*Article* **The Influence of Bi2O<sup>3</sup> Nanoparticle Content on the** γ**-ray Interaction Parameters of Silicon Rubber**

**Mahmoud I. Abbas <sup>1</sup> , Ahmed M. El-Khatib <sup>1</sup> , Mirvat Fawzi Dib <sup>1</sup> , Hoda Ezzelddin Mustafa <sup>2</sup> , M. I. Sayyed 3,4 and Mohamed Elsafi 1,\***


**Abstract:** In this study, synthetic silicone rubber (SR) and Bi2O<sup>3</sup> micro- and nanoparticles were purchased. The percentages for both sizes of Bi2O<sup>3</sup> were 10, 20 and 30 wt% as fillers. The morphological, mechanical and shielding properties were determined for all the prepared samples. The Linear Attenuation Coefficient (*LAC*) values of the silicon rubber (SR) without Bi2O<sup>3</sup> and with 5, 10, 30 and 30% Bi2O<sup>3</sup> (in micro and nano sizes) were experimentally measured using different radioactive point sources in the energy range varying from 0.06 to 1.333 MeV. Additionally, we theoretically calculated the *LAC* for SR with micro-Bi2O<sup>3</sup> using XCOM software. A good agreement was noticed between the two methods. The NaI (Tl) scintillation detector and four radioactive point sources (Am-241, Ba-133, Cs-137 and Co-60) were used in the measurements. Other shielding parameters were calculated for the prepared samples, such as the Half Value Layer (HVL), Mean Free Path (MFP) and Radiation Protection Efficiency (RPE), all of which proved that adding nano-Bi2O<sup>3</sup> ratios of SR produces higher shielding efficiency than its micro counterpart.

**Keywords:** silicon rubber; nano-Bi2O<sup>3</sup> ; *LAC*; RPE; HVL

#### **1. Introduction**

In medical facilities, such as hospitals, clinics, outpatient care centers, radiological centers and dental facilities, where ionizing radiation is widely utilized, planning is compulsory to protect patients and medical staff who are usually exposed to different types of radiation. For this reason, it is important to use radiation protection materials, whether or not these materials are worn, such as eyeglasses, neck guards or an apron [1–4]. Moreover, it is important to utilize specific materials to insulate the walls of the medical facilities in order to prevent radiation leakage into the surrounding environment. This applies not only to the medical facilities, but also all facilities that utilize gamma radiation or X-ray, such as universities and research laboratories, nuclear power plants, and factories [5,6].

The attenuation properties for the radiation protection medium must be accurately known when planning to design any facility that uses gamma rays and X-rays, so that appropriate protection is provided for patients, workers, visitors, and the surrounding environment [7–9]. The radiation protection properties of a medium depend on its density as well as the chemical composition of the medium's constituent materials. Thickness is also considered as another factor that affects the shielding properties of a given medium. The traditional materials that are practically used in radiation protection applications have several drawbacks.

Some of these materials are expensive and some are heavy, and this limits their use in practical applications. For example, tungsten has a higher number of attenuation factors

**Citation:** Abbas, M.I.; El-Khatib, A.M.; Dib, M.F.; Mustafa, H.E.; Sayyed, M.I.; Elsafi, M. The Influence of Bi2O<sup>3</sup> Nanoparticle Content on the γ-ray Interaction Parameters of Silicon Rubber. *Polymers* **2022**, *14*, 1048. https://doi.org/10.3390/ polym14051048

Academic Editors: Jesús-María García-Martínez and Emilia P. Collar

Received: 7 February 2022 Accepted: 3 March 2022 Published: 6 March 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

than lead; it also has a high cost and this prevents tungsten from being widely used in real applications. Recently, researchers turned to the production of radiation protective clothing that is characterized by its low cost, light weight, easy use, comfort and, most importantly, its protection of workers in the field of ionizing radiation [10–12]. In this regard, some additives are usually added to the flexible materials to fabricate protective garments, such as aprons or curtains. In practical applications, vinyl, polymers and rubbers are one of the most widespread materials used as matrix materials to obtain flexible protection materials, while bismuth, tungsten and antimony powders are used as additives. The importance of such additives is to increase the possibilities of the photon interactions with the atoms of the prepared flexible protective materials.

As is known, the polymers and plastics have a relatively low density and this, to a certain degree, produces their ability to attenuate photons. Therefore, these plastic materials are usually used to attenuate low-energy radiation [13–15]. In order to increase its density and thus improve its shielding performance, especially if it is exposed to photons of medium energy, bismuth is used [16,17]. Silicone rubber (SR) is an important matrix material that has good elasticity features. In the past few years, some researchers used silicone rubber to develop flexible radiation shielding materials. Kameesy et al. [18] fabricated SR sheets filled with four concentrations of PbO. They experimentally evaluated the radiation attenuation factors for the prepared SR-PbO campsites using different radioactive sources. They found that adding PbO to the SR enhances the physico-mechanical features. Gong et al. [19] fabricated a novel radiation protection composite based on methyl vinyl silicone rubber. The authors found that when benzophenone is added to the matrix, a notable enhancement in the radiation resistance occurs. Based on their results, the transmission of the photon with energy of 0.662 MeV through a sample thickness of 2 cm is only 0.7. Özdemir and Yılmaz [20] prepared a mixed radiation shielding via 3-layered polydimethylsiloxane rubber composite. The three layers were composed of hexagonal boron nitride, B2O<sup>3</sup> and Bi2O3. They developed a shielding material that possesses a lead equivalent thickness of 0.35 mm Pb. Chai et al. [21] prepared new flexible shielding material using methyl vinyl silicone rubber. They used zinc borate, B4C and hollow beads as filler materials. They evaluated the neutron shielding performance of their flexible material of the thermal neutron transmission technique with the help of an Am–Be radiation source. However, even though different research groups studied the SR as flexible shielding materials, there is still an urgent need for the further development of novel flexible materials using Bi2O<sup>3</sup> as a filler in nano and micro sizes. Therefore, in this study, we develop a new flexible material against X-ray and γ-ray photons based on Bi2O<sup>3</sup> nanoparticle content.

