On the equivalence of neutron source and flux spectra

The equivalence between the neutron source and flux spectra is often implicitly used in practice although many times the users are not even aware of this. This work identifies two conditions under which the equivalence holds. Namely, if the neutron interaction between the source region and the volume where flux is observed is negligible, and the neutron mean track length in the observed volume does not depend on their energy, source and flux spectra are equivalent. Consequently, a flux determined on a closed surface from a full system calculation can be replaced by an equivalent source for a simplified model including only the region contained by the surface.