Nanoparticles as Fillers in Composites for X-Ray and Gamma-Ray Shielding: A Review

Introduction

Significant advancements in nuclear science and technology have hastened the worldwide usage X-rays and -rays in a plethora of industries, including medical diagnostic imaging and radiation therapy, food sterilizing and cultivation, nuclear research laboratories, aeronautical, and space exploration, among others. Prolonged exposure to these radiations poses serious threat to human health, causing genetic damage, leukemia and even death. Although limiting the exposure time and keeping a safe distance from radiation source is instrumental in achieving low dose accumulation, the use of personal protective equipment (PPE) would be the most feasible way to reduce the radiation exposure. Along with personnel protection, it is essential to address the challenges presented by nuclear research labs, aerospace, or observational astronomy, wherein high energy radiations often damage the electronic equipment and instrumentation facilities. Hence, radiation protection and shielding are inevitable for medical personnel and electronic instruments closely associated with radiation field.

From the physics of interactions of radiations with matter, it is known that the attenuation of X-rays and γ-rays increases significantly with increase in atomic number (Z) of the interacting medium. Accordingly, high-Z and high-density materials are preferred for radiation shielding and the conventionally employed materials are lead (Pb), bismuth (Bi), tungsten (W), tin (Sn) and antimony (Sb), manufactured mostly in the in the shape of sheets, fibres, or thin films. Among them, lead with high-Z (82), high density (11.34 g cm³) and higher interaction cross-section has been most widely used for shielding. Similar shielding properties are provided by bismuth and tungsten, however at greater mass thicknesses than Pb. As a result, efforts have been made to develop non-toxic and non-carcinogenic materials to replace lead and minimize the weight and thickness of PPE. Currently, the Today's radiation protective aprons typically have a shielding efficiency of 0.25 to 0.5 mmPb lead equivalent corresponding to 3 mm thickness of overall shielding material. However, increasing the density of the shielding material by reducing porosity and enhancing the particle packing density, is the most practical and simplest technique to reduce the thickness of the shielding material. Reducing the size of the material's constituent particles, or using nanomaterials, is another way to achieve this.

Nanomaterials, especially nanoparticles, are in high demand for a range of practical applications. The design and development of high-performance materials is often facilitated by novel aspects of nanoparticles such small size effect, surface and boundary effect, increased surface-to-volume ratio, and quantum size impact. Nanoparticles offer tunable physicochemical properties, such as soft magnetic properties, tensile strength, and ductility, as well as high melting point, good diffusivity, better wettability, high catalytic activity, greater luminous efficiency, higher electrical and thermal conductivity, compared to their bulk counterparts. Thus, nanoparticles constitute a dynamic area of research and a socio-economic sector that contributes to almost every field of science including physics, chemistry, materials science, biomedical and aerospace engineering, and computer science. One of the most significant advances, in the recent years, has been towards human health with promising results, notably in the field of radiation shielding and protection. Particle size reduction results in shorter interparticle distances, which enhances the material's packing density. In contrast to micron-sized particles, which create holes in the system as seen in Figure 1, nanoparticles occupy every interstitial space. Consequently, attenuation of incident photon energy in nanomaterials is greater than their micro counterparts, mostly due to increased probability of interaction of photons with nanoparticles of the attenuation medium. Therefore, nanoparticles can be either used independently or incorporated as additives/fillers into structural materials like concrete, glass and polymers and molded into desired shape and size according to the end-user application so as to achieve enhanced shielding performance.

Figure 1.

Representative internal structure of materials constituted of micro and nanoparticles