Comparative Theoretical and Experimental Validation of the Shielding Effectiveness of Ceramic Composite-Based Medical Radiation Protection Tools

Abstract

Numerous studies aimed to validate new shielding materials with the transition of medical radiation-shielding tools toward eco-friendly materials. In this study, we assessed the feasibility of ceramic composites, recently adopted in aerospace for internal shielding, as candidates for medical applications. Specifically, three types of ceramic composite mixtures were examined: bismuth oxide-based (Bi₂O₃), cerium oxide-based (CeO₂), and tantalum oxide-based (Ta₂O₅) ceramic composites. Two approaches—theoretical simulations and direct experiments—validated the performance under clinical conditions. Monte Carlo simulation results reveal that CeO₂, with its high linear attenuation coefficient, exhibits the strongest theoretical shielding. In terms of density measurements, Ta₂O₅ composite sheets yielded the highest density (3.318 g/cm³), followed by CeO₂ composites (3.228 g/cm³) and Bi₂O₃ composites (3.091 g/cm³). Although relatively slight differences in density were observed among the fabricated sheets, Ta₂O₅ composites tended to have slightly higher densities. However, Ta₂O₅ composites outperformed the other composites in direct clinical experiments. This discrepancy between the theoretical and experimental results highlights the influence of other factors, such as the energy characteristics of the materials and variations in the fabrication process. Overall, this study supports the development of eco-friendly radiation shields through theoretical and clinical validation.

1. Introduction

Radiation-shielding tools used in medical institutions are lightweight and flexible, and the fundamental condition is that they exhibit a shielding performance comparable to that of lead [1]. An emerging requirement in the development of medical radiation-shielding tools is the use of eco-friendly shielding materials [2]. Many recent studies focused on materials that are harmless to the human body and free from environmentally hazardous elements, with particular attention to blended composites based on material properties [3,4]. Numerous investigations have been conducted to improve the shielding performance by optimizing the internal physical structure of the shielding body or by combining different composite materials [5]. It is well known that the density and atomic number of constituent materials play critical roles in enhancing the effectiveness of radiation shielding [6].

Lead has been the primary material used for radiation shielding and provides effective protection in clinical applications [7]. However, lead poses significant environmental hazards during its manufacturing, use, and disposal because of its heavy-metal properties; therefore, it is being phased out in hospitals and industrial settings [8]. Consequently, eco-friendly shielding materials that are safe for the human body have become the focus of healthcare professionals and engineers in the development of medical devices.

Recently, eco-friendly materials have demonstrated shielding performance comparable to that of lead; however, they are still deficient in terms of cost-effectiveness and processability, driving the ongoing search for alternative materials [9,10]. Consequently, composite approaches that combine different materials are increasingly preferred. Well-known eco-friendly shielding materials, such as bismuth oxide, tungsten, barium sulfate, boron, and antimony, have already been commercialized and are in use; however, their relatively poor processability in comparison with lead necessitates their blending with polymers or with each other to fabricate composite shielding tools with improved performance [11]. However, such composite fabrication processes still face limitations in certain technological aspects because improvements in the shielding performance often require increasing the thickness and density of the shielding body, which raises economic challenges during production [12].

This study aimed to overcome the limitations of conventional material-blending processes by focusing on novel ceramic composites and verifying their performances both experimentally and theoretically. The shielding effects were examined under dual experimental conditions to evaluate ceramic composites—which have recently been widely used as radiation-resistant and protective materials in the aerospace industry—for potential applications as medical radiation-shielding tools [13].

Aging of shielding materials under cumulative radiation dose generally occurs as damage associated with hardening of the microstructure. However, such degradation typically requires relatively high incident energies and sustained high dose rates [14]. In particular, cerium dioxide (CeO₂) exhibits complex structural modifications under irradiation with swift heavy ions; however, these changes are somewhat limited, and point-defect–related damage is typically accommodated or mitigated by the intrinsic structural characteristics of the material [15,16,17]. Therefore, when considering the dose levels applied in medical facilities, radiation stability is not expected to be a critical issue for the polymer–ceramic composites investigated in this study.

