## 3 Results and Discussion  
### 3D-CT Primary Images  
CT imaging can be performed by measuring the decrease in the gamma-ray intensity passing through a CT sample along a series of linear paths and various angles. The amount of decrease depends on the gamma-ray energy, the path lengths, and the material's linear attenuation coefficients. Therefore, one can obtain a gamma-CT image for a sample using  
Figure 2: Schematic diagram of the experimental setup to measure the 3D gamma-CT image at the beamline BL1U in the UVSOR-III synchrotron radiation facility. the gamma-ray transmission factor measurements of the off-resonance energy, known as off-resonance (\(\epsilon_{\text{off}}\)) attenuation, which originates from the atomic effect. The procedures of the 2D gamma-CT images calculation are detailed in references [13; 14; 21]. Since the height of the CT sample holder was 20 mm, we divided it into 20 layers and added two extra layers; one of the two extra layers was located below the bottom of the CT sample and the other above it. These layers were separated by a vertical distance of 1 mm. Therefore, the total number of scanned layers was 22. We assigned the labels L1 to L22 to the layers at the vertical distances of \(y=-1\) mm to \(y=21\) mm, respectively. Consequently, we obtained one 2D gamma-CT image (25 \(\times\) 25) with a resolution of 1 mm/pixel for each row of rods in the \(y\) direction of the CT sample scan. The pixel size was determined by the scanning step in the \(x\) direction. In total, 22 reconstructed 2D gamma-CT images were obtained, which we named L\({}_{off}\)1 to L\({}_{off}\)22. Since Layer L1 was located beneath the CT sample holder and Layer L2 was located at the bottom edge of the holder, the LCS gamma-ray beam traveled underneath the rods. Therefore, the reconstructed images for the first two layers L\({}_{off}\)1 and L\({}_{off}\)2 did not show any characterization of the rods within the CT sample. Figure 3a shows the cross-sectional slices of the CT sample for the horizontal layers from L3 to L8. The total height of these layers was 6 mm, which was identical to the height of each rod within these layers. Two rods of enriched lead isotopes (\({}^{208}\)Pb and \({}^{206}\)Pb) were inserted into two of the holes in the CT sample, and an Fe rod was inserted into the third hole. Figure 3b shows the reconstructed CT images from L\({}_{off}\)3 to L\({}_{off}\)8 due to the atomic attenuation measured by the LaBr\({}_{3}\)(Ce) detector. Because the lead isotope (\({}^{206}\)Pb and \({}^{208}\)Pb) rods had the same atomic attenuation, the two high-attenuation areas in the white-colored area caused by the atomic process corresponding to the locations of these lead rods were plainly evident in all reconstructed images. However, it was difficult to discriminate between the two isotopes. Furthermore, the atomic attenuation caused by the Fe rod was smaller than that caused by the lead rods, making the Fe rod appears as a faint shadow. Figure 4a shows the cross-sectional slices of the CT sample for the horizontal layers from L9 to L14. The total height of these layers was also 6 mm. Rods of \({}^{208}\)Pb, Fe, and Al were inserted into the first, second, and third holes, respectively. The reconstructed CT images from L\({}_{off}\)9 to L\({}_{off}\)14 are shown in Figure 4b. A high-attenuation area caused by the atomic process corresponding to the \({}^{208}\)Pb rod is clearly visible in the reconstructed images. Figure 3: (a) Cross-sectional slice images of the CT sample in the layers from L3 to L8. (b) Reconstructed images from L\({}_{off}\)3 to L\({}_{off}\)8 of the off-resonance attenuation (atomic effect) measured by the LaBr\({}_{3}\)(Ce) detector with a resolution of 1 mm/pixel. The atomic attenuation originating from the Fe rods was less than that caused by the \({}^{208}\)Pb rod, making the Fe rod discernible but not as apparent. Since the CT sample holder was Al, the Al rod was not visible.