## 3 Results and Discussion  
Figure 2 displays the representative LIBS spectra of the surrogate debris (sample #5) measured before and during gamma irradiation. The spectrum during irradiation was recorded when the accumulated dose reached 5.4 kGy at 10 kGy/h. The spectral profiles appear congested, originating from the prominent lines of Ce and Zr, along with the minor lines of Fe and N. As shown in Figure 2, gamma irradiation decreased the LIBS signal, particularly in the UV-Vis region (\(\lambda<800\) nm). The wavelength range of the degradation was consistent with that observed in the transmission attenuation of the optical fiber induced by gamma irradiation. Figure 3a shows a spectrum of the transmission loss of the optical fiber induced by gamma irradiation. It was measured in a separate experiment using a brand-new fiber, a halogen lamp (USS-600, Labsphere, North Sutton, NH, USA), and multichannel spectrometers (EPP2000-HR and BLUE-WAVE, StellarNet, Tampa,  
Figure 1: Overview of the experimental setup. (**a**) Schematic of the fiber-optic laser-induced breakdown spectroscopy (FO-LIBS) system. (**b**) Photograph of the FO-LIBS apparatus. FL, USA). The decrease in the LIBS signal was thus attributed to the damage to the optical fiber. Radiation-induced damage in optical fibers has been the target of several studies [20; 21; 22; 23; 24]; in particular, the gamma-ray dissociates Si-O bonds of silica in the fiber, which yields various types of radical species playing the role of color centers. The formation of these color centers attenuates the optical transmission of the fiber, particularly in the UV-Vis region [24]. By contrast, transmittance at the infrared region proved resistant to gamma irradiation (Figure 3a). Accordingly, transmission at 1064 nm (the wavelength of the excitation laser) remained almost intact, as shown in the temporal change of the transmission loss at 1064 nm (Figure 3b). It was also found that the LIBS spectra recorded immediately before and after stopping the irradiation (10 kGy/h) showed almost identical profiles (not shown here). This indicates that the gamma radiation field scarcely affected the properties of the laser-induced plasma, as expected from the weakly ionizing feature of the gamma rays (1.17 and 1.33 MeV) from \({}^{60}\)Co. As the transmittance of the fiber started recovering after the irradiation was halted (Figure 3b), the LIBS signal gradually increased. The experiments for obtaining the calibration curves described below repeated the LIBS measurements and the sample change. When the samples changed, gamma irradiation was stopped by removing the \({}^{60}\)Co emitter. As a result, the fiber transmittance underwent alternating degradation and recovery throughout the LIBS measurements, further perturbing the detected spectral intensities. Figure 3: Transmission loss of the optical fiber induced by gamma irradiation (10 kGy/h). **(a)** Spectrum of transmission loss after 30 min irradiation. The fluctuations at around 950 nm originated from the inherently low transmittance of the high-OH fiber due to the strong photoabsorption of OH. **(b)** Time evolution of the transmission loss at 500 nm (blue circle), 600 nm (red square), and 1064 nm (black triangle). The gamma irradiation was stopped at 30 min. Figure 2: Emission spectra of the surrogate nuclear fuel debris with and without gamma irradiation at a dose rate of 10 kGy/h. The Gd spectral lines applicable to Gd identification were determined by comparing the LIBS spectra of samples #1(Gd: 0 wt%) and #5(Gd: 0.87 wt%), measured without gamma irradiation. The obtained spectra contained the optical emissions from neutral Gd (Gd-I lines). In contrast, the ionic lines (Gd-II lines) were very weak, probably because the plasma temperature was not high enough to populate a sufficient amount of Gd in its ionic state at a 1 \(\upmu\)s delay time between the laser irradiation and the spectral detection.