|Year : 2019 | Volume
| Issue : 4 | Page : 180-186
Preparation and characterization of bismuth-filled high-density polyethylene composites for gamma-ray shielding
M Sheela1, Vinayak Anand Kamat2, KU Kiran3, KM Eshwarappa4
1 Research and Development Centre, Bharathiar University, Coimbatore, Tamil Nadu; Departments of Physics, Government Science College (Autonomous), Hassan, Karnataka, India
2 Centre for Application of Radioisotopes and Radiation Technology, CARRT, Mangalore University, Mangalore, Karnataka, Ind, India
3 Department of Electronics, Government Science College (Autonomous), Hassan, Karnataka, India
4 Research and Development Centre, Bharathiar University, Coimbatore, Tamil Nadu; Department Studies in Physics, Davanagere University, Davanagere, Karnataka, India
|Date of Submission||05-Sep-2019|
|Date of Decision||09-Oct-2019|
|Date of Acceptance||22-Oct-2019|
|Date of Web Publication||27-Jan-2020|
Dr. K M Eshwarappa
Research Supervisor, Bharathiar University, Coimbatore-641046, Tamilnadu; Associate Professor, Department of Studies in Physics, Davanagere University, Shivagangotri-577007
Source of Support: None, Conflict of Interest: None
An attempt has been made to prepare the nonlead-based gamma shielding materials by adding different weight percentages (0%, 10%, 20%, and 40%) of bismuth in high-density polyethylene (HDPE) matrix. Gamma shielding parameters such as linear attenuation coefficient (μ), mass attenuation coefficient (μm), half-value layer, tenth value layer relaxation length (λ), and percentage attenuation were determined for all synthesized samples using 3“×3” NaI (Tl) detector for 59.54 keV energy. The results obtained from attenuation studies have shown that the shielding efficiency of synthesized HDPE+Bi composites increases with an increase in the weight percentage of bismuth. The experimentally obtained mass attenuation coefficient values are compared with Monte Carlo simulation using Monte Carlo N-particle code and XCOM code and are in good agreement with each other. The mechanical parameters such as tensile strength, tensile modulus, and elongation at the break were also determined for synthesized composites. Among the prepared composites, HDPE containing 40 wt% of bismuth (HDPE+40% Bi) has shown good radiation shielding property for 59.54 keV gamma rays along with good mechanical properties. Therefore, HDPE+40% Bi can be considered as a better lead-free shielding material for low-energy gamma rays.
Keywords: Attenuation coefficient, half-value layer, Monte Carlo N-particle, relaxation length, tenth value layer, tensile strength, XCOM
|How to cite this article:|
Sheela M, Kamat VA, Kiran K U, Eshwarappa K M. Preparation and characterization of bismuth-filled high-density polyethylene composites for gamma-ray shielding. Radiat Prot Environ 2019;42:180-6
|How to cite this URL:|
Sheela M, Kamat VA, Kiran K U, Eshwarappa K M. Preparation and characterization of bismuth-filled high-density polyethylene composites for gamma-ray shielding. Radiat Prot Environ [serial online] 2019 [cited 2020 Aug 14];42:180-6. Available from: http://www.rpe.org.in/text.asp?2019/42/4/180/276921
| Introduction|| |
Ionizing radiation like gamma radiation is widely used in industry and medicine. However, unwanted exposure to these radiations will cause damage to human life depending on its energy. There are three guidelines for controlling exposure to ionizing radiation (i) minimizing exposure time, (ii) maximizing distance from the radiation source, and (iii) shielding from the radiation source. The shielding material is generally used to reduce the intensity of radiation. There are various shielding techniques depending on the type of radiation. The penetrating power of alpha rays is very less, and they can block by human skin because alpha particles are heavy particles. Therefore, there is no need of shielding against alpha particles. The penetrating power of beta particles is higher than alpha particles, so the elements of low atomic number such as aluminum can be used as a shielding material for beta rays. Hydrogenous polymers and boron could play an important role for neutron shielding. Because of high penetrating power, the gamma rays could easily pass through the human body, high-density and high atomic number materials are often used for gamma-ray protection. Lead is particularly well suited for reducing the effect of gamma rays due to its high atomic number, which has its limitations due to its toxicity and heaviness. The polymers loaded with fillers can replace the toxic lead in gamma shielding because they are of low cost, lightweight, mechanical flexibility, and also shielding flexibility, i.e., we can obtain the required shielding effect by varying the filler percentage. These polymer composites can also be used as neutron and X-ray shielding materials.
