## 4 Conclusions  
We experimentally obtained a three-dimensional, isotope-selective CT image of an enriched lead isotope (\({}^{208}\)Pb) distribution, concealed inside a cylindrical aluminum holder together with another enriched lead isotope (\({}^{206}\)Pb), iron and aluminum rods, and vacant areas. We used an LCS gamma ray beam with a maximum energy of 5.528 MeV, a 2 mm beam size, and a total flux density of 10 photons\(\cdot\)s\({}^{-1}\cdot\)eV\({}^{-1}\) to excite the J\({}^{\pi}\) =1\({}^{-}\) state at 5.512 MeV in \({}^{208}\)Pb. For each row of the rods, we obtained two-dimensional CT images based on atomic attenuation, NRF attenuation, and the sum of them with a 4 mm pixel size. In the atomic attenuation images, the enriched lead isotope rods (\({}^{208}\)Pb and \({}^{206}\)Pb) were observed. In the NRF attenuation images, only the \({}^{208}\)Pb rods were visible. The corresponding 3D  
Figure 12: (**a-i**) Consecutive shots of the reconstructed image of the nuclear resonant attenuation (pure NRF) captured from the visualized three-dimensional movie.  
NRF-CT image showed the isotope distribution of \({}^{208}\)Pb with 4 and 8 mm pixel sizes in the horizontal and vertical planes, respectively. The total required time to obtain a 3D isotope-selective image was 48 h. Further improvements in the image quality as well as a reduction in the acquisition time will be feasible by increasing the number of detectors to measure NRF gamma rays, increasing the gamma ray beam intensity and developing advanced data processing techniques, which will be a significant advancement in nuclear engineering for using the 3D NRF-CT technique in real applications.  
The following sub-elementary materials are available online at [[https://www.mdpi.com/article/10.3390/app11083415/s1](https://www.mdpi.com/article/10.3390/app11083415/s1)]([https://www.mdpi.com/article/10.3390/app11083415/s1](https://www.mdpi.com/article/10.3390/app11083415/s1)). Video S1: The three-dimensional movie of the off-resonant attenuation (Gamma-CT image). Video S2: The three-dimensional movie of the nuclear resonant attenuation (Pure NRF image).  
Conceptualization, H.O.; Data curation, K.A. and H.Z.; Formal analysis, K.A.; Funding acquisition, H.O., T.H. and T.S.; Investigation, K.A., H.O. and H.Z.; Methodology, All authors; Project administration, H.O.; Resources, H.O., T.H., T.S., M.K.; Software, K.A., H.Z.; Supervision, H.O.; Validation, All authors; Visualization, K.A., H.Z., H.O.; Writing--original draft, K.A.; Writing--review & editing, All authors. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Japan Society for the Promotion of Science (JSPS) KAKENHI under grants 18H01916, 18H03715, and 17K05482.  
This work was performed at the beamline BL1U of the UVSOR-III Synchrotron Radiation Facility with the approval of the Institute for Molecular Science (IMS), National Institute of Natural Science (NINS), Okazaki 444-8585, Japan. (Proposal No. 19-503 and 20-703).  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.