## 2 Experimental Procedure
To prevent pile-up events, a bismuth absorber is used in front of an LaBr\({}_{3}\)(Ce) scintillation detector, which measures the transmitted gamma rays. detector (SAINT-GOBAIN, Courbevoie, France), which was placed 40 cm downstream from the witness target. A 10 cm thick bismuth absorber was mounted in front of the LaBr\({}_{3}\)(Ce) detector to prevent signal pile-up. Although the bismuth absorber attenuated the gamma rays transmitted from the witness target, this attenuation was annulled during image reconstruction since the relative change of the gamma ray flux, depending on the CT target position and angle, was used in the analysis. The signals from each detector were independently recorded by a multichannel analyzer (APG7400, Techno AP Co., Ltd., Mawatari, Hitachinaka-shi, Ibaraki, Japan). Figure 2a displays a picture of the CT target. A cylindrical aluminum holder with a height of 20 mm and a diameter of 25 mm was used as the target container, as shown in Figure 2b. Three holes with equal diameters of 6.1 mm were drilled into the holder body with a 120\({}^{\circ}\) pitch angle. To construct clear 3D images, the sample holder holes were filled by a set of cylindrical rods with equal dimensions of 6 mm diameter and height, as shown in Figure 2c. In hole 1, the bottom hollow was filled by an iron rod, followed by one of the enriched lead isotope rods (208 Pb), while the third hollow remained empty. In hole 2, the bottom was filled by one of the enriched lead isotope rods (206 Pb), followed by a hollow aluminum rod and a lead isotope rod (208 Pb). In hole 3, the bottom was filled by an enriched lead isotope rod (208 Pb), followed by an iron rod and an enriched lead isotope rod (206 Pb) in the hole top. The CT target was positioned 214 cm downstream from the collimator on a three-axis moving stage which could scan in three directions: vertical (\(z\)), rotational (\(\theta\)), and horizontal (\(x\)). Figure 3 displays the scanning directions of the CT target. We chose the smallest number of the measurement positions that would allow us to obtain a reasonable image in an appropriate time frame and prove the NRF nondestructive analysis in three dimensions using the LCS gamma ray beam. The step sizes in individual directions were determined to obtain a reasonable image resolution, considering the CT target geometry under a limited measurement time. The diameter of the gamma ray beam was 2 mm, which was half the pixel size of the reconstructed image. The CT target had a diameter of 25 mm, and the lead and iron rods in the target had a diameter of 6 mm. The gamma ray beam diameter was sufficiently shorter than the object being studied. Three layers at \(z=3\), 11, and 17 mm from the holder bottom in the \(z\) direction were selected for constructing 2D NRF-CT images. The vertical positions of these layers were selected to obtain a 2D NRF-CT image for each horizontal row of the rods, since there were three rows of the rods in the \(z\) direction, as shown in Figure 2c. The CT target was rotated around the \(\theta\) axis, with an angle step of 30\({}^{\circ}\) in the range from 30\({}^{\circ}\) to 180\({}^{\circ}\). The scanning was also carried out in the \(x\) direction, with a step size of 4 mm in the range from \(-12\) to +12 mm. We collected 126 data  
Figure 2: (**a**) Picture of the CT target. (**b**) Geometry of the CT target holder, having a diameter of 25 mm and a height of 20 mm. The target holder has three holes with equal diameters of 6.1 mm and a pitch angle of 120\({}^{\circ}\). The numbering of the holes is shown on the target holder. (**c**) Arrangement of the rods, including the enriched lead isotope rods (206 Pb and 208 Pb), iron, aluminum, and a vacant area. points with the CT target (7 steps along the \(x\)-axis \(\times\) 6 rotational angles (\(\theta\)) \(\times\) 3 layers in the \(z\) direction). In addition, one data point for each horizontal scan in the \(x\) direction was measured for collecting data in the absence of the CT target. Therefore, a total of 144 points were measured.