## 2 Materials and Methods  
### Materials for Dosimetry  
A commercially available radiochromic film (Gafchromic EBT-XD, ISP, Wayne, NJ, USA) (Figure 1) was employed in the present study. EBT-XD is a Gafchromic film released in 2015 as a recent product of the EBT series model (EBT, EBT2, EBT3 and EBT-XD) which appeared in 2004. EBT-XD has some better features than the prior product Gafchromic EBT-3 [18; 36] in view of delivering beam dosimetry in radiation therapy. For example, the optimal dose range of EBT-XD is 0.4 to 40 Gy and its dynamic dose range is 0.1 to 60 Gy; these ranges are higher than those of EBT-3 (0.1 to 10 Gy and 0.1 to 20 Gy, respectively), while the physical structures of both products are nearly the same. The EBT-XD film have a 25 \(\upmu\)m active layer sandwiched with two 125 \(\upmu\)m-thick polyester substrates. The size of its active particle is 2 to 4 \(\upmu\)m in length with a diameter of 1 to 2 \(\upmu\)m; such small-size particles are considered to work for reducing light scattering and polarization, leading to fewer lateral response artifacts, the so-called 'orientation effect' which is characterized by a change in the measured color intensity of a scanned film depending on its orientation on the scanner [21; 25; 41; 42; 43; 44].  
The color intensities of EBT-XD were measured with a commercially available portable colorimeter (\"nix pro 2\", Nix Sensor Ltd., Ontario, Canada; abbreviated to \"Nix\" hereafter) shown in Figure 1a. While a flatbed scanner has generally been used for acquiring the two-dimensional images of Gafchromic films in radiation therapy [13; 20; 23; 25; 36; 45], the authors first employed the portable colorimeter here with an intention of achieving simple on-site dosimetry that could be utilized in other fields. It should be noted that, since the aperture size of Nix for detecting the reflected light from a sample surface is 14 mm in diameter, the conventional method using a flatbed scanner will be needed for precise two-dimensional dosimetry.  
Nix has the shape of a hexagon with dimensions of 60 mm in height \(\times\) 42 mm in width and a weight of 43 g. It includes a rechargeable lithium-ion battery and can perform more than 100 scans per single charge. Nix measures the color intensities of any flat material cutting the ambient light (Figure 1b), and the data of color intensity measured as RBE in a few seconds and CMYK components are transferred via Bluetooth to a smartphone having the operating system of Android or iOS and immediately displayed on its screen. These data can be saved into the smartphone and exported for further analyses.  
The features of Nix such as the small size, light weight, easiness to handle and relatively low price (350 USD) are preferable for practical application to on-site dosimetry. Particularly, the good portability is quite beneficial for the aim of the present study, as anyone can easily carry this device in their pocket to prepare for a possible occasion of a radiological incident. The high performance of Nix in reading color intensities has been well validated in previous studies for the determination of soil compositions [46; 47; 48; 49] and evaluation of beef freshness [50; 51].