Nuclear cascade class as a function of luminosity L_γ and collision radius R in the internal shock scenario. The injection composition is pure ¹⁶O (left left) or ⁵⁶Fe (right panel). The other parameters are the same as in Fig. 4. Black dots indicate the position of the examples shown in Sect. 3. The gray dashed contours display log ₁₀ (E_i,max[GeV]) in the observer’s frame. Below the photosphere (red solid line), gamma-rays cannot leave the source anymore due to electron-positron pair production. We note that in this figure we fix Γ = 300, and scale t_v ∝ R according to Eq. (8).