The experimental technique of Neutron Scattering is constantly evolving through advances in sources and instrumentation. The latest challenges and opportunities are posed by the European Spallation Source currently under construction in Lund, as both the source dimensions and time structure are far from conventional choices.

Simulation of neutron scattering instrumentation is used when designing instrumentation, but also to understand instrumental effects on the measured scattering data. The Monte Carlo ray-tracing package McStas is among the most popular, capable of simulating the path of each neutron through the instrument using an easy to learn language.

The subject of the defended thesis is contributions to the McStas language in the form of the software package guide_bot and the Union components.The guide_bot package simplifies the process of optimizing neutron guides by writing the McStas instrument file from limited user input. All optimized guides are thoroughly characterized allowing the user to select the most appropriate solution for a specific instrument. The software has been used extensively in connection with ESS, as several guides for instruments accepted for construction have been designed using the software.

The Union components uses a new approach to simulation of samples in McStas. The properties of a sample are split into geometrical and material, simplifying user input, and allowing the construction of complicated geometries such as sample environments. Multiple scattering is simulated between all geometries, providing a rich simulation of background including spurious signals. The MARI (ISIS, UK) powder spectrometer and MACS (NIST, USA) triple axis spectrometer are simulated, and recent measurements are recreated demonstrating the increased fidelity obtainable.