Medical radiation-shielding devices fabricated from ceramic–polymer composites can exhibit stable mechanical properties and ecofriendly characteristics, and they are high-performance multiphase materials tailored for specific structural and functional objectives [18]. Accordingly, we reprocessed validated ceramic-shielding agents by incorporating them into ceramic matrices and then compounded the mixture with a polymer to produce sheet-type laminates. By introducing physical and structural modifications such as multilayer architectures, these sheets are expected to achieve lead-like shielding effects [19]. This is influenced not only by the selection of shielding constituents but also by their physical configuration. In general, composites are multiphase materials in which two or more components are incorporated into a base matrix, and differences in processing technology can lead to improvements in shielding performance [20]. Thus, the specific configuration and mixing ratio of the constituent phases are also important design factors. Accordingly, three ceramic composite models were designed in this study: bismuth oxide-based (Bi₂O₃ ceramic composites), cerium oxide-based (CeO₂ ceramic composites), and tantalum oxide-based (Ta₂O₅ ceramic composites), and their shielding performances were evaluated. Glasses and composites containing Bi₂O₃, CeO₂, and Ta₂O₅ have been reported to exhibit excellent X- and γ-ray attenuation owing to their high density and effective atomic number, as well as good structural compatibility with common base materials [21,22,23]. These characteristics motivated the selection of compositions investigated in this study. For newly proposed shielding materials, the feasibility of their radiation-shielding performance is most often assessed initially via simulation.

Considering these aspects, this study theoretically assessed the shielding performance—the most critical factor in shielding tools—using Monte Carlo simulations and experimentally validated it by evaluating the shielding efficiency of fabricated sheets subjected to clinical X-ray conditions for the material combinations [24]. We developed a methodology for theoretical and experimental evaluations of the functions of various composite-shielding materials based on the shielding performance of ceramic composites. Unlike previous research that assessed shielding properties using a single approach, the combination of theoretical (simulation-based) and experimental methods in this study enabled an objective evaluation and comparison between simulated and experimental attenuation results [25,26]. The proposed approach contributes to the identification of optimal ceramic composite combinations for future radiation-shielding applications in practical medical environments. We note that the particle dispersion or distribution within the fabricated sheets may differ from the idealized simulation assumptions, which can contribute to discrepancies between predicted and measured results. These findings provide guidance for future optimization of shielding performance and practical fabrication technologies of radiation protection tools.

2. Materials and Methods

The performance of radiation-shielding tools used in medical institutions can be evaluated based on the linear attenuation coefficient ($\mu$) or mass attenuation coefficient ($\mu /\rho$) [27]. The linear attenuation coefficient is defined as the probability of interaction when radiation passes through a medium [28]. This parameter can be expressed as Equation (1) based on the Beer–Lambert law [29]:

$$I=I_0{e}^{{-\left(\mu x\right)}_n}$$

(1)

where $I_0$, $I$, $x$, and $\mu$ are the incident radiation intensity, transmitted radiation intensity, sample thickness, and linear attenuation coefficient, respectively. For composites, the linear attenuation coefficient and the density of the shielding material, $\rho ({\mathrm{g}/\mathrm{c}\mathrm{m}}^3$), with ${\omega }_i$ denoting the mass fraction of the i-th element, can be expressed in terms of the mass attenuation coefficient (μ/ρ), as expressed by Equation (2) [30].

$${\left({\displaystyle \frac{\mu }{\rho }}\right)}_n=\sum _i^n{\omega }_i{\left({\displaystyle \raisebox{1ex}{$\mu $}\!\left/ \!\raisebox{-1ex}{$\rho $}\right.}\right)}_i$$

(2)

In particular, the effective atomic number of materials used in composites can describe the radiation interaction characteristics of multielement substances composed of different atomic numbers (${\rm Z}$) [31]. Therefore, a higher effective atomic number indicates a greater probability of interaction with incident radiation, serving as an interaction index for radiation and playing a role similar to that of the atomic number of individual elements [32]. Here, ${{\rm Z}}_i$ and $A_i$ represent the atomic number and atomic weight, respectively, and the effective atomic number can be expressed by Equation (3) [33]. Accordingly, shielding tools can be designed to adjust the shielding performance based on fundamental radiation parameters. Additionally, in this study, the shielding performances of the composites were experimentally evaluated using three basic prototype combinations.

