Unexpected observations of the heavy-ion fusion excitation function above the Coulomb barrier

Physics Division, Argonne National Laboratory, Argonne, Illinois 60439, USA

^* e-mail: nwatwood@anl.gov
^** e-mail: cjiang@anl.gov

Published online: 18 October 2024

Abstract

Two unexpected behaviors have been observed in heavy-ion fusion excitation functions at energies above the Coulomb barrier. The first behavior is observed in overlapping excitation spectra. Fusion excitation functions σ(E) that have different entrance channels but fuse to the same compound nucleus appear to overlap in the energy domain above the barrier. The overlap emerges after scaling the center of mass energy of each excitation function by a constant scaling factor, SF. The second behaviour stems from the structure of the fusion excitation curve. Contrary to descriptions from coupled-channels or other model calculations, heavy-ion fusion excitation functions are not smooth near and above the Coulomb barrier. There appears to be weak but noticeable oscillations or structures within the excitation functions that can be observed clearly in the representation d(σE)/dE and in comparison with theoretical calculations σ(E) - σ_th(E). Moreover, the corresponding d(σE)/dE spectra for systems that form the same compound nucleus also overlap well in this energy range, including their fine structures, but the uncertainty is large. It appears the two behaviors are correlated and the reasoning behind these behaviors are yet unknown, but may be due to the compound-channel effect.