Roncali Emilie, Stockhoff Mariele, Cherry Simon R
Department of Biomedical Engineering, University of California Davis, Davis, CA, United States of America.
Phys Med Biol. 2017 Jun 21;62(12):4811-4830. doi: 10.1088/1361-6560/aa6ca5. Epub 2017 Apr 11.
Accurately modeling the light transport in scintillation detectors is essential to design new detectors for nuclear medicine or high energy physics. Optical models implemented in software such as Geant4 and GATE suffer from important limitations that we addressed by implementing a new approach in which the crystal reflectance was computed from 3D surface measurements. The reflectance was saved in a look-up-table (LUT) then used in Monte Carlo simulation to determine the fate of optical photons. Our previous work using this approach demonstrated excellent agreement with experimental characterization of crystal light output in a limited configuration, i.e. when using no reflector. As scintillators are generally encapsulated in a reflector, it is essential to include the crystal-reflector interface in the LUT. Here we develop a new LUT computation and apply it to several reflector types. A second LUT that contains transmittance data is also saved to enable modeling of optical crosstalk. LUTs have been computed for rough and polished crystals coupled to a Lambertian (e.g. Teflon tape) or a specular reflector (e.g. ESR) using air or optical grease, and the light output was computed using a custom Monte Carlo code. 3 × 3 × 20 mm lutetium oxyorthosilicate crystals were prepared using these combinations, and the light output was measured experimentally at different irradiation depths. For all reflector and surface finish combinations, the measured and simulated light output showed very good agreement. The behavior of optical photons at the interface crystal-reflector was studied using these simulations, and results highlighted the large difference in optical properties between rough and polished crystals, and Lambertian and specular reflectors. These simulations also showed how the travel path of individual scintillation photons was affected by the reflector and surface finish. The ultimate goal of this work is to implement this model in Geant4 and GATE, and provide a database of scintillators combined with a variety of reflectors.
准确模拟闪烁探测器中的光传输对于设计用于核医学或高能物理的新型探测器至关重要。在诸如Geant4和GATE等软件中实现的光学模型存在重要局限性，我们通过实施一种新方法来解决这些局限性，即从3D表面测量计算晶体反射率。反射率保存在查找表（LUT）中，然后用于蒙特卡罗模拟以确定光学光子的命运。我们之前使用这种方法的工作在有限配置下（即不使用反射器时）与晶体光输出的实验表征显示出极好的一致性。由于闪烁体通常封装在反射器中，因此在LUT中包含晶体 - 反射器界面至关重要。在这里，我们开发了一种新的LUT计算方法并将其应用于几种反射器类型。还保存了一个包含透射率数据的第二个LUT，以实现光学串扰的建模。已经针对使用空气或光学油脂耦合到朗伯反射器（例如特氟龙胶带）或镜面反射器（例如ESR）的粗糙和抛光晶体计算了LUT，并使用自定义蒙特卡罗代码计算了光输出。使用这些组合制备了3×3×20 mm的正硅酸镥晶体，并在不同照射深度下实验测量了光输出。对于所有反射器和表面光洁度组合，测量和模拟的光输出显示出非常好的一致性。使用这些模拟研究了光学光子在晶体 - 反射器界面处的行为，结果突出了粗糙和抛光晶体以及朗伯反射器和镜面反射器之间光学特性的巨大差异。这些模拟还展示了单个闪烁光子的传播路径如何受到反射器和表面光洁度的影响。这项工作的最终目标是在Geant4和GATE中实现此模型，并提供结合各种反射器的闪烁体数据库。
