Impact of injection composition: total all-flavor neutrino fluence per shell of a GRB as a function of the energy in the observer’s frame. Different luminosities (panels) correspond to the examples for the prototypes in Sect. 3. The different curves correspond to the injection of pure isotopes: protons, ⁵⁶Fe, or any other isotope in between (see legend). This comparison relies on equal injection luminosities for the primaries with a baryonic loading (luminosity in nuclei versus photons) of ten, and on equal injection spectra ∝ E⁻² .