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.

本工作提出一款用于热中子输运（thermal neutron transport）建模的开源软件包。该软件支持基于蒙特卡洛（Monte Carlo）的输运模拟，初始版本聚焦于镶嵌单晶、多晶材料与粉体中的中子相互作用过程。软件以晶体晶胞（crystal unit cell）的基础参数为输入，可建模相干弹性（布拉格衍射，Bragg diffraction）散射、非相干散射及非弹性（声子，phonon）散射过程。
该软件内置经过验证的晶体定义数据库，同时提供适配C++、C及Python编程语言的独立工具与接口。此外还提供两款主流模拟工具包Geant4与McStas的接口，可实现中子散射装置典型组件的高逼真度模拟，涵盖束流滤波器、单色器、分析器、样品及探测器等。所有接口均配有详细说明文档，同时附带终端用户配置流程——该流程刻意设计为易于上手且在各接口间保持统一规范。
本文提供相关中子散射理论（neutron scattering theory）的概述，并对该软件的物理建模能力展开讨论。其中重点关注了从多源输入加载晶体结构（crystal structures）与形状因子（form factors）的能力；研究通过实验数据与现有晶体学软件对计算结果进行基准测试与验证，结果显示二者吻合度良好。