Several researchers over the globe have fabricated polymer composites for radiation shields and studied the shielding ability. The composites fabricated with 50 wt% aluminum was considered to be good shields for radiological safety aspects. Isophthalic resin bismuth oxide composites are effective gamma radiation shields for the Cs137 source. Hematite can enhance the gamma attenuation behavior of pure ethylene-propylene-diene-monomer for 59.54 keV gamma rays. Therefore, the shielding ability of polymer composites is comparable to that of conventional shielding materials. The addition of sepiolite mineral to concretes may be an alternative option that can be used in several radiation protection applications, and experimental results were in good agreement with Monte Carlo N-particle (MCNP) and XCOM results. The polyester steel composites can be used for both neutrons and gamma rays shielding, and the experimental values of the linear attenuation coefficient are compared with MCNP5 code results. High-density polyethylene (HDPE) composed of mainly low Z materials, and its composites with high Z material are good shielding materials for mixed radiation sites such as high energy particle accelerator and at accelerator-based neutron sources.
In the present work, the authors have aimed at studying gamma-ray shielding ability of the shielding material which contains HDPE as the polymer matrix and bismuth as the filler in different weight% (0%, 10%, 20%, and 40%). Because as bismuth is a naturally occurring heavy element placed next to the lead in the periodic table, it has high value of Z and has high density comparable with lead and less toxic compared to lead, it is a right choice for nuclear radiation shielding as a filler and HDPE has good mechanical, thermal and corrosion resistance properties, it was chosen as a polymer matrix.
| Experimental Methods|| |
In this study, bismuth powder has been used as filler in different weight percentages with the HDPE matrix. This pure bismuth powder (purity-99% and density = 9.8 g/cm3) was purchased from Sarda Industrial Enterprises, Jaipur, Rajasthan, and the thermoplastic HDPE (density = 0.92 g/cm3) of injection grade was purchased from Banu Plast, Mumbai.
Preparation of samples
The HDPE composites were prepared by mixing HDPE powder with different weight percentages (0%, 10%, 20%, and 40%) of bismuth thoroughly by the mechanical mixing method. This mixture was molded to the dimension of 30 cm × 30 cm × 0.25 cm in the hydraulic press. The hydraulic press was maintained at the temperature of 180°C with the load pressure of 1000 kg for about 10 min and then the machine was allowed to turn off and rested till the temperature of the hydraulic press falls to 80°C–90°C. Then released the applied load and remove the sample from the mold. Thus, the HDPE+Bi composites were synthesized and cut into required dimensions for further characterization.
The surface morphology of prepared bismuth-filled HDPE composites was studied using scanning electron microscope (JSM IT 300) at the applied voltage of 15 kV, 3000X resolution.
Gamma attenuation studies
An experimental set up used for attenuation studies is shown in [Figure 1]. The Am241 source is placed inside the cylindrical lead blocks of 50 mm thick with a hole diameter of 8 and 20 mm for NaI (Tl) detector to avoid the additional contribution of scattered rays in the shielding studies.
In this study, the attenuation parameters were determined for energy 59.54 keV bypassing gamma rays through the synthesized polymer composites using well-calibrated 3“×3” NaI (Tl) scintillation detector (Thermo Scientific, Germany) coupled with a photomultiplier tube, preamplifier, amplifier, and PC-based 2k multichannel analyzer (MCA). The entire experimental setup was placed over a wooden table. The data acquisition and analysis were performed by Win (ICx Technologies GmbH, Thermo Scientific, Germany) TMCA 32 software package. The source was placed at a distance of 44 cm from the detector. The polymer composites were placed at a distance of 22 cm from the source. Each measurement was taken for the time duration of 1000 s and 4 trials to reduce the experimental errors by 0.5.
Study of shielding parameters
The parameters such as linear attenuation coefficient (μ), mass attenuation coefficient (μm), half-value layer (HVL), tenth value layer (TVL), percentage of attenuation (%Att), and relaxation length (λ) is necessary to know the effectiveness of shielding material.