#### **2. Materials and Methods**

## *2.1. Matrix*

*2.2. Fillers* 

5 nm.

*2.3. Composites* 

Vulcanized silicone rubber was used as a flexible matrix material. The most common form of silicone is the polydimethylsiloxane polymer, which is liquid in origin. This polymer is a rigid structure of elastomers transformed by catalyzed cross-linking reactions [22]. To obtain catalyzed cross-linking reactions, a stiffener with 4% (by weight) must be added to the silicon rubber. The specific gravity of SR was 1.12 g/mL and the elongation was 350%. The main elements of SR are hydrogen, carbon, oxygen and silicon, as shown in Figure 1. *Polymers* **2022**, *14*, 1048 3 of 14

> Bismuth oxide (Bi2O3) of micro and nano sizes was used as a filler in the composite. Before adding it to the solution, a transmission electron microscope (TEM) is used to im-

> Seven different SR samples were prepared. Codes, compositions and densities of the prepared samples are tabulated in Table 1. The homogenous mixtures (liquid SR + microor nanoparticles of Bi2O3 + stiffener) were poured into cylinder molds, which had a 3 cm diameter and different thicknesses (0.3, 0.66, 0.93 and 1.3 cm). The prepared samples waited for 24 h to become elastic-solid materials. The density of the SR sample is measured via the mass/volume, the volume of the sample is calculated by (ଶ. ), where is the

**Code Compositions (wt%) Density** 

SR-5m 95 5 - 1.301 SR-5n 95 - 5 1.351 SR-10m 90 10 - 1.368 SR-20m 80 20 - 1.509 SR-30m 70 30 - 1.684

**(g/cm3 SR Micro-Bi2O3 Nano-Bi2O3 Stiffener )** 

4

1.191

of the microparticles was 15 ± 5 µm, while the average size of the nanoparticles was 30 ±

**Figure 1.** The structure of silicon rubber. SR-30n 70 - 30 1.713 **Figure 1.** The structure of silicon rubber.

**Figure 2.** TEM images for (**a**) Bi2O3 microparticles and (**b**) Bi2O3 nanoparticles.

radius and is the thickness of the measured sample.

**Table 1.** Codes, compositions and densities of the prepared SR samples.

SR-0 100 - -

#### *2.2. Fillers 2.2. Fillers*

**Figure 1.** The structure of silicon rubber.

Bismuth oxide (Bi2O3) of micro and nano sizes was used as a filler in the composite. Before adding it to the solution, a transmission electron microscope (TEM) is used to image the powder to ensure the size of the particles, as shown in Figure 2. The average size of the microparticles was 15 ± 5 µm, while the average size of the nanoparticles was 30 ± 5 nm. Bismuth oxide (Bi2O3) of micro and nano sizes was used as a filler in the composite. Before adding it to the solution, a transmission electron microscope (TEM) is used to image the powder to ensure the size of the particles, as shown in Figure 2. The average size of the microparticles was 15 ± 5 µm, while the average size of the nanoparticles was 30 ± 5 nm.

*Polymers* **2022**, *14*, 1048 3 of 14

**Figure 2.** TEM images for (**a**) Bi2O3 microparticles and (**b**) Bi2O3 nanoparticles. **Figure 2.** TEM images for (**a**) Bi2O<sup>3</sup> microparticles and (**b**) Bi2O<sup>3</sup> nanoparticles.

#### *2.3. Composites 2.3. Composites*

Seven different SR samples were prepared. Codes, compositions and densities of the prepared samples are tabulated in Table 1. The homogenous mixtures (liquid SR + microor nanoparticles of Bi2O3 + stiffener) were poured into cylinder molds, which had a 3 cm diameter and different thicknesses (0.3, 0.66, 0.93 and 1.3 cm). The prepared samples waited for 24 h to become elastic-solid materials. The density of the SR sample is measured via the mass/volume, the volume of the sample is calculated by (ଶ. ), where is the radius and is the thickness of the measured sample. Seven different SR samples were prepared. Codes, compositions and densities of the prepared samples are tabulated in Table 1. The homogenous mixtures (liquid SR + microor nanoparticles of Bi2O<sup>3</sup> + stiffener) were poured into cylinder molds, which had a 3 cm diameter and different thicknesses (0.3, 0.66, 0.93 and 1.3 cm). The prepared samples waited for 24 h to become elastic-solid materials. The density of the SR sample is measured via the mass/volume, the volume of the sample is calculated by (*πr* 2 · *x*), where *r* is the radius and *x* is the thickness of the measured sample.

