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Radiation shielding materials play a critical role in many commercial and industrial applications such as medical imaging and therapy, nuclear waste storage, space exploration, and food sterilization. Unwanted exposure to ionizing radiation could be hazardous to humans and the environment, as it can cause organ damage or cell mutations in living organisms, mechanical component failure, and other harmful effects.
Researchers at North Carolina State University have developed a novel composite material that contains bismuth trioxide particles and shows enormous potential for replacing the traditional radiation shielding materials as a lightweight and easy to manufacture alternative.
Radiation shielding materials scatter or absorb ionizing electromagnetic (EM) radiation, such as x-rays or gamma-rays, through processes of Compton scattering, photoelectric effect, and through the production of electron-positron pairs in the case of high-energy radiation.
All these processes involve energy transfer between the incident radiation photons and the electron shells surrounding the shielding material's nuclei.
Heavy Atoms for Better Protection from Ionizing Radiation
Materials containing elements with a high atomic number (Z) and a large number of electrons in the atoms, such as lead, copper, and stainless steel, have a better ability to attenuate high-energy EM radiation.
The most widely used material for protection from radiation exposure is lead, but its relatively high price, chemical reactivity, high density, and toxicity make it far from ideal for a wide range of industrial and research applications.
Polymer Composite as a Non-Toxic Alternative for Radiation Shielding Applications
New research conducted by Dr Ge Yang and co-workers at the Department of Nuclear Engineering, North Carolina State University, led to the development of a novel radiation shielding material consisting of bismuth trioxide ceramic particles embedded in a matrix of poly(methyl methacrylate) (PMMA, commonly known as 'acrylic plastic').Bismuth is an element with a high atomic number (Z = 83 compared to Z = 82 for lead) and is slightly less dense than lead (with densities of 9.78 g cm-3 and 11.34 g cm-3, respectively). It can efficiently scatter and absorb EM radiation (better than steel, for instance), and is considered non-toxic.
Bismuth is used in bismuth-impregnated latex shields to minimize exposure during medical examinations such as x-ray imaging or computed tomography scans. However, it is a brittle material with a tendency to crack. This is a significant disadvantage as a crack can compromise its radiation shielding properties.
High-Density Ceramics with High Radiation Attenuation
Bismuth trioxide, on the other hand, is a naturally occurring ceramic material that is widely used in a range of industrial applications such as semiconductor devices, electronic and electro-optical components, magnetic memory, and catalysis.
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Despite being slightly less dense than bismuth (with a density of 8.9 g cm-3), bismuth trioxide nanoparticles are increasingly used as a contrasting agent for medical imaging due to their biocompatibility and excellent EM radiation attenuation.
Cost-Effective Acrylic Matrix Boost Radiation Shielding Performance
The main advantage of using PMMA as a lightweight matrix for the ceramic particles in the new composite material is its low cost, excellent chemical and mechanical resistance, and ease of manufacturing.
Importantly, acrylic plastic is the material of choice when shielding high-energy charged particles (beta-radiation), where the use of high-Z materials is disadvantageous as these create a significant amount of secondary x-rays when charged particles collide with the heavy atoms and lose energy.
This makes the combination of low-Z acrylic plastic with a high-Z ceramic material especially suitable for radiation shielding applications.
Rapid Ultraviolet Crosslinking Can Shorten Manufacturing Times
Another significant advantage of the composite material developed by Dr Yang's team is that it can be manufactured very quickly.
Instead of using heat to solidify the polymer matrix (similar to the traditional thermoset polymers), the researchers have developed a crosslinking process that is initiated by ultraviolet (UV) light. Crosslinking the network of polymer chains gives the material its mechanical strength and enhances its performance in a radiation environment.
Using the UV curing method, the researchers were able to manufacture the new composite material in a matter of minutes at room temperature rather than several hours at high temperature. This demonstrates the rapid manufacturing potential of the new method.
Lightweight Composite for Demanding Industrial Applications
The short manufacturing time allowed the scientists to prepare a range of samples where the loading with bismuth trioxide particles varied between 0% and 44% (wt.) and to subject the samples to a series of gamma-radiation shielding tests.
An important parameter used to evaluate the material's shielding properties is the thickness of the half-value layer (HVL), which is the thickness of the material that causes 50% attenuation of the incident gamma-ray intensity.
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Pure PMMA (0% bismuth trioxide) has HVL of 38 cm, while PMMA with a 44% loading of bismuth trioxide particles has a HVL of only 5 cm.
Comparing the shielding performance of the composite material reinforced with bismuth trioxide particles to traditional radiation shielding materials, such as lead, steel, and concrete, reveal HVL values that lay between those of concrete and steel, with the added benefit of a much-reduced weight.
This makes the new composite material a lightweight, easy to manufacture and effective alternative to the traditional lead-based materials for shielding against ionizing x-ray and gamma radiation in a range of applications such as space exploration, medical imaging, and radiation therapy.
References and Further Reading
D. Cao et al., (2020) Gamma radiation shielding properties of poly (methyl methacrylate) / Bi2O3 composites. Nuclear Engineering and Technology (in press). Available at: https://doi.org/10.1016/j.net.2020.04.026
H. Bi et al., (2018) Bismuth Nanoparticles with "Light" Property Served as a Multifunctional Probe for X‑ray Computed Tomography and Fluorescence Imaging. Chem. Mater. 30, 3301−3307. Available at: https://dx.doi.org/10.1021/acs.chemmater.8b00565
M. R. Ambika et al., (2017) Role of bismuth oxide as a reinforcer on gamma shielding ability of unsaturated polyester based polymer composites. J. Appl. Polym. Sci., 134, 44657. Available at: https://dx.doi.org/10.1002/app.44657
K. D. Hopper et al., (1997) The breast: in-plane x-ray protection during diagnostic thoracic CT-shielding with bismuth radioprotective garments. Radiology, 205, 853–858. Available at: https://dx.doi.org/10.1148/radiology.205.3.9393547
SpecialChem (2020), New Bismuth Trioxide-embedded Polymer Compound for Radiation Shielding. [Online] www.omnexus.specialchem.com Available at: https://omnexus.specialchem.com/news/industry-news/new-bismuth-trioxide-radiation-000221639?lr=iom20051935 (Accessed on 29 May 2020).
North Carolina State University (2020), Polymer composite could serve as lighter, non-toxic radiation shielding. [Online] www.sciencedaily.com Available at: www.sciencedaily.com/releases/2020/05/200511142110.htm (Accessed on 29 May 2020).
North Carolina State University (2020), Study Suggests a Polymer Composite Could Serve as Lighter, Non-Toxic Radiation Shielding. [Online] www.news.ncsu.edu Available at: https://news.ncsu.edu/2020/05/polymer-radiation-shielding/ (Accessed on 29 May 2020).