Beer–Lambert's law I = I0e−μt, where I0 is the intensity of the gamma rays incident on the target material, I will be the intensity of the gamma rays after passing through the sample of thickness t, and μ is the linear attenuation coefficient, gives the following relation:
Where ρ is the density of the material
The percentage of attenuation is evaluated for different thickness of all samples using the following equation:
The percentage of the heaviness of the polymer composites compared with other conventional shielding materials could be evaluated using the relation:
Monte Carlo N-particle simulation
MCNP transport code system is used to simulate various physical interactions at a wide energy range. In the literature, there are various MCNP studies on radiation shielding effectiveness for different materials in recent years. The MCNP input file prepared by incorporating source definition, material card, wherein the weight percentage of various elements of samples under study has given and with suitable tally. The weight percentage of different elements of prepared samples were determined by the elemental composition, which was determined by using energy-dispersive X-ray (EDX) spectroscopy. In this study, the MCNP-A General MCNP transport code of version 4A (Los Alamos National Laboratory Report LA-12625,1993, USA) is used to determine μm values of HDPE+Bi composites using EDX results.
Theoretical μm values can be obtained from the database by Hubbel and Seltzer. Berger and Hubbel developed XCOM software to estimate μm values or photon interaction cross-section of a single element, compound or composite mixture in the wide energy region of 1 keV–100 GeV. Different parameters such as interaction coefficients and total attenuation coefficients are determined by the elemental composition of the composite materials. Recently several studies have been used the XCOM software to calculate the values of μm in the shielding materials. In this study, by using the results of EDX for HDPE+Bi composites, XCOM of version 1.0.1 has used to determine μm values of HDPE/Bi composites for the calculations.
Mechanical properties such as tensile strength, Young's modulus, elongation at the break, and the elongation at the peak were studied for the rectangular bar of area 36 mm2 placed between the grips of Universal Testing Machine (UTM) and was pulled at the speed of 50 mm/min until it broke due to the application of load. The impact strength of the samples was studied using computerized Impact tester and the density of the synthesized HDPE composites was estimated on Archimedes' principle.
| RESULTS and DISCUSSION|| |
Scanning electron microscope analysis
Homogeneous dispersion of the fillers in the polymer matrix is important for the accuracy of measurements. [Figure 2]a, [Figure 2]b, [Figure 2]c, [Figure 2]d. shows surface morphological and dispersion of bismuth within synthesized HDPE composites. [Figure 2]a shows the neat HDPE matrix without any voids and any other impurities on its surface. The surface morphology of HDPE+Bi composites [Figure 2]b confirms the distribution of filler particles on the HDPE matrix. Agglomeration of fillers was observed in [Figure 2]c resulted in a decrease in the mechanical strength of the composite. [Figure 2]d shows the uniform distribution of 40 wt% of the filler in the HDPE matrix. Element analysis for all the composites was performed by using EDX spectroscopy.
|Figure 2: Scanning electron microscope images: (a) Pure high-density polyethylene, (b) High-density polyethylene +10 wt% of bismuth, (c) High-density polyethylene +20 wt% of Bismuth (d) High-density polyethylene +40 wt% of bismuth|
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Gamma shielding parameters
The shielding parameters of synthesized HDPE+Bi composites evaluated from the transmission experimental setup. For Am241 radioactive source, the linear attenuation coefficient (μ) and mass attenuation coefficient (μm) were found to increase with the increase in density and wt % of bismuth of synthesized HDPE/Bi composites as shown in [Figure 3] and [Figure 4] respectively. The HVL, TVL, and the relaxation length found to be decreasing as expected theoretically with the increase in the filler wt% as shown in [Figure 5], [Figure 6], [Figure 7], respectively. These values decide the applicability of these materials for radiation shielding applications. Hence, these parameters are necessary to decide which the better shielding material is. [Figure 8] shows all synthesized composites showed %Att increases and becomes maximum (≈99%) with the increase in the thickness of the samples and increase in the concentration of bismuth (40 wt%) in the HDPE matrix. This shows that as the weight percentage of bismuth increases its attenuation ability increases for 59.54 keV gamma energy.