**Table 1.** Codes, compositions and densities of the prepared SR samples. **Table 1.** Codes, compositions and densities of the prepared SR samples.


#### *2.4. Morphological Images*

A scanning electron microscope (SEM) of JSM-5300, JEOL model, Tokyo, Japan was used for scanning the images of the prepared SR samples. The samples were coated using an ion sputtering coating device (JEOL-JFC-1100E, Tokyo, Japan), and then the samples were placed inside the electron microscope with an operating voltage of 20 keV [23].

SR-30n 70 - 30 1.713

#### *2.5. Mechanical Properties*

The tensile strength, Young's modulus and elongation at break were determined for the present SR samples using an electronic tensile testing machine (model 1425, Germany), according to standard methods with ASTM D412. The Shore hardness was measured according to ASTM D2240.

#### *2.6. Shielding Properties 2.6. Shielding Properties*

*2.4. Morphological Images* 

*2.5. Mechanical Properties* 

cording to ASTM D2240.

The linear attenuation coefficient (*LAC*) was measured for all discussed SR samples using the narrow beam technique of gamma ray spectroscopy in a radiation physics laboratory (Faculty of Science, Alexandria University, Alexandria, Egypt). The devices used in this method were the detector, collimator and radioactive point sources. An NaI (Tl) cylindrical scintillation detector with a (3<sup>00</sup> × 3 00) dimension, a relative efficiency of 15% and an energy value of Cs-137 (0.662 MeV) was used. The inner diameter of the lead collimator was 8mm and the outer diameter was 100 mm. The point radioactive sources were chosen to cover a wide range of energy, where four radioactive sources were used as follows: Am-241 (0.06 MeV), Ba-133 (0.081, 0.356 MeV), Cs-137 (0.662 MeV) and Co-60 (1.173, 1.333 MeV) [24–29]. The illustration of the experimental setup is shown in Figure 3. The linear attenuation coefficient (*LAC*) was measured for all discussed SR samples using the narrow beam technique of gamma ray spectroscopy in a radiation physics laboratory (Faculty of Science, Alexandria University,Alexandria, Egypt). The devices used in this method were the detector, collimator and radioactive point sources. An NaI (Tl) cylindrical scintillation detector with a (3" × 3") dimension, a relative efficiency of 15% and an energy value of Cs-137 (0.662 MeV) was used. The inner diameter of the lead collimator was 8mm and the outer diameter was 100 mm. The point radioactive sources were chosen to cover a wide range of energy, where four radioactive sources were used as follows: Am-241 (0.06 MeV), Ba-133 (0.081, 0.356 MeV), Cs-137 (0.662 MeV) and Co-60 (1.173, 1.333 MeV) [24–29]. The illustration of the experimental setup is shown in Figure 3.

A scanning electron microscope (SEM) of JSM-5300, JEOL model, Tokyo, Japan was used for scanning the images of the prepared SR samples. The samples were coated using an ion sputtering coating device (JEOL-JFC-1100E, Tokyo, Japan), and then the samples were placed inside the electron microscope with an operating voltage of 20 keV [23].

The tensile strength, Young's modulus and elongation at break were determined for the present SR samples using an electronic tensile testing machine (model 1425, Germany), according to standard methods with ASTM D412. The Shore hardness was measured ac-

*Polymers* **2022**, *14*, 1048 4 of 14

**Figure 3.** The experimental setup used to measure the attenuation coefficient. **Figure 3.** The experimental setup used to measure the attenuation coefficient.

The intensity (I0), count rate () or area under the peak () were measured for all energies in a case without the SR sample, and then the sample was placed between the source and detector and the count rate () or area under the peak () was measured at the same time. The *LAC* was measured experimentally using the following equation [30,31]: The intensity (I0), count rate (*N*0) or area under the peak (*A*0) were measured for all energies in a case without the SR sample, and then the sample was placed between the source and detector and the count rate (*N*) or area under the peak (*A*) was measured at the same time. The *LAC* was measured experimentally using the following equation [30,31]:

$$LAC = \frac{1}{\mathfrak{x}} \ln \frac{\aleph\_0}{\aleph} = \frac{1}{\mathfrak{x}} \ln \frac{A\_0}{A} \tag{1}$$

The experimental values of the for SR and the micro filler were compared to the results obtained from the software [32,33]. The relative deviation between the two results is calculated by the following: The experimental values of the *LAC* for SR and the micro filler were compared to the results obtained from the *XCOM* software [32,33]. The relative deviation between the two results is calculated by the following:

$$Dev(\%) = \frac{LAC\_{\text{xcom}} - LAC\_{\text{exp}}}{LAC\_{\text{exp}}} \times 100\tag{2}$$

While the relative increase between the results of the *LAC* of the micro and nano fillers are evaluated via the following: While the relative increase between the results of the *LAC* of the micro and nano fillers are evaluated via the following:

$$R \cdot I(\%) = \frac{LAC\_{nano} - LAC\_{micro}}{LAC\_{micro}} \times 100\tag{3}$$

The other radiation attenuation parameters are based on the *LAC*, such as the Half Value Layer (*HVL*), which represents the thickness needed to reduce the initial intensity to its half value; the Mean Free Path (*MFP*), which represents the path of the photon inside the sample without any interactions; and the Tenth Value Layer (*TVL*), which represents the thickness needed to reduce the initial intensity to its tenth value and can be estimated from the following equation [34–36]:

$$HVL = \frac{\ln 2}{LAC'} \quad MFP = \frac{1}{LAC'} \quad TVL = \frac{\ln 10}{LAC} \tag{4}$$

