## 3 Results and Discussions
### Thermal Effects on the Decolorization Process  
Figure 5 shows the time changes of the green component intensities of PAISiN after irradiation with 5, 10 and 20 Gy \(\gamma\)-rays and being stored in a dark environment at 3 different temperatures: 20 \({}^{\circ}\)C (in a lab), 40 \({}^{\circ}\)C (in an oven) and 5.5 \({}^{\circ}\)C (in a refrigerator). The same set of PAISiN cells was irradiated repeatedly in this order after a complete decolorization was confirmed. As seen in the figures, the colors of PAISiN irradiated with 5 Gy returned to the initial white color within 24 h under room temperature (20 \({}^{\circ}\)C), while it took about 3 days to complete decolorization for 20 Gy irradiation. At 40 \({}^{\circ}\)C, the decolorization process was remarkably accelerated and complete in just 2 h, even for 20 Gy irradiation. In contrast, no decolorization was seen at 5.5 \({}^{\circ}\)C for at least 4 days; some enhancement of the color in the initial hours was observed, which implies that the PVA-I bonding was becoming  
Figure 4: Dose responses of the inverted 8-bit intensities of blue (**left**), green (**middle**) and red (**right**) components of the PAISiN images acquired at room temperature (20 \({}^{\circ}\)C) at 0.25 h, 11.6 h and 70.6 h after \(\gamma\)-ray irradiation. stronger with decreasing temperature. This implication is supported by the fact that the PVA-I matrix becomes a more stable equilibrium and maintains I\({}_{2}\) more firmly below 10 \({}^{\circ}\)C [37]. Such a notable temperature dependence of the decolorization process of PAISiN could be explained by the acceleration or suppression of the chemical reactions triggered by ionizing radiation. The major processes of coloration and decolorization of PAISiN can be described as follows [16; 26; 38; 39; 40; 41]:  
Radiolysis: \[\rm H_{2}O\rightarrow\rm OH^{\bullet},\rm HO_{2}{}^{\bullet},\rm H^{\bullet},\rm etc.\] (1) \[\rm OH^{\bullet}+H^{\bullet}\to H_{2}O_{2}\] (2) Oxidation: \[\rm H_{2}O_{2}+I^{-}\to 0.5I_{2}+OH^{-}+OH^{\bullet}\] (3) \[\rm OH^{\bullet}+I^{-}\to HOI^{-}\] (4) \[\rm HOI^{-}\to HO^{-}+I^{\bullet}\] (5) \[2I^{\bullet}+I^{-}\to I_{3}{}^{-}\] (6) \[\rm Complexation: \rm I_{3}{}^{-}+PVA\to PVA-I_{3}{}^{-}\] (colored) Dissociation: \[\rm PVA-I_{3}{}^{-}\to 3I^{-}+PVA\ (decolorized)\] (8)  
First, ionizing radiation induces the radiolysis of water (H\({}_{2}\)O) and generates radicals such as OH\({}^{\bullet}\), HO\({}_{2}{}^{\bullet}\) and H\({}_{2}\)O\({}_{2}\) (as indicated with Equations (1) and (2)). The rate constant (k) of the Equation (2) reaction is known to be k = 1.1 \(\times\) 10\({}^{10}\) L mol\({}^{-1}\) s\({}^{-1}\)[38]. The same radicals are also made through the electrons and holes in SiNP after ionization [39; 40]. Iodine ions (I\({}^{-}\)) are oxidized by these radicals and changed to I\({}_{3}{}^{-}\) (Equations (3)-(6)). These I\({}_{3}{}^{-}\) molecules are bound to PVA [40] (Equation (7)), which is recognized as red color formation. The rate constant of the overall reaction between PVA and OH\({}^{\bullet}\) is \(9.2\times 10^{10}\) L mol\({}^{-1}\) s\({}^{-1}\)[41], while the reaction of OH\({}^{\bullet}\) and \(\rm I^{\bullet}\) is considered to be dominant because the molar concentration of I\({}^{-}\) (0.3 mol L\({}^{-1}\)) was much higher than that of PVA (0.66 mmol L\({}^{-1}\)). Under the condition of constantly receiving certain thermal energies (e.g., at room temperature), the PVA-I\({}_{3}{}^{-}\) gradually reverts to the iodine ions and PVA (Equation (8)) with time while losing the red color. This reverting, dissociating process is considered to occur on the surface of SiNP, which is negatively charged due to the deprotonated silanol group (-Si-O-) [42]. PVA is adsorbed onto the surface of SiNP through the hydrogen bonding between the hydroxyl groups of the SiNP surface and those in PVA [31].