## 3 Results and Discussion  
The measurements of the gamma ray transmission factors, by including the effects of atomic absorption and nuclear resonance attenuation, yielded 2D NRF-CT images of the \({}^{208}\)Pb rods for each layer in the \(z\) direction [2]. The 2D NRF-CT images were visualized together to obtain reconstructed 3D NRF-CT images. Figure 4 shows a typical gamma ray spectrum obtained from the plastic scintillation detector. The plastic scintillation detector had no energy resolution within the 5 MeV region. Thus, to obtain the relative flux of the incident LCS gamma ray beam, the region of interest (ROI) was chosen in a wide channel range from 50 to 7000. Figure 5 displays a typical energy spectrum measured by the LaBr\({}_{3}\)(Ce) scintillation detector. The energy resolution of the LaBr\({}_{3}\)(Ce) detector was sufficiently high to distinguish between the LCS gamma ray peak and its single escape peak. The ROI, indicated in the shaded area, covered the energy range from 5360 keV to 5885 keV to consider the counts under the full energy peak. Figure 3: Data acquisition geometry of the CT target in three dimensions. In the horizontal direction (\(x\)), the CT target position was scanned from \(-\)12 mm to \(+\)12 mm in steps of 4 mm within the movement range. In the rotational direction (\(\theta\)), the CT target was rotated around the axis of rotation, with angle steps of 30\({}^{\circ}\) in the range from 30\({}^{\circ}\) to 180\({}^{\circ}\). The three layers in the vertical direction (\(z\)) were scanned at 3, 11, and 17 mm from the CT target bottom. For each point of measurement, we calculated the count rates of the plastic scintillator and the LaBr\({}_{3}\)(Ce) detector by integrating the counts in the ROIs and dividing them by the acquisition time. Then, we calculated the transmission factor of the off-resonance gamma rays (\(\varepsilon_{\text{off}}\)) for each \(\theta\) and \(x\) of the CT target. To calculate the normalized count rate of the transmission detector, we used the same procedure as that of previous studies [15; 21]. The count rate of the LaBr\({}_{3}\)(Ce) detector was divided by the total integrated count rate of the plastic scintillator within the ROI for each target position. Then, at each data point, \(\varepsilon_{\text{off}}\) was calculated by the normalized count rate of the transmission detector in the presence and absence of the CT target according to the following formulas:  
\[\varepsilon_{\text{off}}=\text{e}^{\left[-\left(\frac{\mu}{\rho}\right)_{ \text{ave}}\times\rho_{\text{ave}}\times L\right]} \tag{1}\]  
\[\varepsilon_{\text{off}}=\left[\frac{c_{\text{off}}(\text{with CT target})}{c_{\text{off}}(\text{without CT target})}\right] \tag{2}\]  
where \(\left(\frac{\mu}{\rho}\right)_{\text{ave}}\) is the average mass attenuation coefficient, \(\rho_{\text{ave}}\) is the average density, and \(L\) is the length of the path through the CT target. The resonance attenuation effect on the yield measured by the LaBr\({}_{3}\)(Ce) detector was negligible, as the width of the ROI was  
Figure 4: Part of the typical spectrum from the plastic scintillator (0:3000 channel). A few events were found in the channels ranging from 3001 to 8192. Figure 5: Typical LCS gamma ray spectrum obtained from the LaBr\({}_{3}\)(Ce) detector. The shaded area is the region of interest (ROI). wider than that of the resonance attenuation by 4-5 orders of magnitude. Figure 6 shows a typical summed energy spectrum measured by the HPGe detectors. There were several peaks in the HPGe spectrum. Two NRF levels of \({}^{208}\)Pb appeared at the energies of 5512 keV and 5292 keV. The single and double escapes (S.E. and D.E.) of the \({}^{208}\)Pb NRF at 5512 keV were represented by two peaks at the energies of 5001 keV and 4490 keV, respectively. The peaks at 2614 keV and 1460 keV originated from the natural unstable isotopes of \({}^{208}\)Tl and \({}^{40}\)K, respectively.