# Electromagnetic Radiation Shielding Using Carbon Nanotube and Nanoparticle Composites
## 7 Mechanisms for Small Particles to Block Radiation
Radiotherapy was discussed as a common application for materials that protect against radiation. Several applications were presented. For example, high-energy ionizing radiation in the tens of MeV range is used in cancer treatmentto kill tumors. Lower-energy ionizing radiation such as X-rays with energies in the KeV range are used in interventional procedures and in diagnostic radiology such as computed tomography [48]. Nuclear power plant workers and aircraft crews are also susceptible to exposure to radiation. Radiation-protective clothing that is flexible and lightweight can be produced for these applications based on the polymer composite materials discussed. This paper provides references that propose, mechanistically, because of their large surface area-to-volume ratio, that nanoparticles have an increased ability to absorb photons. Photon scattering, the photoelectric effect, and pair production are discussed as methods of radiation blocking. The two most important mechanisms for radiation shielding are described as follows: 1--energy loss due to radiation ionization or excitation of the target material, and 2--nuclear fragmentation of projectile ions, or target material or both. The intensity of secondary radiation is proportional to the square of the atomic number of the target material and inversely related to the mass of the incident particles. This type of secondary radiation is bremsstrahlung radiation. Different-atomic-number materials are used often together to reduce the energy of secondary radiation. Hydrogen has a low atomic number and the smallest atomic diameter. Thus, hydrogen provides many interaction points in a polymer to arrest projectile fragmentation. It is proposed that the absence of elements heavier than carbon will minimize the production of target fragments and hence reduce the secondary radiation. Polyhedral oligosilisesquioxane (POSS) is a particle comprised primarily of inorganic silicon-oxygen cage-like structures ranging from 0.5 to 3 nanometers in diameter [48]. Adding POSS into polyimide can improve the oxidation resistance of the nanocomposite. The improvement is due to a protective silica layer that formed when the nanocomposite was exposed to high levels of radiation composed of atomic oxygen in a low earth orbit. An overview of the different sources of radiation encountered in representative industries is given in ([48], reference Table 1). For cosmic radiation shielding, it is stated that the only practical solution for developing effective shielding materials is by breaking up heavy ions into smaller fragments while minimizing target fragmentation. Also, experimental and simulation studies reported that nanocrystalline materials provided enhanced radiation resistance as compared to polycrystalline materials [48]. The addition of a small weight fraction of nanoclay into different polymers increased radiation resistance by grafting the polymer molecules onto the nano layered clay. The researchers proposed forming nanocomposites of polybenzimidazole and carbon nanofibers or other nanomaterials as a durable polymer material for space applications. It was concluded that composites with a few heavy atoms within the light matrix are a more efficient shield against high-energy photons than a uniform electron-rich material, although the mechanism of the interaction is not described in detail. Regardless of the material used, the absorption was independent of the atomic number of the material and limited by the areal density [48]. Also, intercalation increased the shielding effectiveness because of a higher mass density. Different researchers referenced in the paper explored using CNTs for improving radiation resistance and improving the mechanical strength of hydrogenous polymers. It was stated that the addition of CNT fillers had a dramatic reinforcement effect on the radiation-induced degradation of PMMA and its successful dispersion of the radiation. The CNT material can protect the matrix against degradation from radiation.