The NCrystal library provides a range of models for simulation of both elastic and inelastic scattering of thermal neutrons in a range of material structures. This article presents the available models for elastic scattering, and includes detailed discussion of their theoretical background, their implementation, and in particular their validation. The lineup includes a model for Bragg diffraction in crystal powders as well as one for incoherent elastic scattering, but the main focus is given to models of Bragg diffraction in ideally imperfect single crystals: both for the most widely applicable model of isotropic Gaussian mosaicity, and for a more specific model of layered single crystals which is relevant for materials such as pyrolytic graphite. Although these single crystal models are utilising computationally efficient approximations where appropriate, attention is given to the provision of precise and trustworthy results also for the extreme cases of back-scattering, forward-scattering, and crystals with very large mosaic spreads. Together with NCrystal's other features for crystal structure initialisation and inelastic physics, the presented models enable realistic modelling of components at neutron scattering instruments in frameworks like Geant4 and McStas, including monochromators, analysers, filters, support materials, shielding, and many kinds of samples. As a byproduct of the work, an improved formula for approximating cross sections in isotropic single crystals with Gaussian mosaicity is provided.

Program summary

Program Title: NCrystal

CPC Library link to program files: https://doi.org/10.17632/rzmmzgpg7g.1

Licensing provisions: Apache License, Version 2.0.

Programming language: C++, C and Python

Nature of problem: Thermal neutron transport in structured materials is inadequately supported in popular Monte Carlo transport applications, preventing simulations of a range of otherwise interesting setups.

Solution method: Provide models for thermal neutron transport in open source library NCrystal, to be used standalone or as backend in existing Monte Carlo packages. Specifically, the present publication details the implemented elastic scattering models in crystalline materials.

中文翻译：

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NCrystal 的弹性中子散射模型

所述NCrystal库提供了一系列模型的弹性和非弹性散射的模拟的范围内的材料结构的热中子。本文介绍了可用的弹性散射模型，并详细讨论了它们的理论背景、实现，尤其是它们的验证。阵容包括用于在晶体粉末布拉格衍射以及一个用于非相干弹性散射的模型，但主要焦点是考虑到布拉格衍射的型号在理想情况下不完美的单晶：适用于最广泛适用的各向同性高斯镶嵌模型，以及与热解石墨等材料相关的更具体的层状单晶模型。尽管这些单晶模型在适当的情况下使用了计算效率高的近似值，但也注意为反向散射、前向散射和具有非常大镶嵌分布的晶体的极端情况提供精确和可靠的结果。与NCrystal的其他晶体结构初始化和非弹性物理特性一起，所呈现的模型能够在Geant4和McStas等框架中对中子散射仪器的组件进行真实建模，包括单色仪、分析仪、过滤器、支撑材料、屏蔽和多种样品。作为这项工作的副产品，提供了一种用于近似具有高斯镶嵌性的各向同性单晶横截面的改进公式。

程序概要

程序名称： NCrystal

CPC 库程序文件链接： https : //doi.org/10.17632/rzmmzgpg7g.1

许可条款： Apache 许可，版本 2.0。

编程语言： C++、C和Python

问题的性质：结构材料中的热中子传输在流行的 Monte Carlo 传输应用中没有得到充分支持，从而阻止了对一系列其他有趣设置的模拟。

解决方法：在开源库NCrystal 中提供热中子传输模型，单独使用或作为现有 Monte Carlo 包的后端。具体而言，本出版物详细介绍了晶体材料中实施的弹性散射模型。