# Electromagnetic Radiation Shielding Using Carbon Nanotube and Nanoparticle Composites
## 2 Overview of Shielding Approaches for Different Types of Radiation  
The primary mechanisms of radiation shielding are absorption and scattering, where the radiation interacts with the shielding material by either completely absorbing its energy or deflecting it by changing its direction, effectively reducing its intensity as it travels through the absorber. Radiation shielding primarily occurs through four types of interactions: (i) simple coherent scattering (low energy photons, no absorption), (ii) the photoelectric effect (dominant at low energies), (iii) Compton scattering (dominant at medium energies), and (iv) pair production (dominant at high energies). The interactions depend on the radiation energy and the material composition. Each process contributes to absorbing and deflecting radiation within a shielding material, effectively reducing its intensity by transferring energy to electrons within the material. Materials with high atomic numbers are generally better at shielding due to increased interaction probability with photons. The different mechanisms of X-ray and gamma ray shielding are illustrated in Figure 1 and described briefly next [5; 9; 13; 25; 29]. In simple coherent scattering (Figure 1a), the photon has lower energy than the minimum energy needed to release an electron. Thus, the photon simply scatters, changing its direction, without losing energy. Higher energy (gamma) radiation ionizes atoms through three processes. The first process is the photoelectric effect (Figure 1b). This involves the complete absorption of the photon by an electron causing a secondary electron called a photoelectron to be ejected from the atom. The incoming photon is completely absorbed by an inner shell electron of an atom. This causes the electron to gain enough energy (excluding the binding  
Figure 1: Schematic representation of photon scattering and attenuation mechanisms within a material. Incoming photons can interact through the following: (**a**) simple scattering, photons are scattered by simple scattering; (**b**) the photoelectric effect, photons are absorbed through the photoelectric effect; (**c**) Compton scattering, photons are scattered by interaction with an electron; (**d**) pair production, photons generate electron–positron pairs via pair production depending on their energy and interaction with the material’s electrons. Note the figures are not to scale with respect to the distance between the nucleus and the electrons. Ideally, we would like to arrange different types of atoms in a material to most effectively block radiation. Arranging nanoparticles in a composite material is an attempt to do this at a larger size scale. energy) to escape from its orbit. The free photoelectron will ionize, excite, and produce bremsstrahlung radiation, which is an X-ray photon as it deposits energy along its path in the absorber until it stops. The kinetic energy of the photoelectron is equal to the energy of the incident gamma photon minus the binding energy that held the electron to the atom. The photoelectric effect is the main energy-transfer mechanism for X-ray and gamma ray photons with low energy, below 50 keV. The second process is Compton scattering (Figure 1c). This involves a collision between an incoming photon and an absorber electron in which some of the photon energy is shared with the electron. After the collision, the photon will have less energy and scatters in an arbitrary direction while the electron gains energy and scatters only in the forward direction. The scattered electron will ionize, excite, and produce bremsstrahlung in the absorber until it comes to rest. In Compton scattering, the incident gamma photon shares a fraction of its energy with an outer bound or free electron and is scattered at some direction from its original direction. Compton scattering is considered to be the principal absorption mechanism for gamma rays in the middle energy range 100 keV to 10 MeV. Compton scattering depends on the atomic number of the absorbing material.