GUIMesh: A tool to import STEP geometries into Geant4 via GDML

Detailed radiation analysis of instruments flown in space is critical to ensure mission safety, often requiring the use of state-of-the-art particle transport simulation tools. Geant4 is one of the most powerful toolkits to simulate the interaction and the passage of particles through matter, but it is not prepared to receive Standard for The Exchange of Product data (STEP) files, the most versatile Computer-Aided Design (CAD) format, as input, requiring previous conversion to other CAD formats. This may lead to loss of detail and under or overestimation of the quantities under study, especially when the instruments have complex shapes, and/or a large number of volumes. Though several solutions have been proposed to import complex geometries from STEP files into Geant4, so far, only commercial options are available. In this paper we present a new tool, GUIMesh, that embeds FreeCAD libraries, an open-source CAD editor, to tessellate volumes, and convert them to Geometry Description Markup Language (GDML), a Geant4 readable format, in a straightforward way. Several degrees of freedom are given to the user regarding mesh precision and choice of material. Different geometries were tested for material definition, geometry and navigation errors, and the method used was successfully validated.

对太空飞行仪器开展详细辐射分析，是保障任务安全的关键环节，此类分析往往需要借助最前沿的粒子输运模拟工具。Geant4是模拟粒子与物质相互作用及粒子穿过物质过程的最强大工具包之一，但该工具暂不支持以产品数据交换标准（STEP）文件作为输入——STEP是通用性最强的计算机辅助设计（CAD）格式，因此需要先将其转换为其他CAD格式，这可能导致细节丢失，且对研究量的估算出现偏高或偏低的情况，尤其当仪器形状复杂、或包含大量体元时。尽管已有若干方案可将STEP格式的复杂几何模型导入Geant4，但截至目前仅商业方案可供选择。本文提出一款名为GUIMesh的全新工具，该工具集成了开源CAD编辑器FreeCAD的库函数，可对体元进行网格化处理，并以简便方式将其转换为Geant4可读的几何描述标记语言（GDML）格式。用户可在网格精度与材料选择方面获得多种自由度。我们针对材料定义、几何模型及导航误差测试了多款不同几何模型，并成功验证了所提方法的有效性。