Fig. 1. Experimental principle and onset of complexity in reaction outcomes.

a Depicted is the internuclear potential (blue) of ⁴⁰Ca+²⁰⁸Pb nuclei (with centre-to-centre separation R), the nuclear potential V_nuc (red)⁵², and the sum of the Coulomb and centrifugal potentials V_coul + V_cent (green). The yellow crosses show the deduced distance between the barrier R_B and the distance of closest approach $R_{\min }$, $R_{\min }-R_{\mathrm{B}}$, at each measured energy E_cm, taking into account the change in angular momentum l at the measurement angle. b Left axis: proportion of the reflected flux P/P_reflected that is made up of ⁴⁰Ca (lilac squares), statistical errors are smaller than the points. Right axis: The smallest number of nuclide pairs required to make up 95% of the reflected flux (N₉₅, green diamonds). c The deduced excitation energy distribution E_x. To show the evolution of the excitation energy with decreasing surface separation, the probabilities were normalised to the total reflected flux such that the integral at each $R_{\min }-R_{\mathrm{B}}$ is equal to 1. These distributions will be modulated by absorption at higher energies (above E/V_B = 0.91, below $R_{\min }-R_{\mathrm{B}}=1.93$ fm (Supplementary Fig. 2a)) which will not be equally likely for all E_x. The data has been interpolated between measurement energies using Delaunay triangulation⁵⁷. The black points show the mean excitation energy at each measured energy.