## 3 Results and Discussion
Although the numerous Gd emission lines are present at \(\lambda<700\) nm [25], most of them suffered from severe spectral interference with the lines of the other elements in the surrogate debris. As seen in Figure 4, the lines at 501.5 and 510.3 nm are well separated from the others. Those lines are not associated with resonance transitions, minimizing the influence of self-absorption. Note that the emission lines of uranium are absent in the close vicinity of those Gd lines [25; 26], which can therefore be used for Gd identification. The absolute intensities of those lines were not suitable for Gd quantification in the present work because they were not scaled with the concentration of Gd due to the disturbance from gamma irradiation. In addition, even in the five repeated measurements for a given sample, the relative standard deviation (RSD) values became high at a high dose rate. For example, the RSD of the Gd 501.5 nm line of sample #5 was evaluated as 8.9%, 16.2%, and 27.5% at 0, 0.8, and 10 KGy/h, respectively. To derive Gd calibration curves under such fluctuating conditions, the intensity ratio of Gd and Ce lines were employed as a measure of Gd concentration rather than the absolute intensities. The pair of spectral lines were selected as the 501.5 nm line (Gd) and 474.5 nm line (Ce). This selection is based on the similarity between the upper-level energies for the two emission lines: 3.58 eV for the Gd 501.5 nm line and 3.53 eV for the Ce 474.5 nm line [25; 26]. This mitigates the influence of the fluctuation of the plasma conditions [27]. The intensity ratios were calculated using the peak heights with baseline subtraction, yielding lower RSDs than the absolute spectral intensities: 5.0%, 1.8%, and 5.0% at 0, 0.8, and 10 kGy/h, respectively. Figure 5 presents the calibration curves obtained from the Gd (501.5 nm)/Ce (474.5 nm) intensity ratios, showing good linearity with the Gd concentrations. The results of the linear regression are summarized in Table 2. The limits of detection (LOD) of Gd were calculated as 0.03-0.08 wt% (Table 2) using the expression of LOD = 3\(\sigma\)/\(b\), where \(\sigma\) represents the standard deviation in the blank sample and \(b\) represents the slope of the curve. The similarity of the calibration curves in Figure 5 indicates that the variation of the dose rate exerts only a slight influence on the ratios of the spectral intensities, demonstrating the feasibility of the FO-LIBS in analyzing gadolinium-containing debris in radiation environments. As the present method relies on line intensity ratios, prior knowledge of Ce concentration is required to determine the Gd content. It is therefore more favorable to simultaneously measure the absolute concentrations of these elements. On the basis of the finding that the line intensities ratios  
Figure 4: Emission spectra of surrogate nuclear fuel debris in the neighborhood of Gd lines at (a) 501.5 nm and (b) 510.3 nm. were robust to the radiation field, more elaborate methods such as calibration-free analysis [28; 29] may be implemented to derive the absolute concentrations of Gd and the other constituent elements in the debris.