions. We anticipate that this has important impacts on understanding superheavy element production. The portion of the flux that collides with lower coulomb repulsion will lead to longer sticking times and thus enhanced probability of evolving into a new superheavy nucleus.

Dr. Kaitlin Cook is a Research Fellow in the RSPhys NPAA Department, the Deputy Director of Heavy Ion Accelerators, and an Adjunct Assistant Professor of Physics at the Facility for Rare Isotope Beams at Michigan State University. Kaitlin received her PhD in Nuclear Physics from the ANU in 2017. Her postdoctoral work was at first at the ANU and then later as a Japan Society for the Promotion of Physics International Research Fellow at the Tokyo Institute of Technology. She joined the faculty at the Facility for Rare Isotope Beams at Michigan State University in 2020. In 2022, she returned to the ANU, winning a DECRA fellowship, for which she was awarded the J.G. Russell award of the Australian Academy of Science. Kaitlin’s experimental research program spans multiple aspects of nuclear reaction dynamics with stable and radioactive beams, from understanding the reactions of nuclei that are barely bound, through to investigating new ways to make superheavy elements in the lab.

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