Abstract: Embodiments of the invention comprise a filler to block the radiation, and an organic resin, such as epoxy or modified cyanate ester. Fillers may comprise passivated Gadolinium and Tungsten. The Gadolinium may be passivated by atomic deposition of a nano-layer of silica or alumina. The passivation layer reduces the chemical activity of the filler while retaining the maximum attenuation performance of the pure metal. Radiation performance is optimized by reaching optimum material density, as density is proportional to radiation attenuation performance. The proportion of passivated Gadolinium to Tungsten may be selected based on the radiation shielding performance and environment for such species as X-ray, thermal neutrons, gamma and cosmic rays.