Computational optimization of 161Tb production via neutron irradiation: analytical and MCNPX modeling in Tehran research reactor

161 Tb is a promising radionuclide for targeted beta therapy due to its favorable half-life, β⁻ emission, and co-emission of Auger/conversion electrons. In this study, its production was systematically investigated via three neutron irradiation pathways: direct 159 Tb irradiation, indirect irradiation of natural gadolinium, and enriched 160 Gd targets. An integrated computational framework combining MATLAB-based analytical modeling using Bateman chain equations and MCNPX 2.6.0 Monte Carlo simulations was applied to predict time-dependent activity evolution, assess impurity formation, and evaluate radionuclidic purity under identical irradiation conditions. The analytical model accurately reproduced the overall trends of 161 Tb and co-produced isotopes, while MCNPX provided detailed insights into neutron transport, spectral effects, and self-shielding. Among the routes, enriched 160 Gd yielded the highest 161 Tb activity with minimal long-lived impurities, natural gadolinium provided a balanced compromise between yield and purity, and direct 159 Tb irradiation was limited by significant 160 Tb co-production. This combined approach offers a robust, predictive, and efficient platform for optimizing 161 Tb production, guiding experimental design, and supporting the development of high-purity therapeutic radiopharmaceuticals.