# Electromagnetic Radiation Shielding Using Carbon Nanotube and Nanoparticle Composites
## 7 Mechanisms for Small Particles to Block Radiation  
Different studies have indicated that small particles distributed in a matrix material are more effective in blocking radiation than large size particles on an equal mass basis. However, the literature does not satisfactorily explain the mechanisms whereby small particles in a matrix material can block radiation more effectively than large-size particles. Another open question is whether small particles will block radiation better than an equal mass of the same type of solid material. The literature does not explain mechanistically if small particles can block radiation better than solid metal. This section reviews some of the literature related to the effect of the size or thickness of a small particle on its ability to block radiation. [PERSON] et al. discussed mechanophysical synthesis of core/shell hybrid supraparticles [92]. Supraparticles are formed by embedding hard nanoparticles into soft polymer microparticles. The surface modification of the polymer microparticles is performed by dry mixing and the impact of nanoparticles and polymer microparticles that have different elastic moduli. This is called mechanical physical synthesis and is a new method for the assembly of heterogeneous microparticles that can be used for water filtering, sensing, and other applications. It is possible that the polymer microparticles could also be used for radiation blocking because the multivalent assembly of the hybrid supraparticles can be performed with different materials. Materials in the nanoscale range that can be used include boron nitride, molybdenum disulfide, copper, and different combinations of elements. Hybrid supraparticles can be formed using multiple gas materials assembled onto the same microparticle. This enables the design of multifunctional materials. Overall, this paper offers a new method not yet explored for fabricating multiscale and multifunctional hybrid particles for radiation shielding that provides properties that are improved in many cases from the properties of bulk materials. [PERSON], [PERSON], and [PERSON] studied the effect of the size and the concentration of gold nanoparticles on X-ray attenuation [93]. They used computed tomography as a measurement method and showed that smaller gold nanoparticles have greater X-ray attenuation than larger gold particles. Increasing the concentration of nanoparticles also increased the X-ray attenuation. It was hypothesized that the smaller gold nanoparticles had greater X-ray attenuation than the larger gold particles because of the increase in the surface area to volume ratio for the smaller gold nanoparticles. For the same mass of gold, smaller nanoparticles have a larger surface area. X-ray attenuation is dependent on the target area. Thus, small gold nanoparticles with large surface area should have greater X-ray attenuation. Smaller particles therefore will be more efficient as a computed tomography contrast agent. It was also shown that coated gold nanoparticles with sizes ranging from 4 nm to 60 nm were stable in physiological conditions and non-toxic to certain cells. The 4 nm and 20 nm gold nanoparticles showed higher X-ray attenuation than a commercial contrast agent under the same molarity. X-ray attenuation from gold nanoparticles is therefore both size and concentration dependent. Smaller gold nanoparticles at higher concentrations provided greater attenuation and may be an alternative to commercial contrast agents. A comprehensive study considering different types of materials and size nanoparticles and microparticles needs to be performed. Polymer composite materials were studied for radiation protection by [PERSON] and [PERSON] [48]. Nano or micro tubes, fibers, whiskers, or nano or micro particles or nano platelets were added to a polymer matrix to form a polymer composite material for radiation protection. Polymer-based composites that are candidates for radiation shielding are described for various applications.