NCrystal: A library for thermal neutron transport

An open source software package for modelling thermal neutron transport is presented. The code facilitates Monte Carlo-based transport simulations and focuses in the initial release on interactions in both mosaic single crystals as well as polycrystalline materials and powders. Both coherent elastic (Bragg diffraction) and incoherent or inelastic (phonon) scattering are modelled, using basic parameters of the crystal unit cell as input.

Included is a data library of validated crystal definitions, standalone tools and interfaces for C++, C and Python programming languages. Interfaces for two popular simulation packages, Geant4 and McStas, are provided, enabling highly realistic simulations of typical components at neutron scattering instruments, including beam filters, monochromators, analysers, samples, and detectors. All interfaces are presented in detail, along with the end-user configuration procedure which is deliberately kept user-friendly and consistent across all interfaces.

An overview of the relevant neutron scattering theory is provided, and the physics modelling capabilities of the software are discussed. Particular attention is given here to the ability to load crystal structures and form factors from various sources of input, and the results are benchmarked and validated against experimental data and existing crystallographic software. Good agreements are observed.

本报告介绍了一款开源的软件包，旨在模拟热中子传输过程。该代码通过蒙特卡洛方法支持传输模拟，并在初始版本中聚焦于对马赛克单晶以及多晶材料和粉末的相互作用进行建模。软件模拟了相干弹性（布拉格衍射）以及非相干或非弹性（声子）散射，并以晶体单元细胞的基本参数作为输入。此外，软件包含了经过验证的晶体定义数据库、独立的工具以及针对 C++、C 和 Python 编程语言的接口。提供了两个流行的模拟软件包 Geant4 和 McStas 的接口，从而能够实现对中子散射仪器典型组件的高度逼真模拟，包括束过滤器、单色器、分析器、样品和探测器。所有接口均进行了详细阐述，并附带旨在保持用户友好性和接口之间一致性的最终用户配置程序。报告还提供了相关中子散射理论的概述，并讨论了软件的物理建模能力。特别关注了从各种输入源加载晶体结构和形貌因子的能力，并将结果与实验数据和现有的晶体学软件进行了基准测试和验证，结果显示出良好的吻合度。