Spectral index γ of the energy-per-nucleon spectra in the shoulder region vs. Q/M and the fit γ ∝ (Q/M)^−0.64 (a). Scaling of the proxy of the ion abundance, defined as number of particles above a threshold energy normalized to the total number of particles, with Q/M for threshold energy 10 (red filled circles), 25 (blue plus signs), and 50 (black filled squares) times the initial thermal energy KE_th (b). The straight lines are the fit, abundance ∝(Q/M)^−α, for the data and are plotted with the color corresponding to the data.