# NCrystal : a library for thermal neutron transport

## TL;DR

NCrystal is an open source library that enables detailed Monte Carlo simulations of thermal neutron transport in crystals and powders, supporting multiple programming interfaces and validated against experimental data.

## Contribution

It introduces a comprehensive, validated software package with interfaces for popular simulation tools, facilitating realistic neutron scattering simulations in various materials.

## Key findings

- Validated against experimental data with good agreement
- Supports multiple programming languages and simulation packages
- Models both coherent and incoherent neutron scattering processes

## Abstract

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.