## 2 Materials and Methods  
When a polymer resin is used to make an RSF, the ratio of the shielding material is increased to improve the shielding performance. When the shielding material capacitance is increased to the same base capacitance, the shielding density increases, and the shielding performance improves. However, excessive proportions lower the density of the resin and weaken the tear resistance, which affects the tensile strength and thickness. During composition, the density is affected by the particle packing and porosity of the composing material. Therefore, the bulk density (\(D_{B}\)) includes air gaps. The true density (\(D_{T}\)) is the mass per unit volume of the shielding material, and it indicates the volume excluding air gaps. To increase the shielding effect, the true density must be increased. However, owing to the difficulty in the manufacturing process, the ratio cannot be provided identically. Therefore, it is calculated using the composition ratio of the shielding material. Porosity can thus be calculated using Equation (1):  
\[\text{Porosity}\left(\%\right)=\left(1-\frac{D_{B}}{D_{T}}\right)\times 100 \tag{1}\]  
and an accurate shielding sheet can be manufactured by mixing the materials. The \(D_{B}\) and the single shielding material particle density (\(D_{T}\)) of a shielding sheet, which is a base material, can be expressed by Equation (2). They are directly proportional to the porosity. \[D_{B}=\frac{W_{p}}{V_{p}+V_{air}},\;D_{T}=\frac{W_{p}+W_{m}}{V+V_{p}} \tag{2}\]where \(D_{B}\) is bulk density; \(D_{T}\) is true density; \(W_{p}\) is the weight of the polymer resin in the shielding sheet; \(W_{m}\) is the shielding material weight; \(V_{p}\) is the volume of shielding material; \(V\) is the volume of the shielding sheet; \(V_{air}\) is the pore volume. Therefore, the porosity of the RSF is calculated using Equations (1) and (2) and can indirectly estimate the bulk density; that is, it can be obtained using the mass and density of the shielding material. Furthermore, two mechanisms can be involved in the compounding process of the polymer resin and shielding material to reduce the porosity. The shielding materials used in this experiment were Bi\({}_{2}\)O\({}_{3}\), WO\({}_{3}\), and BaSO\({}_{4}\), which were pulverized in a miller roller with spacing of 0.1 mm for 120 min; a micro-scale particle size was achieved. We blended polyethylene at regular intervals, and the mixing roller was maintained at 50 \({}^{\circ}\)C. Polyethylene is more effective than conventional rubber for heat treatment because thinner flexible sheets can be manufactured. The elasticity is related to the mixing ratio of the shielding material, and shielding sheets with excellent processability can be manufactured. A ratio of 7:3 is the most effective ratio. After this process, the shielding mixture underwent an aging process at 50 \({}^{\circ}\)C for approximately 12 h. Then we manufactured the RSF through compression, vacuuming, and molding with a hydraulic press for 300 s after defoaming. Defoaming is the adequate removal of air by blending with a plasticizer, antioxidants, lubricants, and antistatic agents for 30 min. The shielding sheet shown in the manufacturing process was evaluated for porosity, tensile strength, and shielding ability using the same base material. Accordingly, the composition ratio of the shielding sheets was set to 65% shielding material (Bi\({}_{2}\)O\({}_{3}\), WO\({}_{3}\), and BaSO\({}_{4}\)) and 35% polyethylene. BaSO\({}_{4}\) occurs as the mineral barite, and is the main commercial source of barium used to prepare materials with barium. Bi\({}_{2}\)O\({}_{3}\) is the most industrially important compound of bismuth. It is also used as a raw material for bismuth chemistry. WO\({}_{3}\) powder is a middle stage material obtained during the extraction of tungsten from minerals. The tungsten is treated with an alkali, and WO\({}_{3}\) is obtained, see Table 1.  
Three 60 cm long and 60 cm wide sheets of different thicknesses--0.2, 0.4, and 0.6 mm--were produced.