Abstract: Many traditional Monte Carlo transport programs use the Constructed Solid Geometry (CSG)method for modeling, which is time-consuming and labor-intensive for complex geometries. Therefore, it is necessary to find a method that can simplify the neutron modeling process. This paper explores the key technologies for Monte Carlo transport directly on the CAD model, such as geometric data structure, particle-solid collision algorithms, and the search for the next entity algorithm. Based on these technologies, a preliminary usable direct-method transport program was developed. Moreover, the bounding box acceleration algorithm was introduced to speed up the computation process. The geometric algorithm was validated by a simple model and compared with cosRMC. The results show that the direct method program agrees well with the cosRMC results,which confirms the correctness of the geometric algorithm. The direct method also demonstrates its advantages of high accuracy, intuitive display, and wide applicability by calculating the neutron flux on the surface of the human body model.

Key words: Particle transport, Complex geometry, Direct CAD, Bounding box

摘要： 传统蒙特卡罗输运程序多采用构造实体几何(CSG)方法建模，在应对复杂几何时，建模过程耗费大量时间和人力，因此需要寻找一种能够简化中子学建模过程的方法。基于直接在 CAD 模型上进行蒙特卡罗输运思想，对几何数据结构、粒子-实体碰撞算法以及搜索下一个实体算法等关键技术进行了研究与设计。在此基础上，开发了一套初步可用的直接法输运程序，并引入了包围盒加速算法，运算速度大幅提升。通过简单模型对几何算法进行了初步验证，并使用 cosRMC 对相同模型进行对比计算。结果表明，直接法程序的计算结果与 cosRMC 计算结果吻合良好，验证了几何算法的正确性。通过对人体模型表面中子通量计算，表明直接法还具有结果精细、展示直观、适用性广等优势。

关键词: 粒子输运, 复杂几何, 直接 CAD 法, 包围盒