Upper panel: components of the cross section for photons interacting with nuclei via process l: photoionisation (σ^PI), Compton scattering (σ^C), and pair production (σ^pair). The momentum-transfer (MT) cross section is plotted for Compton scattering, while for catastrophic (PI and pair) processes the cross section coincides with the MT cross section. The grey lines depict the corresponding loss functions near to their crossing (in arbitrary units). The vertical grey stripes, indicating the energy intervals between the crossing points of the corresponding cross sections and loss functions, separate the diffusive and catastrophic regimes of the photon transport (see text for details). Lower panel: the relative average energy lost by a photon per collision, ⟨Δ E_γ ⟩∕E_γ, vs. the photon energy, Eq. (23).