The efficiency of shielding materials is estimated by an important parameter called the Radiation Protection Efficiency (*RPE*) and calculated using the following equation [37–39]: 39]: 

The efficiency of shielding materials is estimated by an important parameter called the Radiation Protection Efficiency (*RPE*) and calculated using the following equation [37–

. ሺ%ሻ <sup>=</sup> −

The other radiation attenuation parameters are based on the *LAC*, such as the Half Value Layer (*HVL*), which represents the thickness needed to reduce the initial intensity to its half value; the Mean Free Path (*MFP*), which represents the path of the photon inside the sample without any interactions; and the Tenth Value Layer (*TVL*), which represents the thickness needed to reduce the initial intensity to its tenth value and can be estimated

1

*Polymers* **2022**, *14*, 1048 5 of 14

$$RPE(\%) = \left[1 - \frac{N}{N\_0}\right] \times 100\tag{5}$$

, =

× 100 (3)

ln 10

(4)

#### **3. Results and Discussion 3. Results and Discussion**

from the following equation [34–36]:

=

ln 2

, =

#### *3.1. SEM Results 3.1. SEM Results*

SEM images of the prepared samples were scanned and showed, in general, a good distribution of Bi2O<sup>3</sup> with the SR composite. On the other hand, the distribution of nanoparticles was better than the microparticles, as shown in Figure 4, which means that the SR containing nanoparticles of Bi2O<sup>3</sup> has a higher surface area and lower porosity, compared to the same contents of SR containing microparticles of Bi2O3. as Additionally, the porosity of SR containing nanoparticles of Bi2O<sup>3</sup> is low, which led to an increase in the mechanical and shielding properties. SEM images of the prepared samples were scanned and showed, in general, a good distribution of Bi2O3 with the SR composite. On the other hand, the distribution of nanoparticles was better than the microparticles, as shown in Figure 4, which means that the SR containing nanoparticles of Bi2O3 has a higher surface area and lower porosity, compared to the same contents of SR containing microparticles of Bi2O3. as Additionally, the porosity of SR containing nanoparticles of Bi2O3 is low, which led to an increase in the mechanical and shielding properties.

**Figure 4.** SEM images of the prepared samples of (**a**) SR, (**b**) SR-5m, (**c**) SR-5n, (**d**) SR-30m and (**e**) SR-30n.

#### *3.2. Mechanical Results*

In the case of free SR, the mechanical properties (MPs) of SR composites are relatively poor. The MPs of the SR/micro- and nano Bi2O<sup>3</sup> composites are plotted in Figure 5A–D. These figures show the variability of tensile strength, Young's modulus and elongation at break with different concentrations of micro- and nano-Bi2O<sup>3</sup> as fillers. The results show that increasing the filler load leads to a significant increase in the tensile strength, Young's modulus and elongation at break of up to 30 wt%.

The results also show that the addition of nano-Bi2O<sup>3</sup> produces a greater an increase in tensile strength, Young's modulus and elongation at break than micro-Bi2O<sup>3</sup> with the same percentage. The concentration of the filler was increased to 40%, and the same mechanical properties were studied as before, and it was found that it was less than 30%, and this is what made us conduct a comprehensive study with a maximum of 30% for micro- and nano-Bi2O<sup>3</sup> as a filler in the SR. Low mechanical properties at 40 wt% of the filler content is

*Polymers* **2022**, *14*, 1048 6 of 14

modulus and elongation at break of up to 30 wt%.

SR-30n.

*3.2. Mechanical Results* 

likely due to the accumulation of filler material in different rubber layers. The hardness was increased with the increase in the filler contents, and this was normal because the addition of filler in the SR leads to an increase in material hardness. content is likely due to the accumulation of filler material in different rubber layers. The hardness was increased with the increase in the filler contents, and this was normal because the addition of filler in the SR leads to an increase in material hardness.

**Figure 4.** SEM images of the prepared samples of (**a**) SR, (**b**) SR-5m, (**c**) SR-5n, (**d**) SR-30m and (**e**)

In the case of free SR, the mechanical properties (MPs) of SR composites are relatively poor. The MPs of the SR/micro- and nano Bi2O3 composites are plotted in Figure 5A–D. These figures show the variability of tensile strength, Young's modulus and elongation at break with different concentrations of micro- and nano-Bi2O3 as fillers. The results show that increasing the filler load leads to a significant increase in the tensile strength, Young's

The results also show that the addition of nano-Bi2O3 produces a greater an increase in tensile strength, Young's modulus and elongation at break than micro-Bi2O3 with the same percentage. The concentration of the filler was increased to 40%, and the same mechanical properties were studied as before, and it was found that it was less than 30%, and this is what made us conduct a comprehensive study with a maximum of 30% for microand nano-Bi2O3 as a filler in the SR. Low mechanical properties at 40 wt% of the filler

**Figure 5.** The mechanical properties of SR with different contents of micro- and nano-Bi2O3. (**A**) Tensile Strength, MPa, (**B**) Young modulus, MPa, (**C**) Elongations (%), (**D**) Hardness,Shore A **Figure 5.** The mechanical properties of SR with different contents of micro- and nano-Bi2O<sup>3</sup> . (**A**) Tensile Strength, MPa, (**B**) Young modulus, MPa, (**C**) Elongations (%), (**D**) Hardness, Shore A.

