# Electromagnetic Radiation Shielding Using Carbon Nanotube and Nanoparticle Composites
## 2 Overview of Shielding Approaches for Different Types of Radiation
The third process is pair production (Figure 1d). Interactions of photons with energy greater than 1.02 MeV in an absorber may result in the annihilation of the photon producing a positive and negative electron each with half the energy of the photon. The positive and negative electrons will ionize, excite, and produce bremsstrahlung in the absorber until they stop. Following pair production, when the positron comes to rest in the absorber, it will annihilate with a free electron producing two 0.511 MeV photons that travel in opposite directions, Figure 1d. These two photons will also ionize, excite, and produce bremsstrahlung in the absorber. Pair production mainly occurs at energies greater than 5 MeV. In pair production, the incident photon interacts with the electric field of a nucleus. The energy of the incident photon is converted into the mass of an electron-positron pair. Gamma ray energy greater than the equivalent rest mass of the two particles appears as the kinetic energy of the pair and in the recoil of the emitting nucleus. This interaction mechanism suggests that in the design of nanocomposites, the nano-particles should be distributed to maximize the probability that the incident photons would interact with the nucleus of the atoms in the nanoparticles. At the end of the positron's range, it combines with a free electron. The two annihilate, and their entire mass is then converted into two gamma photons of 0.51 MeV each or higher energy depending on the kinetic energy of the annihilated particles. This suggests the use of a matrix material in a nanocomposite that can intercept secondary radiation. The secondary electrons or positrons produced in any of these three processes frequently have enough energy to produce ionization themselves. Another effect is that gamma rays having energy generally in the range of 10 MeV or higher may cause the target atom to undergo photodisintegration where a nucleus absorbs a high-energy gamma ray and ejects subatomic particles such as a proton, neutron, or alpha particle. In some cases, photodis fission also occurs where a nucleus splits into two nearly equal fragments after absorbing a photon. Although photodisintegration can attenuate radiation, it also produces secondary radiation such as neutrons or protons that need to be shielded in materials prone to this reaction. Lead (Pb) and other heavy metals have traditionally been used for radiation shielding in industries such as healthcare, aerospace, defense, and nuclear energy [32; 33]. However, due to its toxicity, lead can cause several health problems, including cancer, slowed growth, memory loss, and learning disabilities [32; 33; 34]. Lead is also heavy, which is a limitation for many applications, especially for use in space travel. Consequently, developing suitable materials that can effectively absorb radiation to replace lead is crucial. For optimal radiation protection, shielding designs often incorporate multiple layers of materials to effectively attenuate radiation across a range of energy. Scientists and engineers have been extensively investigating various alternatives to lead for radiation shielding. As an illustration, polydimethylsiloxane (PDMS), a versatile polymer composed mainly of silicon, oxygen, and carbon, has been widely considered in studies of radiation-shielding composites. PDMS has excellent physical properties such as resistance to heat, moisture, and chemicals, along with flexibility, transparency, stability, biocompatibility, and non-toxicity [35; 36; 37; 38; 39]. An introduction explaining various approaches to shielding different types of radiation is given below. According to [PERSON] et al. [35], dispersing nanocomposites of single-walled carbon nanotubes (SWCNTs) in PDMS can enhance proton shielding. When tested under monoenergetic proton beams, the combination of PDMS and SWCNTs was lower in weight compared to adding other materials like aluminum (Al) to achieve the same proton-stopping capability.