###### Abstract  
Combining the nuclear resonance fluorescence (NRF) transmission method with computed tomography (CT) can be a novel method for imaging the isotope distributions, which is indispensable in nuclear engineering. We performed an experiment to reconstruct a three-dimensional NRF-CT image with isotope selectivity of enriched lead isotope rods (\({}^{208}\)Pb) together with a set of different rods, including another enriched isotope (\({}^{206}\)Pb), iron, and aluminum rods, inserted into a cylindrical aluminum holder. Using a laser Compton scattering (LCS) gamma ray beam with a 5.528 MeV maximum energy, 2 mm beam size, and 10 photon-s\({}^{-1}\)-eV\({}^{-1}\) flux density, which is available at the BLU beamline in the ultraviolet synchrotron orbital radiation-III (UVSOR-III) synchrotron radiation facility at the Institute of Molecular Science at the National Institutes of Natural Sciences in Japan, and we excited the \(\mathrm{J}^{\pi}=1^{-}\) NRF level at 5.512 MeV in \({}^{208}\)Pb. An isotope-selective three-dimensional NRF-CT image of the \({}^{208}\)Pb isotope distribution was experimentally obtained for the first time with a pixel resolution of 4 mm in the horizontal plane. Keywords:gamma rays; laser Compton scattering (LCS); nondestructive inspection; nuclear resonance fluorescence (NRF); isotope-selective computed tomography (CT) +
Footnote †: journal: journal: Applied Physics Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. NDI technique in safeguard applications lies in its ability to directly measure a specific isotope in an assay target without having to unfold the combined responses of several fissile isotopes, as is often required by other NDI methods [12; 13]. Because of its high penetrability, an MeV energy region monochromatic gamma ray is an effective probe for the NDI of high-density materials. In NRF measurements, laser Compton scattering (LCS) gamma rays [14] are used extensively to excite the nuclei to be detected because of their characteristics, such as directivity, a quasi-monochromatic energy spectrum, tunable energy, controllable polarization, and high flux [15]. [PERSON] et al. [6] performed a proof-of-principle experiment for NRF nondestructive analysis with LCS gamma ray beams. A lead isotope of \({}^{208}\)Pb inside of an iron box was measured as a high-\(Z\) material instead of fissionable isotopes. CT is one of the cross-sectional imaging modalities which has numerous applications in various fields, such as medical imaging. CT systems are also used for checking luggage at airports worldwide to detect explosives [16; 17; 18; 19]. For imaging isotopes inside massive materials, the integration of CT and NRF (NRF-CT) has been proposed [20]. [PERSON] et al. [21] employed a GEANT4 Monte Carlo simulation code to perform a simulation study of the CT imaging of nuclear distribution by NRF processes induced by quasi-monoenergetic LCS gamma rays. Recently, we carried out a proof-of-experiment for the NRF-CT using an LCS gamma ray beam available at the beamline BL1U in the ultraviolet synchrotron orbital radiation-III (UVSOR-III) synchrotron radiation facility at the Institute of Molecular Science at the National Institutes of Natural Sciences in Japan [15]. We measured a two-dimensional (2D) NRF-CT image of a natural lead isotope (\({}^{208}\)Pb) implanted in an aluminum sample based on the transmission method (notch detection method) [15]. One of the advantages of this technique is the ability to measure the CT image of an isotope of interest. Therefore, the isotope-selective capability was demonstrated using an LCS gamma ray beam for enriched lead isotope rods (\({}^{208}\)Pb and \({}^{206}\)Pb) [2]. The reconstructed image had a resolution of 2 mm in pixel size. This result suggests that when a three-dimensional visualization of a specific isotope within an assay volume is provided, we get the full perception of the hidden isotopes.