## 3 Proposed Methodology  
### 950 keV/3.95 MeV X-ray Sources  
We used an X-band (9.3 GHz) LINAC-based 950 keV/3.95 MeV X-ray source to inspect the actual bridge [14; 15; 16]. These systems are shown in Figure 9a,b, respectively.  
Figure 8: Other inspection methods for the inner structure of bridges. (**a**) RADAR (1.6–3 GHz) [10]; (**b**) ultrasonic testing (50 kHz) [11]; (**c**) magnetic testing [12]; (**d**) X-ray tubes (200–400 keV) [13].  
Figure 9: Portable X-band LINAC-based X-ray sources. The systems are composed of three units: X-ray head, magnetron, and power units. (**a**) 950 keV; (**b**) 3.95 MeV.  
The electrons are accelerated up to 950 keV by radio frequency (RF) fields in the first source. We also adopted a side-coupled standing-wave-type accelerating structure. The electrons are injected into the tungsten target, which generates bremsstrahlung X-rays. A tungsten collimator makes the generated X-rays to the shape of a cone with an opening angle of 17\({}^{\circ}\). The X-ray intensity is 50 mSv/min at 1 m for a full magnetron RF power of 250 kW. The system consists of a 50 kg X-ray head, a 50 kg magnetron box, and a stationary unit of an electric power source and water chiller. The X-ray head and the magnetron box are portable. Because a flexible waveguide connects them, only the position and angle of the X-ray head can be finely tuned. We optimized the design based on the X-ray intensity, compactness, and weight. The parameters of the 950 keV X-ray source are listed in Table 1. We placed an X-ray detector on the opposite side of the object to detect the transmitted X-rays. We used a flat panel detector (FPD) of a 0.4 thick GOS scintillator, with 16\({}^{\prime\prime}\)\(\times\) 16\({}^{\prime\prime}\) detector size and 200 \(\upmu\)m pixels.  
The 3.95 MeV system appears in Figure 9b. This system consists of a 62 kg X-ray head with a target collimator of 80 kg, a magnetron box weighing 62 kg, electric power sources at 116 kg, and a water-cooling system weighing 30 kg. The X-ray head and magnetron box are portable, and the position and angle of the former are finely tuned. The X-ray intensity of the system was 2 Gy/min at 1 m.  
The calculated attenuation in concrete for the X-rays from the 950 keV/3.95 MeV sources are shown in Figure 10. The results indicate that concrete with thicknesses up to 400 mm and 800 mm can be penetrated by the 950 keV/3.95 MeV sources, respectively.  
Figure 11 depicts the procedure of X-ray inspection, flaw evaluation, and structural analysis. Poor construction of unfilled grout in PC sheaths, early degradation such as rainwater intrusion, and finally serious degradation of thinning and disconnection of PC wires are detected with a spatial resolution of 1 mm within minutes by X-ray transmission imaging inspection. The iron components of PC, such as wires, can be clearly seen with a good contrast to concrete. Thinning and disconnection were observed with a spatial resolution of 1 mm. Measured flaws such as unfilled grout and thinning/disconnection are input to the structural analysis described in References [16; 17]. Thus, the initial poor construction  
\begin{table}
\begin{tabular}{c c c} \hline  & **950 keV** & **3.95 MeV** \\ \hline Operating Frequency & 9.3 GHz & 9.3 GHz \\ Beam Energy & 950 keV & 3.95 MeV \\ Beam Current & 130 mA & 100 mA \\ Electron Gun Voltage & 20 kV & 20 kV \\ Electron Gun Current & 300 mA & 300 mA \\ Pulse Width & 2.5 μs & 4 μs \\ Pulse Frequency & 330 pps & 200 pps \\ RF Power & 250 kW & 1.5 MW \\ \hline \end{tabular}
\end{table}
Table 1: Specifications of X-ray sources.  
Figure 10: Calculated results of attenuation of the X-rays in concrete from the 950 keV/3.95 MeV X-ray sources.  
of unfilled grout and serious thinning and disconnection of PC wires are diagnosed, and their effect on the lack and degradation of strength is quantitatively evaluated by structural analysis. Finally, maintenance, repair, and reconstruction were planned.