#### *3.3. Shielding Results 3.3. Shielding Results*

In order to obtain the linear attenuation coefficient experimentally, we represented the relation between Ln (I/I0) and the thickness of the samples, according to the Lambert– In order to obtain the linear attenuation coefficient experimentally, we represented the relation between Ln (I/I0) and the thickness of the samples, according to the Lambert–Beer law. The slope of the straight line is the absolute value of the *LAC*. We represent the reduction in the intensity of the photons as a function of the thickness for four samples in Figure 6a–d. In this figure, we show the results for the following samples: SR-5m, SR-5n, SR-30m and SR-30n. The other samples have the same trend shown in this figure, so we did not present the data for the remaining samples in Figure 6a–d.

As one can see in this figure, the slope is negative, which implies that the transmission of the photons through the prepared silicone rubber samples decrease with increasing the thickness from 3 to 13 mm. The slope of the rubber silicon sample, SR-5m, is −0.1502 at 0.356 MeV, and, as can be seen from Table 2, the *LAC* for this sample at 0.356 MeV is 0.1502 cm−<sup>1</sup> . The *LAC* values for all the prepared rubber silicon with different amounts of microand nano-Bi2O<sup>3</sup> is summarized in Table 3.

did not present the data for the remaining samples in Figure 6a–d.

**Figure 6.** Graphical representation of the reduction in the intensity of the photons as a function of the thickness for (**a**) SR-5m, (**b**) SR-5n, (**c**) SR-30m and (**d**) SR-30n. **Figure 6.** Graphical representation of the reduction in the intensity of the photons as a function of the thickness for (**a**) SR-5m, (**b**) SR-5n, (**c**) SR-30m and (**d**) SR-30n.

Beer law. The slope of the straight line is the absolute value of the *LAC*. We represent the reduction in the intensity of the photons as a function of the thickness for four samples in Figure 6a–d. In this figure, we show the results for the following samples: SR-5m, SR-5n, SR-30m and SR-30n. The other samples have the same trend shown in this figure, so we


0.060 0.3097 0.3059 1.25 0.9620 0.9464 1.65 1.7499 1.7215 1.65 0.081 0.2456 0.2384 3.01 0.544 0.5305 2.52 0.9042 0.8860 2.05



**Table 3.** Linear attenuation coefficient of bulk and nano samples.

In this study, the linear attenuation coefficient (*LAC*) values of the SR without Bi2O<sup>3</sup> and with 5, 10, 20 and 30% Bi2O<sup>3</sup> (in micro and nano sizes) were experimentally measured using different radioactive point sources in the energy range varying from 0.06 to 1.333 MeV. Additionally, we theoretically calculated the *LAC* for the SR with micro-Bi2O<sup>3</sup> using XCOM software. The other parameters based on *LAC* were calculated, such as HVL, MFP and TVL, and tabulated in Table 4.

**Table 4.** The HVL, MFP and TVL for all the micro and nano prepared samples.


The comparison between the experimental and theoretical *LAC* for the SR (free Bi2O3) and SR with 20% micro-Bi2O<sup>3</sup> is plotted in Figure 7. We notice good comparability between both *LAC* values measured in the lab and those calculated by XCOM. This is true for most tested energies, however, we found some minor differences between both approaches, and this is acceptable since usually anyone can find some small errors in the experimental part, but generally the experimental results match the XCOM results in an acceptable manner. This is an essential and important step since it provides confidence in the accuracy of the geometry utilized in the lab for the determination of *LAC* for the SR and SR/micro-Bi2O<sup>3</sup> samples. We also calculated the error (Dev.%) between the experimental and XCOM data.

XCOM data.

**Figure 7.** Comparison between the experimental and XCOM *LAC* for the SR 0 and 20% of micro-**Figure 7.** Comparison between the experimental and XCOM *LAC* for the SR 0 and 20% of micro-PnO.

part, but generally the experimental results match the XCOM results in an acceptable manner. This is an essential and important step since it provides confidence in the accuracy of the geometry utilized in the lab for the determination of *LAC* for the SR and SR/micro-Bi2O3 samples. We also calculated the error (Dev.%) between the experimental and

PnO. We found that the Dev.% for SR (free Bi2O3) is confined between 0.25 and 2.55%, while the Dev.% for the SR with 10 micro-Bi2O3 is limited between 0.62 and 3.25%. The Dev.% also ranges between 0.28 and 2.14% for the SR with 20 micro-Bi2O3, while the Dev.% for the SR with 30 micro-Bi2O3 is limited between 1.22% and 2.54%. These results confirm that the Dev.% is small (less than 3%), which reaffirms the compatibility of the practical We found that the Dev.% for SR (free Bi2O3) is confined between 0.25 and 2.55%, while the Dev.% for the SR with 10 micro-Bi2O<sup>3</sup> is limited between 0.62 and 3.25%. The Dev.% also ranges between 0.28 and 2.14% for the SR with 20 micro-Bi2O3, while the Dev.% for the SR with 30 micro-Bi2O<sup>3</sup> is limited between 1.22% and 2.54%. These results confirm that the Dev.% is small (less than 3%), which reaffirms the compatibility of the practical and theoretical results.

