## 2 Experimental Procedure  
The experimental setup was similar to that of a previous study [2], which was performed at the BL1U beamline of the UVSOR-III synchrotron radiation facility [22; 23]. Figure 1 displays the schematic diagram of the experimental setup. The LCS gamma rays were generated by a head-on collision between high energy electrons in the UVSOR-III storage ring and a laser beam via Compton scattering. The electron energy in the storage ring was 746 \(\pm\) 1 MeV, and the average beam current was 300 mA in the top-up mode [22]. A continuous wave (CW) Tm-fiber laser system (TLR-50-AC-Y14, IPG Photonics GmbH, Kottinger Weg 188 Wissen, Germany) with a lasing wavelength of 1.896 \(\upmu\)m (0.6539 eV) and a maximum average power of 50 W was used as the laser source. The laser beam was randomly polarized. In this setup, the maximum gamma ray energy and the total flux with a 100% energy bandwidth were 5528 keV and 10\({}^{8}\) photons\(\cdot\)s\({}^{-1}\), respectively. The maximum energy of the LCS gamma ray beam was able to excite the state \(J^{\pi}=1^{-}\) at 5.512 MeV in \({}^{208}\)Pb [24]. To define the LCS beam diameter and the energy spectrum on a CT target, a lead collimator (20 cm \(\times\) 10 cm \(\times\) 10 cm) with a 2 mm internal hole diameter was positioned at the LCS gamma ray beam path. As was numerically simulated using the EGS5 Monte Carlo simulation code [25], the gamma ray flux density passing through the collimator was found to be 10 photons-s\({}^{-1}\)-eV\({}^{-1}\) at the resonance energy (5.512 MeV). In this experiment, the gamma ray flux density was double that of our previous study, because we used a collimator with a hole diameter twice that of the previous setup [2]. A 5 mm thick plastic scintillation detector (OHYO KOKEN KOGYO Co., Ltd. Kumagawa, Fussa City, Tokyo, Japan), which was located 160 cm downstream from the lead collimator, was used to measure the flux of the incident LCS gamma ray beam. After traversing the plastic scintillation detector, the transmitted LCS gamma ray beam was injected into the CT target. Subsequently, the LCS gamma ray beam transmitted from the CT target irradiated a witness target made of an enriched lead isotope rod (\({}^{208}\)Pb) with a diameter of 6 mm and a length of 6 mm, which was positioned 65 cm away from the CT target. Two high-purity germanium (HPGe) detectors with efficiencies of 100% (AMETEK ORTEC, Tennessee, USA) and 130% (Mirion Technologies (CANBERRA), Connecticut, USA), relative to a 3'' \(\times\) 3'' NaI (TI) scintillator, were arranged at two positions with an angle of 120 deg with respect to the gamma ray beam axis and at a vertical distance of 8 mm from the witness target. The HPGe detectors were used to measure the gamma rays scattered from the witness target (\({}^{208}\)Pb). A 6 mm thick lead absorber plate was installed between each HPGe detector and the witness target. Moreover, thick lead shields were positioned around the gamma ray beam axis in the arrangement to shield the HPGe detectors. The lead shields were helpful in preventing photon accumulation on the HPGe detectors and reducing the detector's dead time. To evaluate the atomic attenuation in the CT target, we measured the flux of the gamma rays that were transmitted from the witness target by a 3.5'' \(\times\) 4'' LaBr\({}_{3}\)(Ce) scintillation  
Figure 1: Schematic diagram of the experimental setup at the BL1U beamline in the ultraviolet synchrotron orbital radiation-III (UVSOR-III) synchrotron radiation facility. A lead collimator is positioned at the laser Compton scattering (LCS) gamma ray beam path. A plastic scintillator is used to measure the flux of the incident LCS gamma ray beam. The LCS gamma ray beam path is intercepted by a computed tomography (CT) target, which is positioned on a three-dimensional moving stage. Two high-purity germanium (HPGe) detectors, shielded by lead blocks, are used to measure the nuclear resonance fluorescence (NRF) gamma rays scattered from a witness target (\({}^{208}\)Pb).