$$Z_{eff}={\displaystyle \frac{\mathsf{\Sigma }{\omega }_i{{\rm Z}}_i{\left(\frac{\mu }{\rho }\right)}_i}{\mathsf{\Sigma }{\omega }_i{A}_i{\left(\frac{\mu }{\rho }\right)}_i}}$$

(3)

Silicon and aluminum oxides, such as SiO₂ and Al₂O₃, were used as the matrix materials, whereas high-density ceramic fillers, including Bi₂O₃, CeO₂, and Ta₂O₅, were combined to produce the final shielding composites. These composites were further mixed with polymer compounds to ensure their stability and structural flexibility, resulting in shielding tools with effective radiation protection properties. The basic shielding materials used in this study exhibited the following characteristics.

First, in Bi₂O₃ ceramic composites, bismuth—which serves as the primary shielding material—has an atomic number of 83 and a density of 8.9 g/cm³. Bismuth is one of the densest eco-friendly materials, with a K-edge of approximately 90.5 keV, despite its density being lower than that of lead (11.34 g/cm³). Hence, it exhibits excellent attenuation performance in the diagnostic energy range and is a known cost-effective material [34]. Second, in CeO₂ ceramic composites, cerium has an atomic number of 58, a density of 7.2 g/cm³, and a K-edge of 40 keV. Its properties contribute to dose reduction in the diagnostic energy region [35]. Third, in Ta₂O₅ ceramic composites, tantalum has an atomic number of 73, density of 8.2 g/cm³, and K-edge of 67.4 keV, which lies within the diagnostic X-ray spectrum range [36]. Therefore, all three materials exhibit shielding effectiveness within the medical diagnostic energy range, and oxide-based composites fabricated using these materials should exhibit the physicochemical characteristics required for shielding tools used in medical institutions [37,38]. In addition, the formulations were designed on a same mass basis, with the polymer base (high-density polyethylene [HDPE]) and the proposed ceramic composite being 20 wt% and 80 wt%, respectively.

Theoretical shielding efficiency was investigated using Geant4 (version 10.5) [39]. The evaluation comprised two stages. In the first stage, a laboratory X-ray system was numerically modeled (Figure 1). A total of 2.1 × 10⁹ electrons were accelerated to 120 keV and directed onto a tungsten (W) target at an anode angle of 30° to generate the X-ray beam (Figure 2). The beam then passed through a 2.5 mm-thick aluminum filter, after which the spectrum was recorded at a source-to-detector distance of 1500 mm using a box-type detector with dimensions of 400.0 mm × 400.0 mm × 150 mm, as shown in Figure 1. The transmitted beam subsequently traversed an ionization chamber and was finally absorbed by a concrete floor. The obtained photon spectrum was normalized to 10⁷ photons while preserving its shape. In the second stage, the photons generated in the first stage were emitted isotropically from the focal spot toward the detector. The shielding sheet was positioned 30 mm upstream of the detector. Shielding efficiency was assessed by comparing the exposure dose—defined here as the charge per unit mass of air in the detector—between cases with and without the shielding sheet. The charge generated in the detector was computed by dividing the deposited kinetic energy by the average ionization energy of air (34 eV, for the detector medium). The unattenuated primary beam yielded a charge of 2.549 nC.

The G4EmLivermorePhysics list was used in both stages to model electron–matter and photon–matter interactions. Across five independent simulation runs, the mean deviation of the computed charge was <3%, and the relative percent error was ≤3%. As illustrated in Figure 1, the concrete floor was the only scattering body considered in the geometry. The densities of the composite constituents—Ta₂O₅–Al₂O₃: 3.318 g·cm⁻³, CeO₂–SiO₂: 3.228 g·cm⁻³, and Bi₂O₃–SiO₂: 3.091 g·cm⁻³—were obtained from the manufacturers’ specifications. Each composite sheet was fabricated by mixing the shielding filler and polyethylene at a mass ratio of 0.8:0.2.

In this study, theoretical shielding performance was predicted using Monte Carlo simulations, and experimental validation was conducted by fabricating shielding sheets and evaluating their performance under clinical X-ray conditions. For the ceramic composites, shielding materials (particle sizes of 100–400 μm) were mixed with the polymer HDPE [40]. HDPE was selected to ensure that the material exhibited the minimal mechanical strength required for the experiments. However, the fabricated sheets did not possess sufficient tensile strength or dimensions for direct use in clinical practice.

