## 2 Experimental Details  
### Digital Detector Array Quality Assessment Using Radioactive Isotopes (Se-75 and Ir-192)  
Typically, nondestructive testing of welds using radiation is performed using radioactive isotopes and X-ray generators. Naturally occurring radioactive isotopes are advantageous for inspecting thick objects because their energy decreases in intensity and dissipates over time. Furthermore, they have a wider energy range than that of X-rays. Various X-ray generators are available depending on the principle of generating X-rays. The most common type is the vacuum-tube X-ray generator, which can be used continuously until the occurrence of malfunction. Users can select and use the desired tube voltage (kV) and tube current (mA) depending on the specifications of the device, which shortens the inspection time and ensures optimal results [6; 7; 8]. However, most vacuum-tube X-ray generators are large and not portable. Therefore, using such devices in narrow spaces between pipes to inspect pipe welds in the field is not feasible. In such cases, radioactive isotopes are practical for inspecting installed pipes. To address this problem, this study developed an inspection device in which radioactive isotopes are used instead of X-rays to inspect pipe welds when automatically rotating the device in narrow spaces between installed pipes. Therefore, as explained in this section, the performance of a digital detector array was evaluated using Se-75 and Ir-192, which are radioactive isotopes typically used to inspect welds. Tests were conducted to satisfy the quality specifications of the inspection according to international quality standards and the results were used to confirm the image characteristics produced by the gamma rays. Because this study was focused on developing a device for inspecting pipes installed in the field, digital detector array (DDA) quality assessment was conducted using gamma rays because this approach is easy to use. Furthermore, a bendable DDA bent according to the pipe diameter was used in the experiment. Table 2 presents the geometries and specifications of the bendable DDA used in this study. Tables 3 and 4 summarize the DDA quality assessment results using gamma rays (Se-75 and Ir-192) and evaluate whether the image quality indicator (IQI) satisfies the quality standards through tests of varying thicknesses according to the types of pipe and plate. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{3}{c}{\(\bullet\)**Radioactive isotopes: Se-75 (72 Ci)**} \\ \multicolumn{3}{c}{\(\bullet\)**IQI: ASTM 1A, 1B EN 10 FE (ISO 19232-1) [9]**} \\ \multicolumn{3}{c}{\(\bullet\)**Related standards:**} \\ \multicolumn{3}{c}{(1) ASME BPVC Section V: Nondestructive Examination**} \\ \multicolumn{3}{c}{(2) ISO 17636-2: Nondestructive testing of welds—Radiographic testing—Part 2: X- and gamma-ray techniques with digital detectors [10]**} \\ \hline \multirow{2}{*}{**No. Size/Type**} & **Thickness (mm)** & **Material** & **IQI Requirement** & **Acquired IQI** \\  & & & **(mm)** & **(mm)** \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Specifications of the bendable digital detector array used in this study. \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{3}{c}{\(\bullet\)**Scintillator type**} & **Gadox** \\ \multicolumn{3}{c}{\(\bullet\)**Pixel size**} & **140 \(\upmu\)m** \\ \multicolumn{3}{c}{\(\bullet\)**Pixel matrix**} & **768 \(\times\) 2304 mm** \\ \multicolumn{3}{c}{\(\bullet\)**Active area**} & **327 \(\times\) 107 mm** \\ \multicolumn{3}{c}{\(\bullet\)**Internal shielding**} & **300 kV** \\ \multicolumn{3}{c}{\(\bullet\)**Wireless**} & **Battery and WLAN** \\ \multicolumn{3}{c}{\(\bullet\)**Bendable**} \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Digital detector array (DDA) quality assessment results using gamma rays (Se-75).