## 2 Experimental Setup
### 3D Gamma-CT Image Measurement  
Figure 2 shows a schematic diagram of the experimental setup at the beamline BL1U in the UVSOR-III facility used to measure the 3D gamma-CT image. A lead collimator with a quadrangular prism shape that was 200 mm in length, 100 mm in width, and 100 mm in height, which had a circular hole with a diameter of 1 mm, was positioned in the LCS gamma-ray beam's path to define the diameter and the energy spectrum of the LCS gamma-ray beam on the CT sample. We operated the laser system at a typical average power of 2.4 W during measurement of the gamma-CT images to generate an LCS gamma-ray beam with an intensity of 7 \(\times\) 10\({}^{6}\) photons/s with 100% energy bandwidth at the full width at half maximum (FWHM) before collimation. The EGS5 Monte Carlo simulation code [19] was used to estimate the LCS gamma-ray beam's properties after the collimation [13]. The estimated flux of the LCS gamma-ray beam after collimation was 0.7 photons/s/eV at a maximum energy of 5528 MeV with 1.1% energy bandwidth at the FWHM. Since the LCS gamma-ray beam's divergence was small (on the order of 10\({}^{-3}\)), the diameter of the LCS beam on the CT sample was almost the same as the size of the hole in the collimator: approximately 1 mm. The flux of the incident LCS gamma-ray beam was measured using a 5-mm-thick plastic scintillator (PL) located 180 cm downstream from the collimator. After passing through the PL, the transmitted LCS gamma-ray beam was injected into the CT sample, which was installed on the three-axis traveling stage located 213 cm downstream from the collimator. The flux of the transmitted gamma rays was measured using a 3.5\({}^{\prime\prime}\times\) 4\({}^{\prime\prime}\) LaBr\({}_{3}\)(Ce) scintillation detector positioned 82 cm downstream from the CT sample. This transmitted flux was used for evaluating the atomic attenuation of the sample. We chose the LaBr\({}_{3}\)(Ce) scintillation detector due to a better energy resolution, 2.7% at 662 KeV, than the NaI(Tl) scintillation detector whose typical energy resolution is approximately 6% to 8% at 662 keV. In addition, the availability of a high counting rate of the LaBr\({}_{3}\)(Ce) scintillation detector due to the short decay time is suitable for our measurement. The intrinsic radiation of the LaBr\({}_{3}\)(Ce) scintillation detector contributes significantly to the background radiation in the region below 1.6 MeV [20], which is far below our energy of interest (around 5.5 MeV). To avoid signal pile-up on the LaBr\({}_{3}\)(Ce) detector, a 5-cm-thick bismuth (Bi) absorber was placed in front of the detector. We used the bismuth as an absorber because of its high density. In addition, in the NRF-CT imaging technique, we measure the scattered NRF gamma rays from a witness target made from the isotope of interest such as the lead isotope \({}^{208}\)Pb in our experiments. If we used lead material as an  
Figure 1: (**a**) The cylindrical CT sample holder’s geometry, with the arrangement of the rods within the holder. (**b**) Picture of the CT sample during the experiment. absorber, the NRF gamma rays would be contaminated by the scattered NRF gamma rays from the absorber material. Therefore, it is preferable to avoid using the isotope of interest material as an absorber. Other high-density materials, such as tungsten, can be used as absorbers as well; however, bismuth is less expensive than tungsten. The CT sample was scanned along three axes as follows: (i) the vertical position (\(y\)) was fixed at a set position; (ii) 2D gamma-CT images were scanned in the horizontal direction (\(x\)) across the range from \(-12\) to +12 mm with a step size of 1 mm. The sample was scanned in each horizonal position in the direction of rotation (\(\theta\)) across the range from 0 to 150 degrees with a step size of 30 degrees; (iii) (i) and (ii) were repeated for different vertical positions (\(y\)) from 1 to 22 mm with a step size of 1 mm.