Channel couplings redirect absorbed flux from peripheral loss to fusion in weakly bound nuclear reactions

Abstract

In reactions of weakly bound nuclei, the absorption cross section mixes two physically distinct contributions: inner capture associated with compound-nucleus formation, and peripheral losses from breakup, transfer, and other direct reactions. Within a framework that combines an ingoing-wave boundary condition (IWBC) at an inner radius with a complex potential in the external region, we derive the exact flux identity $\sigma_{\rm abs}=\sigma_{\rm fusion}+\sigma_W$ from the radial continuity equation. The resulting partition is exact within the adopted CC/CDCC model space and provides a practical diagnostic of where absorbed flux is removed. Applied to $^6$Li+$^{209}$Bi, the analysis reveals that channel couplings qualitatively reorganize the absorbed flux: the dominant absorption mechanism shifts from peripheral loss at sub-barrier energies to inner capture above the barrier, whereas the single-channel baseline remains peripheral-loss dominated throughout. The resulting IWBC-defined inner-capture cross section tracks the measured complete-fusion excitation function with only a modest dependence on the chosen boundary radius. Together with the exact identity $\sigma_{\rm abs}=\sigma_{\rm fusion}+\sigma_W$, this agreement supports interpreting the peripheral term $\sigma_W$ as a major spatial contributor to the well-known CF suppression in weakly bound systems.