二维电离室阵列探测器的室壁效应研究

Study on Chamber Wall Effect of 2D Ion-chamber Array Detector

由于二维电离室阵列探测器在剂量测量精度、实时性、可重复性上具有优势,其在放射治疗中的应用日益广泛。二维电离室阵列探测器由平行板电离室单元组成的二维平面阵列构成,阵列单元由充空气的腔室（空腔）及其室壁构成。根据空腔理论,空腔中的电离几乎全部由来自室壁的次级电子产生,因此研究室壁效应是提高二维电离室阵列探测器性能与优化其结构设计的重要环节。利用蒙特卡罗方法,分析研究了不同能量的入射光子在不同部位的室壁中产生的次级电子数量随室壁厚度的变化规律。结果表明,进入空腔的大部分次级电子由电离室前壁产生,且随入射光子能量增加,次级电子数达到最大所需的室壁厚度也增加;对于侧壁和后壁,大部分次级电子产生于空腔附近2～3mm厚的室壁中,且对入射光子能量不敏感。同时还研究了不同探测器结构对各探测单元之间信号串扰的影响。结果表明,串扰程度与侧壁厚度密切相关,且随入射光子能量的增加而增大,而与空腔尺寸的关系不大。研究结果对二维电离室阵列探测器的设计具有重要的指导价值。

The 2D ion-chamber array detector is widely used for dose measurement in radiotherapy due to its accuracy,instantaneity and repeatability.The 2Dion-chamber array detector is made of an array of small parallel-plate ion-chambers,which mainly comprise air cavity with wall surrounded.According to cavity theory on dose measurement,most ionization electrons in cavity come from the interaction of secondary electrons produced from chamber wall and air in cavity.Thus studying the effect of chamber wall in 2Dion-chamber array detector plays an important role in optimization design of2Dion-chamber array detector.The Monte Carlo simulation method was used to analyze the relationship between the wall thickness and the amount of secondary electronsproduced in different parts of chamber wall irradiated by incident photons of different energy and the influence of array structure on chamber-chamber crosstalk.It is shown that most secondary electrons are produced in the front wall whose thickness should be greater with photon energy increasing.It is also shown that the surface zone with thickness of only 2-3mm in the side and back walls make a contribution to the amount of secondary electrons entering into air cavity,which is not sensitive to the incident photon energy.And the chamber-chamber crosstalk is directly related to the side wall thickness and the photon energy rather than the air cavity size.The results are of significance to the development of new design criterions on 2D ion-chamber array detector.