and theoretical results. In Figure 8a, we plot the *LAC* for the SR and the RS with different concentrations of micro-Bi2O3 (5, 10, 20 and 30%). Using this figure, we aim to understand the influence of adding some fractions of Bi2O3 into the SR on the attenuation performance of the prepared samples. Evidently, the lowest *LAC* is found in the SR and the *LAC* progressively increases as the amount of Bi2O3 increases from 5 to 30%, where the maximum *LAC* is reported for the SR + 30% Bi2O3 sample. The reason for this enhancement in *LAC* is due to the high density and atomic number of bismuth, and it is known that adding high atomic number elements to the materials increases the probability of the interaction between the photons In Figure 8a, we plot the *LAC* for the SR and the RS with different concentrations of micro-Bi2O<sup>3</sup> (5, 10, 20 and 30%). Using this figure, we aim to understand the influence of adding some fractions of Bi2O<sup>3</sup> into the SR on the attenuation performance of the prepared samples. Evidently, the lowest *LAC* is found in the SR and the *LAC* progressively increases as the amount of Bi2O<sup>3</sup> increases from 5 to 30%, where the maximum *LAC* is reported for the SR + 30% Bi2O<sup>3</sup> sample. The reason for this enhancement in *LAC* is due to the high density and atomic number of bismuth, and it is known that adding high atomic number elements to the materials increases the probability of the interaction between the photons and the electrons in the materials. Consequently, incorporating Bi2O<sup>3</sup> into the SR sample leads to the enhancement of the radiation protection performance.

and the electrons in the materials. Consequently, incorporating Bi2O3 into the SR sample leads to the enhancement of the radiation protection performance. In order to compare the effect of Bi2O3 size on the attenuation performance of the SR, we plotted the *LAC* for the SR with 5% micro- and nano-Bi2O3 in Figure 8b, and SR with 30% micro- and nano-Bi2O3 in Figure 8c. The *LAC* values for the SR-5n is higher than the *LAC* of SR-5m, and the same for 30% (i.e., the *LAC* for the SR with nanoparticles is higher than micro-Bi2O3). These results imply that the radiation interaction probability increases when the micro-Bi2O3 is replaced with nano-Bi2O3 in the SR. From Equation (3), we define a parameter called relative increase (R.I), which shows the enhancement in the *LAC* due to the replacement of micro-Bi2O3 by nano-Bi2O3. The R.I is higher than 1, which reaffirms the importance of using nanosized Bi2O3 to develop an effective attenuation barrier (see In order to compare the effect of Bi2O<sup>3</sup> size on the attenuation performance of the SR, we plotted the *LAC* for the SR with 5% micro- and nano-Bi2O<sup>3</sup> in Figure 8b, and SR with 30% micro- and nano-Bi2O<sup>3</sup> in Figure 8c. The *LAC* values for the SR-5n is higher than the *LAC* of SR-5m, and the same for 30% (i.e., the *LAC* for the SR with nanoparticles is higher than micro-Bi2O3). These results imply that the radiation interaction probability increases when the micro-Bi2O<sup>3</sup> is replaced with nano-Bi2O<sup>3</sup> in the SR. From Equation (3), we define a parameter called relative increase (R.I), which shows the enhancement in the *LAC* due to the replacement of micro-Bi2O<sup>3</sup> by nano-Bi2O3. The R.I is higher than 1, which reaffirms the importance of using nanosized Bi2O<sup>3</sup> to develop an effective attenuation barrier (see Figure 8d). Additionally, the RI for 30% of Bi2O<sup>3</sup> is higher than that with 5% of Bi2O3. Accordingly, SR with 30% of Bi2O<sup>3</sup> has interesting radiation shielding features, compared to the SR with micro-Bi2O3.

Figure 9a,b shows the measured gamma photon transmission through the RS with micro-Bi2O<sup>3</sup> and nano-Bi2O3, respectively. We call this T micro and T nano. It can be seen that both T micro and T nano exponentially decrease with increasing the thickness from 3 to 13 mm. In Figure 9a, the T micro is lower than that of SR (free Bi2O3), which means that the transmission of the photons through the sample with Bi2O<sup>3</sup> is lower than the transmission of photons through the free Bi2O<sup>3</sup> SR sample. This means that the addition of Bi2O<sup>3</sup> reduces the transmission of the photons through the prepared silicon rubber.

Additionally, we can see that the T micro depends on the amount of Bi2O<sup>3</sup> incorporated into the SR. The more Bi2O<sup>3</sup> in the SR, the lower the T micro. Hence, the incorporation of Bi2O<sup>3</sup> has a positive influence on the attenuation performance of the SR. If we observe Figure 9b, we can conclude the same results obtained in Figure 9a. In the other words, the photon's transmission through free Bi2O3-SR is higher than the SR with nano-Bi2O3. This result reaffirms that the increase in the weight fraction of Bi2O<sup>3</sup> in the SR can efficaciously diminish the photon's transmittance. Therefore, a high amount of Bi2O<sup>3</sup> (micro or nano sized) in the SR is a good choice to improve the gamma ray shielding performance for the prepared SR. *Polymers* **2022**, *14*, 1048 10 of 14 Figure 8d). Additionally, the RI for 30% of Bi2O3 is higher than that with 5% of Bi2O3. Accordingly, SR with 30% of Bi2O3 has interesting radiation shielding features, compared to the SR with micro-Bi2O3.

