| Summary: | 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 (<sup>208</sup>Pb) together with a set of different rods, including another enriched isotope (<sup>206</sup>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<sup>−1</sup>·eV<sup>−1</sup> flux density, which is available at the BL1U 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 J<sup>π</sup> = 1<sup>−</sup> NRF level at 5.512 MeV in <sup>208</sup>Pb. An isotope-selective three-dimensional NRF-CT image of the <sup>208</sup>Pb isotope distribution was experimentally obtained for the first time with a pixel resolution of 4 mm in the horizontal plane.
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