Theoretical Determination of High-Energy Photon Attenuation and Recommended Protective Filler Contents for Flexible and Enhanced Dimensionally Stable Wood/NR and NR Composites

Abstract

This work aimed to theoretically determine the high-energy-photon-shielding properties of flexible wood/natural rubber (NR) and NR composites containing photon protective fillers, namely Pb, Bi2O3, or Bi2S3, using XCOM. The properties investigated were the mass attenuation coefficient (µm), linear attenuation coefficient (µ), and half value layer (HVL) of the composites, determined at varying photon energies of 0.001–5 MeV and varying filler contents of 0–1,000 parts per hundred parts of rubber by weight (phr). The simulated results, which were in good agreement with previously reported experimental values (average difference was 5.3%), indicated that overall shielding properties increased with increasing filler contents but decreased with increasing incident photon energies. The results implied the potential of bismuth compounds, especially Bi2O3, to replace effective but highly toxic Pb as a safer high-energy-photon protective filler, evidenced by just a slight reduction in µm values compared with Pb fillers at the same filler content and photon energy. Furthermore, the results suggested that the addition of 20 phr wood particles, primarily aimed to enhance the rigidity and dimensional stability of Pb/NR, Bi2O3/NR, and Bi2S3/NR composites, did not greatly reduce shielding abilities; hence, they could be used as dimensional reinforcers for NR composites. Lastly, this work also reported the optimum Pb, Bi2O3, or Bi2S3 contents in NR and wood/NR composites at photon energies of 0.1, 0.5, 1, and 5 MeV, with 316–624 phr of filler being the recommended contents, of which the values depended on filler type and photon energy of interest.

X-ray Shielding, Mechanical, Physical, and Water Absorption Properties of Wood/PVC Composites Containing Bismuth Oxide

“…On the other hand, Table 3 showed that increasing the wood content from 20 pph to 40 pph resulted in slightly lower values of µ and µ m , although the reduced values were less than 10% (determined for the same Bi 2 O 3 content). This lower X-ray shielding property for samples with more wood particles added to the composite was due to the much lower X-ray interaction probability for the wood particles (mostly comprised of C, H, and O) compared to Bi 2 O 3 (µ m-wood = 0.162 cm 2 /g and µ m-Bi2O3 = 5.162 cm 2 /g for 100 keV X-rays [ 45 ]), resulting in suppressed effects of Bi 2 O 3 in X-ray attenuation and thus, less X-ray shielding capability for those samples containing 40 pph wood particles [ 20 ]. Additionally, notable from Table 3 was that the values of µ and µ m decreased at higher X-ray energies (determined at the same Bi 2 O 3 and wood contents).…”

“…Additionally, notable from Table 3 was that the values of µ and µ m decreased at higher X-ray energies (determined at the same Bi 2 O 3 and wood contents). This could have been due to higher-energy X-rays being less likely to interact with materials through dominant and effective photoelectric absorption, which rapidly decreased with increasing X-ray energies/frequencies, as depicted in Equation (5) [ 20 , 46 ]: where σ pe is the photoelectric cross section, Z is the atomic number of the element, h is Planck’s constant, and ν is the frequency of the X-rays that is directly related to X-ray energy through Equation (6): …”

“…It is also notable that the values of µ and µ m for a neat PVC, calculated using a web-based software (XCOM) [ 20 , 47 ], were similar to WPVC composites without the addition of Bi 2 O 3 , due to the low σ pe of C, H, and Cl in PVC, leading to low interaction probabilities between the X-rays and the neat PVC.…”

“…In order to determine recommended Bi 2 O 3 contents in the WPVC composites for actual use, especially in medical applications, Pb eq values obtained from the datasheets of three commercial X-ray shielding products based on a gypsum board (XRoc board) [ 48 ], plasterboard (GIB X-Block board) [ 49 ] and Knauf Safe board [ 50 ], for which the average Pb eq values of the commercial products were 0.45, 0.70, and 0.40 mm Pb for the 60, 100, and 150-kV X-rays, respectively, were compared with those from the current work. It should be noted that the Pb eq values of the commercial products for 100 kV X-rays were higher than other X-ray energies due to the sharp increase in X-ray interaction probabilities at the K-edge absorption of barium (Ba), which was used as the main X-ray protective filler for the commercial products, that occurs at 37.4 keV [ 20 ]. Hence, the 100 kV X-rays, with their average energy around 40–60 keV depending on the type and setup of the X-ray machine, were just above the binding energy of the electron K shells inside the Ba atoms, leading to immensely enhanced probabilities of X-ray interaction through photoelectric absorption and, subsequently, higher Pb eq values at these particular energies [ 51 ].…”

“…To minimize the risks of Pb exposure, there has been a constant effort to identify Pb-free X-ray shielding materials having comparable or better attenuation abilities than Pb-containing materials, with additional preferred properties such as transparency, flexibility, and self-healable. Examples of recent Pb-free X-ray shielding materials are: Bi 2 O 3 /polyvinyl chloride (PVC) composites [ 17 ], W/Bi 2 O 3 /high-functional methyl vinyl silicone rubber (VMQ) composites [ 18 ], W 2 O 3 /ethylene propylene diene monomer (EPDM) composites [ 19 ], Bi 2 O 3 /wood/natural rubber (NR) composites [ 20 ], and Bi/high-density polyethylene (HDPE) composites [ 21 ], in which Bi and W compounds act as alternative X-ray protectively fillers to Pb. It should be noted that the attenuation efficiencies of the mentioned materials vary depending on the filler types, contents, and the manufacturing process of the materials [ 22 ].…”