## 1 Introduction  
Since the accident at the Fukushima Daiichi Nuclear Power Station (F1-NPS) in 2011, a great deal of effort has been devoted to the decommissioning of F1-NPS. The biggest challenge facing this process is the retrieval of the nuclear fuel debris, which was formed in the reactor vessels during the meltdown accident [1]. The fuel debris is presumed to contain uranium oxide (UO\({}_{2}\)) from the fuel itself, zirconium (Zr) from the cladding, and stainless steel (Fe, Ni, Cr) from the surrounding structural material, while the actual composition ratio may vary depending on where the debris was formed [2,3]. Besides those elements, another key element is gadolinium (Gd). A trace amount of gadolinium oxide (Gd\({}_{2}\)O\({}_{3}\)) was incorporated into some of the fuel rods to control fuel reactivity efficiently; Gd acts as a burnable absorber of thermal neutrons with the aid of the high neutron absorption cross-sections of \({}^{155}\)Gd and \({}^{157}\)Gd isotopes [4,5]. The relative abundance of Gd in the debris is an important factor for criticality control of the debris and a risk assessment of the retrieval process. Although it is crucial to determine its elemental compositions, the nuclear fuel debris is currently inaccessible and has yet to be identified because of the high radioactivity in the reactor vessels. Thus, there is a compelling need for methods to remotely analyze the debris and detect the above elements in a high-radiation field.  
Laser-induced breakdown spectroscopy (LIBS) is an elemental analysis technique based on plasma emission spectroscopy [6, 7, 8]. In LIBS, the output of a pulsed laser is focused onto the target material to generate a luminous plasma. The optical emission from the plasma is then collected with a spectrometer for qualitative and quantitative elemental analysis. Because LIBS allows us to perform remote and rapid measurements with little or no sample preparation, there have been continuous efforts to apply LIBS to nuclear material analysis [9; 10; 11; 12]. In particular, fiber-optic LIBS (FO-LIBS), which employs fiber optics to deliver laser light and collect plasma light [13], is considered suitable for the direct analysis of radioactive debris because it provides flexible access to a variety of samples in hazardous environments. We recently developed a portable FO-LIBS system originally aimed at the remote analysis of underwater samples in a harsh environment [14]. While several of our studies have demonstrated the capability of our system for analyzing various samples [14; 15; 16], measurements in a radiation field have yet to be explored. Therefore, further examination is necessary for the FO-LIBS approach to the analysis of the nuclear fuel debris.  
This paper reports on the application of our FO-LIBS system for the identification and quantification of Gd in surrogate nuclear fuel debris under gamma irradiation. As real fuel debris is not available as a sample, the LIBS measurements were performed using surrogate debris synthesized from mixed oxide materials containing Gd, Ce, Zr, and Fe. Cerium was employed as a surrogate for uranium based on the similarity of their electronic structures [17]. Detection of Gd in such complex materials is challenging because of the presence of a high density of spectral lines from the rare earth elements that severely interfere with each other [18; 19]. Another aspect to consider is the damage to the optical components of the LIBS system caused by gamma irradiation; exposure to gamma rays deteriorates the optical throughput, eventually hindering stable LIBS measurements. This study aims to reveal Gd emission lines applicable to Gd identification in the nuclear fuel debris and obtain calibration curves that will prove robust against gamma irradiation.