Cobalt-60 and iridium-192 sources are both generated in nuclear reactors through the irradiation of stable target materials. Variation in: neutron energy, flux and irradiation time; target material characteristics and purity; the activation cross sections of the desired neutron reactions; decay and daughter progeny in-growth; and any post-irradiation processing, can all play a key part in determining the isotopic and chemical composition of the material produced. These isotopic ratios, together with any activated elemental impurities, could potentially be used as signatures in order to provide information relating towards not only the material’s production date, but also the source’s production route, irradiation history and original elemental and isotopic composition. All of which are key indicators for material attribution in the event of a nuclear forensics incident. Computational studies to evaluate the effects of neutron flux spectra on nuclide production within these materials have indicated a number of impurity nuclides that have the potential to be used as nuclear forensics signatures. These signatures though show significant variability depending on the irradiation position and reactor conditions used. When combined with the various issues that investigations into target material purity and manufacturing conditions highlighted regarding signature development, the complexity of the problem starts to become a strong impediment towards the identification of a material’s source of origin. It is therefore unlikely that these impurity signatures could ever be used in nuclear forensics investigations, because investigators would need to know the specific processes and procedures employed by manufacturers in order to attribute materials back to their source of origin. This is an unlikely scenario based on the number of manufacturers and the variety of processes that are employed during source production.