## 3 Results and Discussion
\[\varepsilon_{\text{off}}=\text{e}^{\left[-\left(\frac{\mu}{\rho}\right)_{ \text{ave}}\times\rho_{\text{ave}}\times L\right]} \tag{1}\]1 (**a**) CT target cross-sectional slice image at \(z=17\) mm, with reconstructed CT images of the (**b**) atomic attenuation, (**c**) on-resonance attenuation, and (**d**) pure NRF case. \[\varepsilon_{\text{off}}=\text{e}^{\left[-\left(\frac{\mu}{\rho}\right)_{ \text{ave}}\times\rho_{\text{ave}}\times L\right]} \tag{1}\]2 (**a**) CT target cross-sectional slice image at \(z=11\) mm, with reconstructed CT images of the (**b**) atomic attenuation, (**c**) on-resonance attenuation, and (**d**) pure NRF case. Figure 11a-i shows consecutive shots of the three-dimensional reconstructed image of the off-resonance attenuation (i.e., atomic attenuation used in normal gamma ray CT), which were captured from the visualized 3D movie (see video S1 in the supplementary information). These figures clearly show five high-attenuation areas caused by the atomic process, corresponding to the positions of the five lead isotope rods (\({}^{208}\)Pb and \({}^{206}\)Pb) within the CT target. In contrast, the atomic attenuation induced by the iron rod, aluminum rods, or the empty area were almost negligible, and thus it was difficult to identify their locations. Figure 11a-i shows consecutive shots of the visualized reconstructed image of the NRF attenuation caused only by the lead isotope rods (\({}^{208}\)Pb, pure NRF) in three dimensions. These figures clearly show the locations of the three enriched lead isotope rods (\({}^{208}\)Pb) in the three \(z\) layers. In contrast, \({}^{206}\)Pb, the iron and aluminum rods, and the empty areas were invisible (see video S2 in the supplementary information). In the current study, we demonstrated isotope-selective CT imaging based on NRF in three-dimensional measurements. A measurement time of approximately 48 h was required for obtaining a single 3D NRF-CT image of the examined sample with a 4 mm pixel resolution in the horizontal plane for three vertical positions. Although the resolution of the 3D NRF-CT image in the current study (4 mm pixel size) was lower than the resolution of the 2D NRF-CT which we obtained in the previous study (2 mm pixel size) [2], the improvements in the experimental set-up were positively reflected in the image acquisition time, whereas the current 3D NRF-CT image with three layers in the \(z\) direction required a shorter acquisition time compared with the 2D NRF-CT image [2]. One of the objectives of this study was to look at the possibility of applying the NRF-CT technique in three dimensions using the current experimental setup of the UVSOR-III synchrotron radiation facility. Consequently, the lack of the obtained image resolution was not proven to be an impediment so far. \[\varepsilon_{\text{off}}=\text{e}^{\left[-\left(\frac{\mu}{\rho}\right)_{ \text{ave}}\times\rho_{\text{ave}}\times L\right]} \tag{1}\]3 (**a**–i) Consecutive shots of the reconstructed image of the off-resonant attenuation (gamma-CT image) captured from the visualized three-dimensional movie. We agree that the 3D NRF-CT imaging method is still time-consuming. However, some solutions have been proposed to enhance the obtained image quality and reduce the measurement time. Increasing the number of HPGe detectors to measure the NRF gamma rays from the witness target is one of the possible solutions. Another solution is to increase the gamma ray beam intensity. The Extreme Light Infrastructure Nuclear Physics (ELI-NP) project is currently being constructed as a new LCS gamma ray facility in Romania, where the expected flux density of the LCS gamma ray beam (5000 photons\(\cdot\)s\({}^{-1}\)-eV\({}^{-1}\)) [30] will be 500 times higher than the present one. The development of a new laser-based inverse Compton scattering gamma beam system at the ELI-NP project, which features extremely high intensities at very narrow bandwidths, opens new and important opportunities in nuclear science research and for 3D NRF-CT applications.