## 1 Introduction  
In general, it is necessary to develop residual radioactivity measurement technology that can easily access the site, and the technology in radiological measurements that can safely and effectively evaluate the environment, contaminated by a nuclear safeguard or serious accident at a nuclear facility [1, 2, 3, 4, 5, 6, 7]. In addition, there must be verified that the site is not contaminated for reuse of the site after the nuclear facilities are dismantled. Until now, a lot of studies have been conducted to secure technologies that can prepare for serious accidents such as Chernobyl and Fukushima [8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61,of Ba-137m with maximum electron energy of 0.514 MeV. The Ba-137m is a metastable state that decays with a 2.552 min half-life to the ground state of Ba-137 with emission of a 0.662 MeV gamma ray photon. Second, 5.4% of \(\beta^{-}\) decay proceeds directly from Cs-137 to the ground state of Ba-137 with maximum electron energy of 1.176 MeV. Cs-137 has a half-life of 30.17 \(\pm\) 0.03 years [15], which can cause long-term damage when exposed to the environment [16; 17; 18; 19; 20; 21; 22; 23; 24; 25]. Cesium has similar chemical properties to potassium (K), so it is easily concentrated in crops such as rice. In addition, it occupies a large amount of artificial radioactive gamma-emitting isotopes that have been leaked through nuclear tests and nuclear accidents. In this regard, cesium is an important radionuclide for detection and monitoring in environments and contaminated sites. Plastic detectors are widely used as large size detectors. Additionally, over the past few decades, many studies have been conducted on plastic scintillators composed of polymer matrix containing nanomaterials as an alternative to conventional inorganic scintillators [26]. Plastic scintillators are generally manufactured by thermal polymerization, doping, or coating by adding materials such as fluorescent dye. However, direct mixing of nanomaterials and polymers is less efficient in producing transparent nanocomposites. Because nanomaterials tend to aggregate due to their high specific surface area and surface energy, transparency is lost by Rayleigh scattering. To increase the transparency, research is required to reduce the aggregation of nanomaterials [27]. The plastic detector is composed of a low atomic number material such as C, H, O, so its accuracy is low due to low stopping power and light yield. Because of these properties, plastic scintillators are used in applications such as fast neutron detection, charged particle tracking, and non-destructive testing. Plastic scintillators have advantages over inorganic scintillators such as faster decay time (about 10 ns), non-hygroscopicity, relatively low manufacturing cost, robustness, and processability. Using these advantages, it is possible to develop a plastic scintillator with improved detection efficiency by adding nanomaterials of high atomic number to the plastic matrix [28; 29; 30; 31; 32; 33; 34; 35; 36]. In general, Monte Carlo codes are widely used for radiation instrument design, performance evaluation of prototypes, and detector calibration [37; 38; 39; 40]. The performance \begin{table}
\begin{tabular}{c c c c c c} \hline  & \({}^{137}\)**Cs** & **(E\({}_{k}\))\({}_{max}\)** & **(E\({}_{k}\))\({}_{avg}\)** & **(E\({}_{Y}\))** & **Probability** \\ \hline \�1 & \(\beta^{-}\) decay & 0.514 MeV & 0.170 MeV & - & 94.6\% \\ \�2 & \(\beta^{-}\) decay & 1.176 MeV & 0.420 MeV & - & 5.4\% \\ \�3 & Internal conversion & - & - & - & 7.8\% \\  & (K shell) & - & - & - & 1.80\% \\  & Internal conversion & - & - & 0.662 MeV & 85\% \\  & (L, M shell) & - & - & 0.662 MeV & 85\% \\ \hline \end{tabular}
\end{table} 
Table 1: \(\beta^{-}\) decay process, electron energy, and probability of Cs-137. Figure 1: Decay of diagram of \({}^{137}\)Cs.