## 3 Results  
The composition of the three types of shielding sheets manufactured using the same process is reported in Table 2. The appearance and mixing ratio of the sheets were the same, but there was a difference in the particle packing of tungsten and the weight per unit area. In general, a larger amount of shielding material can be added to a sheet composed of only nanoparticles.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{3}{c}{**Value**} \\ \cline{2-4}
**Item** & **A** & **B** & **C** \\ \hline Sheet structure & & Single structure & \\ Shielding material & & Tungsten & \\ Mixing ratio (polymer: tungsten) & & 1:3 & \\ Sheet weight (kg/m\({}^{2}\)) & 1.0 & 1.2 & 1.1 \\ Sheet thickness (mm) & 0.30 \(\pm\) 0.005 & 0.32 \(\pm\) 0.005 & 0.31 \(\pm\) 0.005 \\ Solvents (g) & 11.2 & 12.1 & 11.4 \\ Polyethylene (g) & 23.4 & 22.5 & 25.1 \\ Tungsten (g) & 70.2 & 76.4 & 73.1 \\ \hline \hline \end{tabular}  
* A, microparticle sheet; B, nanoparticle sheet; C, mixed microparticle and nanoparticle sheet.  
\end{table}
Table 2: Fabrication composition of radiation-shielding sheet.  
Figure 3: Evaluation of radiation-shielding performance of shielding sheets.  
Figure 4 shows the cross-sectional structure of the three shielding sheets with different tungsten particle contents, sizes, and arrangements. The comparison of the cross-sectional structures at the same magnification of the electron microscope shows that the array was composed of a dense structure when the particles were small. Figure 4C shows that it could not be arranged by particle size because it was artificially arranged, and that the dispersion of the shielding material according to particle size was not achieved during the stirring process.  
During sheet manufacturing, HDPE, the base material, was used to hold the particles of the shielding material. Figure 5 shows that the clustering effect between shielding material and polymer was more pronounced when nanoparticles were used. This shows that, when nanoparticles are used, it is ultimately effective in blocking pinholes through which radiation particles are transmitted. The clustering effect [34] between particles inside the shielding sheet explains the phenomenon in which particles are attached because the gap between particles is narrow, as shown in Figure 5A. Figure 5B shows that the gap between particles is narrow, but the phenomenon of particles sticking together is less than that of Figure 5A. Therefore, as confirmed by observations of the internal structure, if the size of the particles was small, a multilayered structure was formed in the sheet because of the clustering effect, and shielding efficiency improved. This is expected to increase the probability of particle interaction.  
Figure 4: Structure of inner particle arrangement of shielding sheet. (**A**) Nanoparticle shielding sheet; (**B**) microparticle shielding sheet; (**C**) nano-micro particle mix sheet.  
Figure 6 shows the shielding-efficiency comparison of the three types of shielding sheets. Overall, the larger the particle size was, the greater the shielding efficiency; however, the higher the X-ray tube voltage was (i.e., the higher the energy intensity), the lower the shielding efficiency. With nanoparticles, the particles were small, and particle packing was high, making shielding efficiency high at high tube voltage. However, with microparticles, shielding efficiency tended to drop by approximately 5% at a high tube voltage. In the mixed nanoparticle and microparticle sheet, shielding efficiency was similar to that of the microparticle sheet.