University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital, Carl von Ossietzky University, Oldenburg, Germany.
Phys Med Biol. 2017 Dec 14;63(1):015001. doi: 10.1088/1361-6560/aa9b46.
The well-known field-size dependent overresponse in small-field photon-beam dosimetry of solid-state detectors equipped with very thin sensitive volumes, such as the PTW microDiamond, cannot be caused by the photon and electron interactions within these sensitive layers because they are only a few micrometers thick. The alternative explanation is that their overresponse is caused by the combination of two effects, the modification of the secondary electron fluence profile (i) by a field size too small to warrant lateral secondary electron equilibrium and (ii) by the density-dependent electron ranges in the structural detector materials placed in front of or backing the sensitive layer. The present study aims at the numerical demonstration and visualization of this combined mechanism. The lateral fluence profiles of the secondary electrons hitting a 1 µm thick scoring layer were Monte-Carlo simulated by modelling their generation and transport in the upstream or downstream adjacent layers of thickness 0.6 mm and densities from 0.0012 to 3 g cm, whose atomic composition was constantly kept water-like. The scoring layer/adjacent layer sandwich was placed in an infinite water phantom irradiated by circular Co, 6 MV and 15 MV photon beams with diameters from 3 to 40 mm. The interpretation starts from the ideal case of lateral secondary electron equilibrium, where the Fano theorem excludes any density effect. If the field size is then reduced, electron tracks potentially originating from source points outside the field border will then be numerically 'cut away'. This geometrical effect reduces the secondary electron fluence at the field center, but the magnitude of this reduction also varies with the density-dependent electron ranges in the adjacent layers. This combined mechanism, which strongly depends on the photon spectrum, explains the field size and material density effect on the response of detectors with very thin sensitive layers used in small-field photon-beam dosimetry.
著名的小束宽光子束辐照下固体探测器的响应过度问题，这种过度响应与探测器内非常薄的灵敏体积（如 PTW 微钻石探测器）中的光子和电子相互作用无关，因为这些灵敏体积只有几微米厚。另一种解释是，探测器的过度响应是由两种效应的结合引起的，即（i）由于束宽太小而不能保证侧向次级电子平衡，从而导致次级电子注量轮廓的改变，以及（ii）位于灵敏层前后的结构探测器材料中的电子射程随密度变化。本研究旨在对这种联合机制进行数值演示和可视化。通过模拟生成和传输位于厚度为 0.6mm 的上下游相邻层中的次级电子，对撞击 1μm 厚的评分层的次级电子的横向注量轮廓进行了 Monte-Carlo 模拟，这些相邻层的密度从 0.0012 到 3gcm-3 不等，其原子组成始终保持水相似。将评分层/相邻层夹层放置在无限大的水中体中，由直径为 3 到 40mm 的圆形 Co、6 MV 和 15 MV 光子束辐照。从侧向次级电子平衡的理想情况开始解释，在这种情况下，Fano 定理排除了任何密度效应。如果进一步减小束宽，那么源自场边界外源点的电子轨迹将在数值上“被切断”。这种几何效应会降低场中心的次级电子注量，但这种降低的幅度也随相邻层中电子射程的密度依赖性而变化。这种强烈依赖于光子谱的联合机制，解释了在小束宽光子束剂量测量中使用非常薄的灵敏层的探测器的响应与束宽和材料密度的关系。
Zotero 插件