Shruti Nambiar

Polymer-Composite Materials for Radiation Protection

Unwanted exposures to high-energy or ionizing radiation can be hazardous to health. Prolonged or accumulated radiation dosage from either particle-emissions such as alpha/beta, proton, electron, neutron emissions, or high-energy electromagnetic waves such as X-rays/γ rays, may result in carcinogenesis, cell mutations, organ failure, etc. To avoid occupational hazards from these kinds of exposures, researchers have traditionally used heavy metals or their composites to attenuate the radiation. However, protective gear made of heavy metals are not only cumbersome but also are capable of producing more penetrative secondary radiations which requires additional shielding, increasing the cost and the weight factor. Consequently, significant research efforts have been focused toward designing efficient, lightweight, cost-effective, and flexible shielding materials for protection against radiation encountered in various industries (aerospace, hospitals, and nuclear reactors). In this regard, polymer composites have become attractive candidates for developing materials that can be designed to effectively attenuate photon or particle radiation. In this paper, we review the state-of-the-art of polymer composites reinforced with micro/nanomaterials, for their use as radiation shields.

Bismuth Sulfide Nanoflowers for Detection of X-rays in the Mammographic Energy Range

The increased use of diagnostic x-rays, especially in the field of medical radiology, has necessitated a significant demand for high resolution, real-time radiation detectors. In this regard, the photoresponse of bismuth sulfide (Bi2S3), an n-type semiconducting metal chalcogenide, to low energy x-rays has been investigated in this study. In recent years, several types of nanomaterials of Bi2S3 have been widely studied for optoelectronic and thermoelectric applications. However, photoresponse of Bi2S3 nanomaterials for dosimetric applications has not yet been reported. The photosensitivity of Bi2S3 with nanoscale “flower-like” structures was characterized under x-ray tube-potentials typically used in mammographic procedures. Both dark current and photocurrent were measured under varying x-ray doses, field sizes, and bias voltages for each of the tube potentials – 20, 23, 26 and 30 kV. Results show that the Bi2S3 nanoflowers instantaneously responded to even minor changes in the dose delivered. The photoresponse was found to be relatively high (few nA) at bias voltage as low as +1 V, and fairly repeatable for both short and long exposures to mammographic x-rays with minimal or no loss in sensitivity. The overall dose-sensitivity of the Bi2S3 nanoflowers was found to be similar to that of a micro-ionization chamber.

PMMA/MWCNT nanocomposite for proton radiation shielding applications

Radiation shielding in space missions is critical in order to protect astronauts, spacecraft and payloads from radiation damage. Low atomic-number materials are efficient in shielding particle-radiation, but they have relatively weak material properties compared to alloys that are widely used in space applications as structural materials. However, the issues related to weight and the secondary radiation generation make alloys not suitable for space radiation shielding. Polymers, on the other hand, can be filled with different filler materials for reinforcement of material properties, while at the same time provide sufficient radiation shielding function with lower weight and less secondary radiation generation. In this study, poly(methyl-methacrylate)/multi-walled carbon nanotube (PMMA/MWCNT) nanocomposite was fabricated. The role of MWCNTs embedded in PMMA matrix, in terms of radiation shielding effectiveness, was experimentally evaluated by comparing the proton transmission properties and secondary neutron generation of the PMMA/MWCNT nanocomposite with pure PMMA and aluminum. The results showed that the addition of MWCNTs in PMMA matrix can further reduce the secondary neutron generation of the pure polymer, while no obvious change was found in the proton transmission property. On the other hand, both the pure PMMA and the nanocomposite were 18%-19% lighter in weight than aluminum for stopping the protons with the same energy and generated up to 5% fewer secondary neutrons. Furthermore, the use of MWCNTs showed enhanced thermal stability over the pure polymer, and thus the overall reinforcement effects make MWCNT an effective filler material for applications in the space industry.

Effects of particle size on X-ray transmission characteristics of PDMS/Ag nano- and microcomposites

Polymers reinforced with nano- or micro-material(s) are increasingly being considered for the development of lightweight, conformable shielding materials against diagnostic X-rays. However, the choice of the optimal particle-size and the concentration for the filler material is not yet well studied in terms of their role in effective attenuation of the desired range of X-ray energies. In this study, X-ray transmission properties of silver nano- and micro-particles were investigated at relatively very low concentrations of 0.5, 2.73 and 5.5 weight percentage (wt%) under a wide range of X-ray energies (20, 23, 26, 30, 40, 60 and 80 kV). With increase in concentration, silver nanoparticles showed enhanced X-ray attenuation properties with respect to silver microparticles over the whole range of X-ray energies, especially at the low energy X-rays (up to 9% more attenuation at 20 kV).

Polymer nanocomposite for space applications

Polymer nanocomposites, synthesized using acid-functionalized multi-walled carbon nanotubes (MWCNTs) as filler material, are proposed for space radiation-shielding applications. The morphology and quality of the functionalized MWCNTs (f-MWCNTs) were evaluated using field emission scanning electron microscopy (FESEM), Raman spectroscopy, and Fourier transform infrared spectrometer (FTIR). It is well-established that polymer reinforced with MWCNTs enhances of the overall material properties. With the significant progress in the field of polymer composites, there is an increasing demand to use them as an alternative solution; especially in space industry wherein the launch cost can dramatically be reduced by decreasing the overall payload using lightweight, cost-effective, multifunctional polymer composites. In this work, we reinforced a high performance polymer, polybenzimidazole (PBI), with acid-functionalized MWCNTs and evaluated the effects of electron beam radiation on the PBI nanocomposites.