Optical fiber X-ray sensors have the potential to realize real-time dose monitoring of precision radiotherapy. However, an over-response phenomenon can occur when using an optical fiber sensor (OFXS) filled with inorganic scintillator (Gd₂O₂S:Tb) to measure the off-axis ratio (OAR) curve. The aim of this paper is to study the mechanism responsible for the over-response. Due to the complex particle distribution present in water phantoms, the Monte Carlo-based code GEANT4 was used to model the response of the scintillator. The energy response of the scintillator to photons and electrons was initially simulated, which subsequently allowed the OAR curve to be simulated and the results were compared with experiment. To analyze the energy distribution of particles in different positions, electron spectroscopy was simulated together with the photon spectrum at the position from the central axis to a distance of 14.5 cm away from the center. Finally, three metal (Al, Cu, Sn) caps were made for the OFXS to prevent the low-energy photons penetrating the OFXS, and the OARs measurements were repeated. The results show that the scintillator exhibits higher sensitivity to photons with energy below 0.5 MeV, while for electrons, the scintillator has a higher sensitivity to high-energy electrons. Simulations for electron spectroscopy and the photon spectrum show that there are many low-energy photons with relatively few low-energy electrons. The OARs measured using the OFXS with metal caps show that the over-response can be mitigated using a high-Z metal cap. The measurements demonstrate that the OAR cure measured using an OFXS fitted with a Sn cap exhibits the closest response to that measured using an IC.

光纤X射线传感器具有实现精准放疗实时剂量监测的潜力。然而，当使用填充无机闪烁体（Gd ₂ O ₂ S:Tb）的光纤传感器（OFXS）测量离轴比（OAR）曲线时，可能会出现过度响应现象。本文的目的是研究过度反应的机制。由于水体模中存在复杂的粒子分布，基于蒙特卡罗的代码 GEANT4 用于模拟闪烁体的响应。最初模拟了闪烁体对光子和电子的能量响应，随后模拟了 OAR 曲线，并将结果与​​实验进行了比较。为了分析粒子在不同位置的能量分布，对从中心轴到距中心14.5 cm位置处的电子能谱和光子能谱进行了模拟。最后，为 OFXS 制作了三个金属（Al、Cu、Sn）帽，以防止低能光子穿透 OFXS，并重复 OAR 测量。结果表明，闪烁体对能量低于0.5 MeV的光子表现出较高的灵敏度，而对于电子，闪烁体对高能电子表现出较高的灵敏度。电子能谱和光子能谱的模拟表明，存在许多低能光子，而低能电子相对较少。使用带有金属帽的 OFX​​S 测量的 OAR 表明，使用高 Z 金属帽可以减轻过度响应。测量表明，使用装有 Sn 帽的 OFX​​S 测量的 OAR 固化表现出与使用 IC 测量的最接近的响应。