**Figure 8.** The linear attenuation coefficient for the SR (**a**) with 0, 5, 10, 20 and 30 micro-Bi2O3, (**b**) with 5% of micro- and nano-Bi2O3, (**c**) with 30% of micro- and nano-Bi2O3, and (**d**) the relative increase in the *LAC*. **Figure 8.** The linear attenuation coefficient for the SR (**a**) with 0, 5, 10, 20 and 30 micro-Bi2O<sup>3</sup> , (**b**) with 5% of micro- and nano-Bi2O<sup>3</sup> , (**c**) with 30% of micro- and nano-Bi2O<sup>3</sup> , and (**d**) the relative increase in the *LAC*.

Figure 9a,b shows the measured gamma photon transmission through the RS with micro-Bi2O3 and nano-Bi2O3, respectively. We call this T micro and T nano. It can be seen that both T micro and T nano exponentially decrease with increasing the thickness from 3 to 13 mm. In Figure 9a, the T micro is lower than that of SR (free Bi2O3), which means that the transmission of the photons through the sample with Bi2O3 is lower than the transmission of photons through the free Bi2O3 SR sample. This means that the addition of Bi2O3 reduces the transmission of the photons through the prepared silicon rubber. Additionally, we can see that the T micro depends on the amount of Bi2O3 incorporated into the SR. The more Bi2O3 in the SR, the lower the T micro. Hence, the incorporation of Bi2O3 has a positive influence on the attenuation performance of the SR. If we observe Figure 9b, we can conclude the same results obtained in Figure 9a. In the other words, the photon's In order to understand the influence of the thickness of the prepared SR on the attenuation performance, we plotted the radiation shielding efficiency (RPE) for the SR, SR-5m and SR-30m, in Figure 10a, and SR, SR-5n and SR-30n, in Figure 10b, at the same energy value of 0.081 MeV. In both subfigures, it is evident that the RPE for the SR is less than the RPE for the SR with Bi2O<sup>3</sup> (micro or nano sized), which reaffirms that adding Bi2O<sup>3</sup> to the SR causes an improvement in the attenuation performance. Most importantly, we can see that the RPE for the SR with a thickness of 13 mm is higher than that with a thickness of 3 mm. This is correct for the SR incorporating micro- or nano-Bi2O3. For instance, from Figure 10a, for the SR-5m, the RPE is 10% and this is increased to 40% for the same sample with a thickness of 13 mm. Therefore, we can conclude that the thickness is an important parameter that affects the attenuation competence of the prepared SR. The

transmission through free Bi2O3-SR is higher than the SR with nano-Bi2O3. This result reaffirms that the increase in the weight fraction of Bi2O3 in the SR can efficaciously diminish

SR.

*Polymers* **2022**, *14*, 1048 11 of 14

high thickness SR is recommended as a good attenuator barrier. Moreover, we found that the RPE for the SR with nano-Bi2O<sup>3</sup> is higher than that of the corresponding micro-Bi2O3. parameter that affects the attenuation competence of the prepared SR. The high thickness SR is recommended as a good attenuator barrier. Moreover, we found that the RPE for the SR with nano-Bi2O3 is higher than that of the corresponding micro-Bi2O3.

In order to understand the influence of the thickness of the prepared SR on the attenuation performance, we plotted the radiation shielding efficiency (RPE) for the SR, SR-5m and SR-30m, in Figure 10a, and SR, SR-5n and SR-30n, in Figure 10b, at the same energy value of 0.081 MeV. In both subfigures, it is evident that the RPE for the SR is less than the RPE for the SR with Bi2O3 (micro or nano sized), which reaffirms that adding Bi2O3 to the SR causes an improvement in the attenuation performance. Most importantly, we can see that the RPE for the SR with a thickness of 13 mm is higher than that with a thickness of 3 mm. This is correct for the SR incorporating micro- or nano-Bi2O3. For instance, from Figure 10a, for the SR-5m, the RPE is 10% and this is increased to 40% for the same sample with a thickness of 13 mm. Therefore, we can conclude that the thickness is an important

**Figure 9.** The measured gamma photon transmission through the RS with micro-Bi2O3 and nano-Bi2O3, respectively, (**a**) RS with micro-Bi2O3 at 0.06 MeV, (**b**) RS with micro-Bi2O3 at 0.662 MeV, (**c**) RS with nano-Bi2O3 at 0.06 MeV and (**d**) RS with nano-Bi2O3 at 0.662 MeV **Figure 9.** The measured gamma photon transmission through the RS with micro-Bi2O<sup>3</sup> and nano-Bi2O<sup>3</sup> , respectively, (**a**) RS with micro-Bi2O<sup>3</sup> at 0.06 MeV, (**b**) RS with micro-Bi2O<sup>3</sup> at 0.662 MeV, (**c**) RS with nano-Bi2O<sup>3</sup> at 0.06 MeV and (**d**) RS with nano-Bi2O<sup>3</sup> at 0.662 MeV.

