## 4 Discussion  
The medical radiation shield is mostly used as a material for radiation-shielding aprons, and the thickness and weight of the shielding fabric are important factors to ensure user safety. This study compared shielding performance by adjusting particle size to reduce the thickness and weight of the shielding sheet, and shielding performance was not efficient considering particle processing cost.  
When a lightweight shielding suit for protection against low-dose exposure in medical contexts is manufactured, there is a sufficient protective effect even if it is not produced of nanosized particles. Radiation energy produces photoelectric and Compton effects during interactions within a shielded sheet [35].  
The photoelectric effect is a phenomenon in which incidental photons interact with shielding particles and transfer the total energy of the photons to internal electrons, and internal electrons collide with surrounding atoms, causing scattering [36; 37]. A large number of particles in the material increases the occurrence of scattering, and absorption in the shielding sheet accordingly increases because of the photoelectric effect in the low  
Figure 5: Comparison of nanoparticles and microparticles in cross-section of shielding sheet. (**A**) Nanoparticle shielding sheet; (**B**) microparticle shielding sheet (red circle is where clustering effect was observed).  
Figure 6: Change in shielding efficiency based on particle size (A: nanoparticle, B: microparticle, C: mixture of microparticles and nanoparticles).  
energy region; thus, good shielding efficiency is achieved. However, in the high-energy region, shielding is less effective because of scattering due to transmission rather than due to absorption. To improve shielding performance, more particles are required to increase the probability of interaction within the same area. Therefore, the shielding efficiency of the nanoparticle sheet has a temporary synergistic effect at high-energy intensity, as shown in Figure 6. That is, because the number of particles per unit area is large in the shielding sheet, density is high, and uniform dispersion is achieved [38]. By reducing DBP, there was an increase in the probability of interaction with X-ray photons in the particles. This result was caused by the clustering effect, and it can be assumed that the base material directly affects the particle distribution of the shielding material.  
In particular, when mixing with a polymer series, the clustering effect can be predicted to be high, as the particle size is small. Future studies should compare the effects with other materials, such as rubber [39]. Furthermore, prior studies showed that the mixing of particle size and the base material that disperses it is crucial in the shielding-sheet manufacturing process, and that small particles generally exhibit high dispersion forces [40].  
In this study, the change in shielding efficiency according to particle size and particle-size mixing was quantitatively analyzed through an experiment using tungsten particles, an ecofriendly material that could replace lead. In addition, in the case of producing lightweight shielding suits used by medical institutions, experiments demonstrated which particle composition of shielding materials is advantageous according to the energy domain. Consequently, when the shielding sheet was manufactured under the same conditions, the change in shielding efficiency relative to the particle size was not large; however, the smaller the particle size was, the more effective the high-energy shielding. Therefore, when manufacturing a shielding body used for resisting high-energy radiation, such as gamma rays, the particle size of the shielding material should be small. In addition, the mixability and combination of the base material that could narrow the gap between particles are important.