Magnetite Nanocomposite Films for Radiation Shielding
August 11, 2015
Author(s): Sayed M. Badawy
Exposure to high-energy or ionizing radiation may result in cell mutation, cancer, and death. As a result, much research has been devoted to developing shielding materials to attenuate high-energy radiation such as x-rays and γ-rays. However, existing radio-protective gear made from lead and other ‘high-Z’ materials is heavy and bulky.1 (Z is atomic mass.)
Polymer-based composites show promise for lighter radiation shielding. They are lighter than their metal counterparts and can be processed to achieve effective shielding for radiation associated with specific industries.2 They also combine the excellent functional properties of the nanoparticles with those of the host polymers. Incorporating even just a few percent of nanoparticles can dramatically change the properties.3 The surface area and dangling or unsaturated bonds on the surface of the nanoparticles together lead to interface polarization and multiple scattering, which contribute to shielding.4 There are three main mechanisms of high-energy photon interactions: photon scattering (elastic or inelastic), the photoelectric effect, and pair production.1
Recently, we developed magnetite Fe3O4 nanocomposite films by co-precipitation with polyvinyl alcohol (PVA) for applications in radiation protection and absorption of gamma radiation.5 The co-precipitation method consists of mixing highly basic solutions of ferric and ferrous ions in a 2:1 molar ratio at room temperature in the presence of PVA in solution.
X-ray diffraction (XRD) spectra of the magnetite/PVA nanocomposite films matched crystal planes of magnetite Fe3O4 with a cubic crystal structure. We used the Davis and Mott expression6 to obtain values for the size of the indirect and direct energy bandgaps of 1.82eV and 2.81eV, respectively, from the UV-visible spectra of the magnetite nanocomposite thin film. These bandgaps classify this sample as a semiconductor. Transmission electron microscopy (TEM) images of the magnetite/PVA nanocomposite films showed that the nanoparticles incorporated into the polymer matrix were well dispersed within a smooth polymer matrix (see Figure 1). The magnetite nanoparticles were 6–11nm in diameter.
Transmission electron microscopy images of a magnetite nanocomposite thin film.
We evaluated the magnetic properties of the films using a vibrating sample magnetometer (VSM). They showed typical superparamagnetic behavior at room temperature. The saturation magnetization (Ms) at room temperature was 0.4176memu/cm2 (8.1emu/g). This is much smaller than that of pure magnetite nanoparticles, most likely because the magnetite nanoparticles are incorporated into a non-magnetic polymer matrix. Ms, defined as the maximum of the magnetization value achieved in a sufficiently large magnetic field, is one of the most important and controversial properties of magnetic nanocrystals.
We compared the radiation shielding properties of the magnetite/PVA nanocomposite with those of pure PVA and lead for radiation with a photon energy of 0.662MeV from a cesium-137 source (see Table 1). Cesium-137 is used in medical and industrial radiography, radiation gauges, food irradiators, and soil testing. The magnetite/PVA nanocomposite outperforms pure PVA, which has only 8% of the shielding ability of lead. In contrast, the magnetite/PVA nanocomposite has nearly 59% of lead's shielding ability, in terms of the linear attenuation coefficient, the fraction of photons that interact with the shielding medium per centimeter of shielding. However, the magnetite nanocomposite surpasses lead for lightness and flexibility, being only 16% as dense as lead. It is worth noting that photon scattering (reflection at various surfaces or interfaces in the shield) requires the shield to have a large surface area or interface area, such as that provided by the surface of the nanoparticles.Table 1.
Radiation shielding properties and density of the magnetite/polyvinyl alcohol (PVA) nanocomposite compared to pure PVA and lead at photon energy of 0.662MeV.
|Materials||Density, g/cm3||Linear attenuation coefficient, cm−1||Half-value thickness, cm|
In summary, we have shown that a light, flexible magnetite/PVA nanocomposite successfully shields from high-energy radiation. We are now investigating the shielding properties for a variety of radiation types of similar nanocomposites prepared with different nanoparticles and host polymers.
AuthorSayed M. Badawy
Sayed M. Badawy studied chemistry at Cairo University, and obtained his PhD in physical chemistry in 2001. He has been employed as a fellow of chemistry at Cairo University since 2008. He is the author or co-author of about 22 peer-reviewed publications including polymer and environmental chemistry.
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