## 1 Introduction  
In the medical field, radiation is the most important tool for the diagnosis and treatment of diseases, and it is developing into a specialization. As the medical and industrial radiation fields are expanding, exposure to radiation from medical [1, 2] and industrial [3, 4] diagnostic devices is also increasing. Exposure to medical radiation occurs at low doses and only in some parts of the body. However, owing to the inadequate protection and carelessness of medical workers, the amount of exposure increases [5]. In addition, there is an increasing risk of cumulative radiation exposure due to the increasing frequency of examinations and photographs [6]. The medical radiation-exposure level is primarily determined by secondary scattered radiation, which is known as a low-dose region. Recently, attempts were made to reduce the exposure of medical workers to low-dose radiation while ensuring safety with a lightweight shielding suit [7, 8, 9]. Shielding clothing in medical institutions is made primarily of lead; however, because of issues such as the weight of lead, its toxic potential, manufacturing contaminations, and disposal costs, interest in developing a shield using ecofriendly materials is rising [10, 11]. The most common ecofriendly radiation-shielding materials include tungsten, bismuth, barium sulfate, tin, and antimony [12, 13]. Shields produced using these materials are primarily manufactured in the form of sheets, fibers, and thin films, and they have similar shielding performance to that of lead; thus, they are highly effective. Research into developing an ecofriendly shielding material to replace lead, and reduce the weight and thickness of clothing is underway [14]. This is important for expanding the range of users, ensuring that they can wear such clothing for a long time, and to maintain activity. Research into increasing shield density to create lightweight and thin films is in progress; density depends on the mixing process and the process technology of the shielding material [15]. The shielding sheet, which is the most common apron material among medical radiation shields, is produced by either extrusion molding, calendering, coating, or injection molding by mixing a shielding material with a polymer resin. In addition, recent manufacturing practices include a laminated structure to absorb and control radiation, thus maintaining stable shielding performance [16]. Shielding sheets for medical radiation are primarily used as apron materials, and generally feature a shielding performance of 0.25 and 0.50 mmPb, lead-equivalent [17]. The thickness of the shielding sheet is around 3 mm, and is primarily determined by the amount of polymer and shielding materials. In previous studies, researchers investigated a method for reducing the thickness of the shielding sheet to increase density by reducing the particles of the shielding material rather than the type of polymer material [18; 19; 20]. To increase the density of the shielding sheet, process technology for lowering the porosity and increasing the particle packing is required [21]. To reduce the porosity, a method that narrows the gap between particles by reducing the particle size is used. However, most of the particles of commercial shielding sheets use microsized particles because they are more cost-effective, and these shielding sheets offer shielding performance that is similar to that of shielding sheets that use nanosized particles [22; 23]. Therefore, this study evaluated shielding performance according to the particle size of the shielding material, and explains its quantitative effect in the process technology. In addition, this study attempts to confirm, through the cross-section of the shielding sheet, the clustering effect that narrows the gap between particles that occurs frequently when nanoparticles and microparticles are used. Thus, the effect of particle size on shielding performance is explained through testing the shielding effect.