|Figure 3: Effect of the density of high-density polyethylene+ Bi composites on linear attenuation coefficient|
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|Figure 4: Effect of wt% of Bi on mass attenuation coefficient of high-density polyethylene|
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|Figure 5: Effect of the wt% of filler on half-value layer of high-density polyethylene|
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|Figure 6: Effect of wt% of Bi on tenth value layer of high-density polyethylene|
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|Figure 7: Effect of the wt% of Bi on relaxation length of high-density polyethylene|
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|Figure 8: Variation of % Att with the thickness of high-density polyethylene +Bi composites|
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The main advantage of polymer composites is its lightness. To verify the heaviness of the HDPE+Bi composites the lead was assumed as standard and normalized to 100%. The results of % of heaviness using the relation (7) are as shown in [Figure 9]. With the lead at 100% heavier than other shielding material under consideration, copper is at 78.83%, aluminum is at 23.8%, carbon is at 19.92%, and iron is at 69.22% heavier of lead. In the case of HDPE+Bi composites, even 40% bismuth-filled composite is only 11.37% of lead, while pure HDPE polymer is only 8.11% of lead. These results prove that the HDPE/Bi composites exhibit excellent lightness when compared with the conventional shielding materials such as lead, copper, aluminum, carbon, and iron and also their performance as radiation shielding materials are appreciable particularly at higher filler concentration (40 wt%) and for low-energy gamma rays.
XCOM and Monte Carlo N-particle simulation results
For the study of shielding properties of the material/composites, the main parameter that has to be determined is the mass attenuation coefficient. The mass attenuation coefficient measures the probability of interaction of a photon with the material. The experimentally obtained mass attenuation coefficient values of HDPE/Bi composites were found to be quite close to those with MCNP and XCOM calculated values as shown in [Table 1]. The relative error in MCNP simulated results was kept below 5% based on the null hypothesis method. The slight disagreement between calculated and measured values is due to inaccurate nuclear reaction cross-section data, errors in measurement of physical quantities such as dimensions, densities, the elemental composition of the material, and the intensity of sources and it can be seen from [Table 1] that the experimental values are less than the theoretical values. It is due to the fact that, uncertainty in experimentally determined weight percent of various elements and also due to uncertainty in reaction cross-section values based on which MCNP and XCOM programs calculate parameters of interest. It could be also due to minute difference geometry of experimental setup and geometry described in the MCNP input file. Although it was tried to design the experiment setup precisely, there were some inaccuracies that were always present, and these affected factors were eliminated in simulation and theoretical calculations.
|Table 1: Comparison between the experimental, XCOM database, and Monte Carlo N-particle calculated values of µm|
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The results of the mechanical properties of the prepared HDPE+Bi composites were tabulated in [Table 2]. This study shows that as the filler concentration increases its tensile strength and Young's modulus of the material of the composites increases, and there is a decrease in elongation at the peak, elongation at the break, and impact strength. As tensile strength represents the resistance of a material to breaking under tension and Young's modulus of the material measures the resistance of a material to elastic deformation under load, the present study results that as the bismuth concentration increases its mechanical strength increases. The details of the study could be found elsewhere.
|Table 2: Results of mechanical properties of high-density polyethylene +Bi composites|
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| Conclusions|| |
In this study, pure bismuth-filled HDPE composites were fabricated successfully by using hydraulic press and their gamma shielding performance, mechanical properties were analyzed. It was found that bismuth was distributed uniformly within the HDPE matrix, as the concentration of bismuth increases its mechanical strength increases. Results obtained from attenuation studies have shown that linear attenuation coefficient, mass attenuation coefficient and percentage attenuation coefficient increase with the increase in filler concentration, HVL, TVL, and λ are observed to decrease with the increase in filler concentration in HDPE matrix for Am241 source of energy 59.54 keV. In this study, HDPE with 40 wt% of bismuth shows good shielding properties for 59.54 keV gamma rays. Furthermore, it was found that the experimental values of μm are in good agreement with MCNP and XCOM calculated values. This indicates as the filler wt% increases its shielding ability increases and is comparable with the conventional shielding materials such as aluminum and carbon. As per the shielding properties, HDPE+Bi polymer composites can be used as an effective shielding material for 59.54 keV (Am241) energy gamma radiations. Since241 Am has found application in power generation, spacecraft, and radiotherapy, the lightweight HDPE+Bi composite with 40 wt% of Bi can be used as a flexible coat for radiation workers as well as flexible shielding material in space applications.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9]
[Table 1], [Table 2]