## 3 Results and Discussion
The CT reconstructed image of the off-resonance attenuation, measured by the LaBr\({}_{3}\)(Ce) detector, is shown in Figure 8b. The two high-attenuation areas that were induced by the atomic process corresponding to the \({}^{206}\)Pb and \({}^{208}\)Pb rods were clearly visible in the reconstructed image. However, it was impossible to distinguish between these two isotope rods, because the atomic attenuations by \({}^{208}\)Pb and \({}^{206}\)Pb were almost the same. In comparison with these lead rods, the atomic attenuation caused by the iron rod was significantly less. Thus, it was difficult to see the iron rod in the reconstructed image. Figure 8c shows the distribution of the on-resonance gamma ray attenuation. Although the \({}^{208}\)Pb rod was clearly seen, the intensity of the \({}^{206}\)Pb rod was comparatively less. Moreover, the iron rod almost vanished. The distribution of NRF attenuation by \({}^{208}\)Pb (pure NRF) after subtraction of the atomic attenuation is shown in Figure 8d. Although the \({}^{208}\)Pb rod was clearly visible, both the other lead-enriched isotope (\({}^{206}\)Pb) rod and iron rod almost vanished. Figure 9a shows the CT target arrangement at \(z\) = 11 mm. This figure shows an enriched lead isotope rod (\({}^{208}\)Pb), iron rod, and hollow aluminum rod inserted into the first, second, and third holes, respectively, of the aluminum holder. The CT reconstructed image of the off-resonance attenuation, measured by the LaBr\({}_{3}\)(Ce) detector, is shown in Figure 9b. A high-attenuation area, induced by the atomic process corresponding to the \({}^{208}\)Pb rod, appeared clearly in the reconstructed image. Because the atomic attenuation that was caused by the iron rod or the hollow aluminum rod was much smaller compared with that caused by the lead rod, the former two rods disappeared in the reconstructed image. Similarly, in the case of the layer at \(z\) = 3 mm, only the lead isotope rod (\({}^{208}\)Pb) was visible in the on-resonance reconstructed image (Figure 9c) and pure NRF image (Figure 9d), whereas the iron and aluminum rods vanished almost completely. Figure 10a shows a cross-sectional slice image of the CT target at \(z\) = 17 mm. This figure shows that two enriched lead isotope rods (\({}^{206}\)Pb and \({}^{208}\)Pb) were inserted into two holes of the aluminum holder, whereas the third hole seems empty. The CT reconstructed image of the off-resonance attenuation, measured by the LaBr\({}_{3}\)(Ce) detector, is shown in Figure 10b. The two high-attenuation areas, induced by the atomic process corresponding to the positions of the lead isotope rods (\({}^{208}\)Pb and \({}^{206}\)Pb), were clearly visible, whereas the low intensity at the third hole position indicated an empty area. Figure 10c indicates the distribution of \(e_{\text{off}}\). Although the \({}^{208}\)Pb was clearly visible, the \({}^{206}\)Pb rod appeared blurred. Figure 10d shows the distribution of the NRF attenuation caused by \({}^{208}\)Pb, namely pure NRF. Although the \({}^{208}\)Pb rod was clearly visible in the reconstructed image, the \({}^{206}\)Pb rod and empty area almost vanished. As in the previous study [2], these results show that the NRF-CT method can clearly distinguish the two enriched lead isotope rods (\({}^{208}\)Pb and \({}^{206}\)Pb) within the same sample with isotope-selective capability. A 3D-CT image can essentially be constructed from 2D-CT images measured at different axial positions. In this study, three measured 2D NRF-CT images were used to construct one 3D NRF-CT image. We used the MicroAVS data visualization tool [29] to visualize the 3D NRF-CT images with a 4 mm pixel size in the horizontal plane. \[\varepsilon_{\text{off}}=\text{e}^{\left[-\left(\frac{\mu}{\rho}\right)_{ \text{ave}}\times\rho_{\text{ave}}\times L\right]} \tag{1}\]0 (**a**) CT target cross-sectional slice image at \(z\) = 3 mm, as well as reconstructed CT images of the (**b**) atomic attenuation, (**c**) on-resonance attenuation, and (**d**) pure NRF case.