Polyethylene has a molecular weight greater than 4 million and a density of 0.91 g/cm³. Because a solid-state polymer was employed, N-dimethylformamide (DMF, 99.5%) was used as the solvent to prepare the casting solution [41]. HDPE was dissolved in DMF at 10 wt% using a mechanical stirrer, after which the shielding material (80 wt%) was added and dispersed by stirring at 5000 rpm. Diisononyl phthalate was used as a plasticizer at 0.85–0.95 wt% to eliminate pores inside the sheet and improve density.

The final casting solution was filtered and degassed to ensure uniform shielding performance. Sheet fabrication required a forming step, for which a calendering process was employed [42]. During calendering, the compaction temperature was set to approximately 180 °C, and the roll speed was set to 7.0–8.5 m/min (75–85 MPa). Cooling was performed at 50 °C [43,44].

The fabricated shielding sheets had dimensions of 150 mm × 150 mm, as illustrated in Figure 3. The sheet thicknesses were 0.5, 1.0, 1.5, and 2.0 mm, which were selected to reflect the accessibility and flexibility requirements for potential practical applications. Therefore, shielding sheets with different thicknesses were fabricated following the same process with a fixed shielding material content of 80 wt%, and their shielding performance was evaluated in the diagnostic X-ray energy range.

Thin-film cross-sections were captured using an optical microscope (field emission-scanning electron microscope (FESEM, S-4800, Hitachi High-Technologies Corp., Tokyo, Japan) to examine the particle distribution inside the shielding sheets [45]. The experimental method for evaluating the radiation-shielding performance was applied in accordance with the Korean Industrial Standard for lead equivalent test methods for X-ray protective devices (KS A 4025:1990, confirmed in 2009), as shown in Figure 4 [46]. This setup is identical to that of the X-ray-shielding experiment used for the theoretical analysis. X-rays were generated using an X-ray source (E7239 X-ray tube, Tochiba Corp., Tokyo, Japan; 150 kV, 500 mA), and each experiment was repeated 10 times, with the mean value used for analysis. A dose detector (DosiMax Plus 1, 2019; IBA Dosimetry GmbH, Schwarzenbruck, Germany) was used after calibration [47]. The shielding sheet performance was evaluated in terms of shielding efficiency by considering the radiation protection efficiency (RPE) [48]. The shielding efficiency in the experiment was determined using Equation (4):

$$\mathrm{R}\mathrm{E}\mathrm{F}=\left(1-{\displaystyle \frac{e}{e_0}}\right)\times 100$$

(4)

where REF is the shielding rate, $e$ is the exposure dose measured with the shielding sheet placed between the X-ray beam and detector, and $e_0$ is the exposure dose measured without the shielding sheet.

3. Results

The physical properties of the fabricated shielding sheets were evaluated by measuring the densities of sheets produced with Bi₂O₃, CeO₂, and Ta₂O₅. The densities of the Ta₂O₅, CeO₂ and Bi₂O₃ composite sheets were 3.318, 3.228, and 3.091 g/cm³, respectively. Thus, only slight differences in density were observed among the sheets, although the Ta₂O₅-based composite possessed a slightly higher density. As depicted in Figure 5, these density differences can be explained in terms of the particle distribution.

In Figure 5, panels (a-1–a-3), (b-1–b-3), and (c-1–c-3) correspond to Ta₂O₅, CeO₂, and Bi₂O₃ composites, respectively, showing the particle distribution in each sheet. The particle distributions of the shielding materials were similar, which was likely owing to the comparable physicochemical properties of the three materials. The spacing between the particles is possibly influenced by the affinity of the particles with the polymer matrix; the higher the affinity, the narrower the interparticle spacing. All three materials exhibited good compatibility with the polymers. This indicates that no filler–filler agglomeration was present, and the polymer phase formed only small, well-dispersed domains, with no evidence of polymer–polymer clustering [8,49,50].

A comparison between Figure 5(a-3,b-3) shows that their morphologies are nearly identical. However, Figure 5(c-3) indicates that smaller particles fill the gaps between the larger particles, which may account for the slightly higher density.

