# Electromagnetic Radiation Shielding Using Carbon Nanotube and Nanoparticle Composites
## 1 Introduction
### Alpha, Beta, X-Ray, and Gamma Radiation Processes  
Depending upon the isotope, the process of radioactive decay may result in the emission of an alpha or beta charged particle from the nucleus of the atom. X-ray and gamma radiation may also accompany alpha and beta decay events [14]. Alpha radiation occurs when an unstable nucleus ejects a positively charged particle that has the same characteristics of a helium nucleus consisting of two neutrons and two protons. Alpha particles are emitted from a nucleus with unique energy ranging from approximately 4 MeV up to approximately 9 MeV. Even with such high energy, the alpha particle is easily stopped by the outer layer of dead skin. However, once inhaled or ingested, the alpha particle will interact and deposit energy in cells of living tissue that are likely to cause damage that may disrupt cellular function [14; 15; 16].  
The process of beta decay is the result of a neutron in the nucleus being transformed into a proton, an electron, and an antineutrino. The electron is a called a beta particle and shares the decay energy with the antineutrino such that beta particles exhibit a range of kinetic energy up to some maximum that is unique to the isotope. Beta particles are much smaller than alpha particles and exhibit greater penetration in an absorber, such as fabrics or living tissue. A major challenge in shielding beta radiation is that beta particles can ionize, excite, and produce bremsstrahlung radiation as they interact with the absorber atoms. An understanding of X-rays for medical imaging and their detrimental effects on human health is discussed in [17; 18; 19; 20; 21; 22; 23]. Using lead aprons for shielding also has detrimental effects [24]. The effects of gamma ray bursts are discussed in [25].  
Gamma radiation is emitted when nuclei transition from a higher energy state to a lower one [25]. Gamma rays are bundles of energy (i.e., photons) that have no mass and travel as a wave at the speed of light. They also exhibit interactions that are characteristic of particles having a mass equal to E/c\({}^{2}\), where E is the photon energy and c is the speed of light. In deep space, natural sources of gamma radiation include neutron stars and pulsars, making radiation shielding critical for space travel [25; 26; 27; 28; 29; 30]. On the surface of the Earth, gamma radiation is generated by cosmic particles interacting with atmospheric gases. The ability of a material to shield radiation is a combination of its density and the radiation interaction probability known as the attenuation coefficient. Although high-density materials, such as lead and tungsten, are good absorbers for radiation, their weight makes them unsuitable as shields as solid materials in space applications [31; 32].  
\begin{table}
\begin{tabular}{l l l l} \hline  & **Frequency (Hz)** & **Wavelength (m)** & **Energy (J)** \\ \hline Radio Frequency (RF) & below \(3\times 10^{9}\) & above \(1\times 10^{-1}\) & below \(2\times 10^{-24}\) \\ \hline Microwave (MW) & \(3\times 10^{9}\)–\(3\times 10^{11}\) & \(1\times 10^{-3}\)–\(1\times 10^{-1}\) & \(2\times 10^{-24}\)–\(2\times 10^{-22}\) \\ \hline Infrared (IR) & \(3\times 10^{11}\)–\(4\times 10^{14}\) & \(7\times 10^{-7}\)–\(1\times 10^{-3}\) & \(2\times 10^{-22}\)–\(3\times 10^{-19}\) \\ \hline Optical & \(4\times 10^{14}\)–\(7.5\times 10^{14}\) & \(4\times 10^{-7}\)–\(7\times 10^{-7}\) & \(3\times 10^{-19}\)–\(5\times 10^{-19}\) \\ \hline Ultraviolet Light (UV) & \(7.5\times 10^{14}\)–\(3\times 10^{16}\) & \(1\times 10^{-8}\)–\(4\times 10^{-7}\) & \(5\times 10^{-19}\)–\(2\times 10^{-17}\) \\ \hline X-Ray & \(3\times 10^{16}\)–\(3\times 10^{19}\) & \(1\times 10^{-11}\)–\(1\times 10^{-8}\) & \(2\times 10^{-17}\)–\(2\times 10^{-14}\) \\ \hline Gamma-Ray & above \(3\times 10^{19}\) & below \(1\times 10^{-11}\) & above \(2\times 10^{-14}\) \\ \hline \end{tabular}
\end{table}
Table 1: Regions of the electromagnetic spectrum. Data rearranged from [13].