It is important to compare the radiation attenuation performance for the prepared SR doped with Bi2O3 with a similar material, in order to check the possibility of using these samples in real applications. For this purpose, we compared the half value layer of the SR It is important to compare the radiation attenuation performance for the prepared SR doped with Bi2O<sup>3</sup> with a similar material, in order to check the possibility of using these samples in real applications. For this purpose, we compared the half value layer of the SR with 30% of micro- and nano-Bi2O<sup>3</sup> with 3 samples: SR 30, 40 and 50% of magnetite iron [40], as shown in Figure 11. We selected one energy value in the comparison, i.e., 0.662 MeV. Evidently, the SR with 30% of Bi2O<sup>3</sup> (micro and nano sized) have a lower HVL and thus better attenuation competence than the SR with 30% of magnetite iron. The SR with 30% of micro-Bi2O<sup>3</sup> has an HVL of 4.52 cm and this is close to 4.62 cm, which was reported for the SR with 40% of iron. The SR with 30% nano-Bi2O<sup>3</sup> is lower than that of the SR with 30 and 40% of iron, but has an almost similar HVL, with the SR being in contact with 50% of the magnetite iron.

with 30% of micro- and nano-Bi2O3 with 3 samples: SR 30, 40 and 50% of magnetite iron [40], as shown in Figure 11. We selected one energy value in the comparison, i.e., 0.662 MeV. Evidently, the SR with 30% of Bi2O3 (micro and nano sized) have a lower HVL and thus better attenuation competence than the SR with 30% of magnetite iron. The SR with 30% of micro-Bi2O3 has an HVL of 4.52 cm and this is close to 4.62 cm, which was reported for the SR with 40% of iron. The SR with 30% nano-Bi2O3 is lower than that of the SR with 30 and 40% of iron, but has an almost similar HVL, with the SR being in contact with 50%

with 30% of micro- and nano-Bi2O3 with 3 samples: SR 30, 40 and 50% of magnetite iron [40], as shown in Figure 11. We selected one energy value in the comparison, i.e., 0.662 MeV. Evidently, the SR with 30% of Bi2O3 (micro and nano sized) have a lower HVL and thus better attenuation competence than the SR with 30% of magnetite iron. The SR with 30% of micro-Bi2O3 has an HVL of 4.52 cm and this is close to 4.62 cm, which was reported for the SR with 40% of iron. The SR with 30% nano-Bi2O3 is lower than that of the SR with 30 and 40% of iron, but has an almost similar HVL, with the SR being in contact with 50%

*Polymers* **2022**, *14*, 1048 12 of 14

**Figure 10.** The measured radiation protection efficiency at 3 and 13 mm through (**a**) the RS with micro-Bi2O3 and (**b**) nano-Bi2O3 at 0.081 MeV. **Figure 10.** The measured radiation protection efficiency at 3 and 13 mm through (**a**) the RS with micro-Bi2O<sup>3</sup> and (**b**) nano-Bi2O<sup>3</sup> at 0.081 MeV. **Figure 10.** The measured radiation protection efficiency at 3 and 13 mm through (**a**) the RS with micro-Bi2O3 and (**b**) nano-Bi2O3 at 0.081 MeV.

**Figure 11.** The HVL of 30% micro and nano prepared samples compared with SR filled with magnetite iron. **Figure 11.** The HVL of 30% micro and nano prepared samples compared with SR filled with magnetite iron. **Figure 11.** The HVL of 30% micro and nano prepared samples compared with SR filled with magnetite iron.

#### **4. Conclusions 4. Conclusions 4. Conclusions**

of the magnetite iron.

of the magnetite iron.

Flexible materials were prepared from SR and different sizes of Bi2O3. The morphological, mechanical and shielding properties were determined. The SEM results indicate that the nano filler is significantly better than micro filler. The mechanical results conclude that the flexibility of the materials decreases as we increase the Bi2O3 filler with 30 wt%. Therefore, the attenuation study was controlled by the flexibility results. The *LAC* was determined experimentally and the results show good agreement with the theoretical results. The attenuation coefficients of the prepared SR samples showed a clear superiority Flexible materials were prepared from SR and different sizes of Bi2O3. The morphological, mechanical and shielding properties were determined. The SEM results indicate that the nano filler is significantly better than micro filler. The mechanical results conclude that the flexibility of the materials decreases as we increase the Bi2O3 filler with 30 wt%. Therefore, the attenuation study was controlled by the flexibility results. The *LAC* was determined experimentally and the results show good agreement with the theoretical results. The attenuation coefficients of the prepared SR samples showed a clear superiority Flexible materials were prepared from SR and different sizes of Bi2O3. The morphological, mechanical and shielding properties were determined. The SEM results indicate that the nano filler is significantly better than micro filler. The mechanical results conclude that the flexibility of the materials decreases as we increase the Bi2O<sup>3</sup> filler with 30 wt%. Therefore, the attenuation study was controlled by the flexibility results. The *LAC* was determined experimentally and the results show good agreement with the theoretical results. The attenuation coefficients of the prepared SR samples showed a clear superiority in lower energy levels over other energies, and the SR's nano-Bi2O<sup>3</sup> was better than the corresponding SR's micro-Bi2O<sup>3</sup> at all discussed energies for the shielding materials.

**Author Contributions:** Conceptualization, M.E. and A.M.E.-K.; methodology, M.E.; software, M.I.S.; validation, H.E.M., M.F.D. and M.I.A.; formal analysis, M.F.D.; investigation, M.I.S.; resources, M.E.; data curation, M.I.A.; writing—original draft preparation, M.F.D.; writing—review and editing, M.I.S.; visualization, A.M.E.-K.; supervision, M.I.A.; project administration, M.E.; funding acquisition, M.F.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**