The shielding performances of the fabricated sheets were evaluated under conditions equivalent to those used in clinical X-ray environments. The results of the performance evaluations are summarized in

Table 1. Experimental X-ray evaluation of shielding-sheet performance.

Radiation Type	X-Ray Energy (kVp) Tube Voltage	Composites_RPE * (%)
		Ta2O5_AI2O3				CeO2_SiO2				Bi2O3_SiO2
		Thickness (mm)
		0.5	1.0	1.5	2.0	0.5	1.0	1.5	2.0	0.5	1.0	1.5	2.0
X-ray	40	80	84	88	92	74	77	82	87	71	76	79	82
	60	76	80	84	90	71	75	80	86	67	72	75	80
	80	72	75	78	86	68	72	77	80	65	68	70	74
	100	68	71	74	82	67	69	73	76	62	64	68	71
	120	65	68	70	80	63	65	68	74	59	60	67	70

* RPE: Radiation Protection Efficiency.

. Unlike density, differences were observed in the X-ray-shielding performance. The Bi₂O₃ composites exhibited the lowest performance, whereas the Ta₂O₅ composites yielded the best results. Even at a relatively small thickness (approximately 0.5 mm), the shielding efficiency remained between 70% and 80%, indicating an effective shielding performance. The theoretical evaluation assessed using Monte Carlo simulation is presented in

Table 2. Performance evaluation of shielding sheets based on Monte Carlo simulation at an acceleration voltage of 120 kV.

Thickness (mm)	Ta2O5_AI2O3		CeO2_SiO2		Bi2O3_SiO2
	Charge (nC)	RPE * (%)	Charge (nC)	RPE * (%)	Charge (nC)	RPE * (%)
0.5	1.332	47.7	1.205	52.7	1.302	48.9
1.0	0.779	69.4	0.671	73.7	0.789	69.0
1.5	0.481	81.1	0.409	84.0	0.511	79.9
2.0	0.309	87.9	0.265	89.6	0.346	86.4

* RPE: Radiation Protection Efficiency.

. Although the experimental X-ray results showed similar patterns, the overall shielding efficiency was lower than that of the simulations. A distinct difference was that the CeO₂ composites demonstrated the highest shielding performance in the simulation, which was inconsistent with the experimental findings. This discrepancy exists because the interactions occurring within the shielding sheet, particularly in the lower energy range, involve scattering and absorption, which do not directly affect the measured dose. Consequently, the experimental shielding sheets demonstrated a slightly higher shielding efficiency than those of the simulation. Nevertheless, no significant differences in shielding performance were observed.

The shielding performance results are presented in Figure 6. Figure 6a presents a comparison of the differences in shielding performance according to sheet thickness, and Figure 6b–d present the fitted curves obtained from the data. The variation in thickness corresponded to an increase in the shielding efficiency, and the fitting analysis revealed that the trend of improvement was nearly identical across the different materials, confirming that the shielding performance consistently improved with increasing thickness.

4. Discussion

Shielding tools used in medical institutions have traditionally been manufactured using lead, which has the advantage of excellent workability [51]. However, tungsten is the most widely used eco-friendly non-lead material. Although it offers superior shielding effectiveness, its limited processability requires its fabrication in composite forms with other materials [52]. Therefore, when eco-friendly shielding tools are fabricated using a single material, their shielding performance may be limited [53]. Accordingly, novel materials comparable to lead can be produced using composites [54].

In this study, the shielding effectiveness of ceramic composites for medical radiation protection was verified using theoretical and experimental approaches. The objective was to compare and predict the differences between the results obtained from Monte Carlo-based theoretical simulations, which serve as a preliminary screening method for new materials; additionally, the shielding performances were validated under clinical X-ray conditions. The shielding materials employed were Bi₂O₃, CeO₂, and Ta₂O₅, which were selected because of their proven cost-effectiveness and processability [55].

Therefore, sheets composed of ceramic composites prepared by mixing the proposed shielding materials with silicon oxides and aluminum oxide and further combining with polymer materials can be considered applicable as medical radiation-shielding tools in practical settings [56]. These sheets are expected to be valuable materials owing to their flexibility as shielding devices and economic feasibility, which is comparable to that of lead [57]. Moreover, the dual evaluation of shielding performance provides a methodological reference for future assessments of similar materials and contributes to the comparative analysis of theoretical and experimental results.

