McStas

McStas is a widely used Monte Carlo ray-tracing simulation package specifically designed for neutron scattering instrumentation. It provides a virtual neutron source and allows users to simulate the trajectories of neutrons through a complex instrument design, taking into account various physical processes such as scattering, absorption, and refraction.

Developed primarily at the Technical University of Denmark (DTU), McStas allows scientists and engineers to model the performance of neutron scattering instruments before they are built, optimize existing instruments, and analyze experimental data. The software is open-source and freely available under a license that permits redistribution and modification.

The simulation process in McStas involves defining a virtual instrument consisting of interconnected components, each described by a specific component definition. These definitions specify the geometry, material properties, and scattering behavior of the component. Neutrons are then "launched" from the source and their paths are traced through the instrument. At each component, the neutron's fate is determined by a Monte Carlo sampling process, taking into account the probabilities of different interactions.

McStas provides a comprehensive library of pre-defined components representing common neutron optics elements, such as guides, monochromators, detectors, and samples. Users can also create custom components to model specific experimental setups or novel instrument designs. The output of a McStas simulation typically includes distributions of neutron intensity, wavelength, and scattering angle, which can be used to assess the instrument's performance and optimize its design.

The software supports different simulation modes, including steady-state simulations for determining instrument performance and time-of-flight simulations for modeling pulsed neutron sources. McStas is actively developed and maintained by an international community of users and developers.