The densities of Ta₂O₅, CeO₂, and Bi₂O₃ composites analyzed in this study were 3.315, 3.228, and 3.091 g/cm³, respectively. Based on curve fitting of the shielding efficiency, the linear attenuation coefficients of Ta₂O₅, CeO₂, and Bi₂O₃ were 1.178, 1.337, and 1.163 mm⁻¹, respectively. CeO₂ exhibited the highest shielding capability owing to its relatively higher linear attenuation coefficient, despite the shielding efficiencies obtained from the experiments indicating similar performance among the materials. The K-absorption edges (K-edges) of Ta and Ce are approximately 67 and 40 keV, respectively. In the experiments, the higher shielding efficiency observed for Ta₂O₅ relative to CeO₂ indicates that the shielding sheet effectively attenuated photons with energies above ~67 keV. From the spectral perspective, the Geant4-generated X-ray spectrum contains more photons near ~40 keV than above ~67 keV [58]. However, in the actual X-ray tube, additional inherent (intrinsic) filtration hardens the beam by preferentially attenuating low-energy photons. As low-energy X-rays are removed, a relatively higher fraction of higher-energy X-rays contributes more efficiently to the shielding evaluation. By contrast, in the Geant4 calculations, only a 2.5 mm-thick aluminum filter is used to obtain the spectrum; consequently, as shown in Figure 2, the bremsstrahlung distribution exhibits a maximum near ~40 keV. Thus, the Ce K-edge aligns with the spectral maximum in the simulation, leading to more efficient removal of photons above ~40 keV and yielding a higher apparent shielding efficiency for Ce-containing composites under the simulated conditions. However, in the experimental evaluations using practical tools, Ta (O) exhibited the best shielding performance. This contrast demonstrates that experimental assessments are as important as theoretical simulations for predicting shielding performance.

In the design of composite shielding tools for medical applications, the primary criterion is to enhance density while avoiding the need for special processing techniques during fabrication [59]. Excessive complexity and variability in the manufacturing process may hinder the consistency of shielding performance. In practice, the reproducibility of processing technology should be ensured by controlling parameters, such as particle arrangement and material density, and thus, should be achievable solely through a consistent material composition [57]. Therefore, the design of new composites requires the thorough blending of materials with similar microstructural characteristics, such as density, which is a critical condition for achieving reliable shielding performance.

In composites, the dispersion of shielding material particles onto a single structure, such as a base or matrix, can directly influence the shielding performance of the composite [56]. Efforts have been made to improve shielding performance using multiphase structures by introducing physical and structural modifications [60]. Previous research has attempted to improve density by mixing polymer materials and controlling particle size; however, owing to limitations, new materials are now being investigated [61].

The limitations of this study include the need for further improvements in shielding performance through more detailed control of particle size, as well as the lack of dual- and cross-evaluations of shielding performance under different material-mixing conditions. Nevertheless, similar results were obtained for the theoretical simulation and experiment, and the causes of the observed differences could be identified.

5. Conclusions

The theoretical and experimental shielding performances of Bi₂O₃, CeO₂, and Ta₂O₅ ceramic composites, which are applicable as radiation-shielding tools in medical institutions, were investigated in this study. The main conclusions are as follows:

1.

The obtained densities of Ta₂O₅, CeO₂, and Bi₂O₃ composites were 3.315, 3.228, and 3.091 g/cm³, respectively.

2.

In theoretical simulations, all three composites exhibited similar shielding performance; however, CeO₂—with a relatively higher linear attenuation coefficient—demonstrated the best shielding effectiveness.

3.

In experimental evaluations, similar shielding performance was also observed, although Bi₂O₃ exhibited comparatively lower performance, whereas Ta₂O₅ yielded the most superior results.

Ceramic composites have sufficient potential as new materials for radiation shielding based on the excellent test and simulation results obtained in this study. In the future, factors such as the consistency, reproducibility, and simplicity of the shielding tool manufacturing process should be considered to ensure that the direct experimental results of the ceramic composite shielding performance in clinical environments and the results of theoretical simulations are consistent. These challenges highlight the need for standard processing technologies to ensure uniform shielding performance as well as further studies on the quantitative regularity of material mixing.