4  The McStas kernel and meta-language

Instrument definitions are written in a special McStas meta-language which is translated automatically by the compiler mcstas into a C program which is in turn compiled to an executable that performs the simulation. The meta-language is custom-designed for neutron scattering and serves two main purposes: (i) to specify the interaction of a single neutron ray with a single optical component, and (ii) to build a simulation by constructing a complete instrument from individual components.

For maximum flexibility and efficiency, the meta-language is based on C. Instrument geometry, propagation of neutrons between the different components, parameters, data input/output etc. is handled in the meta-language and by the compiler mcstas. Complex calculations are written in C embedded in the meta-language description of the components. However, it is possible to set up an instrument from existing components and run a simulation without writing a single line of C code, working entirely in the meta-language.

Apart from the meta-language, McStas also includes a number of C library functions and definitions that are useful for neutron ray-tracing simulations. The definitions available for component developers are listed in appendix A.2. The list includes functions for

• Computing the intersection between a flight-path and various objects (such as planes, cylinders, boxes and spheres)

• Functions for generating random numbers with various distributions

• Functions for reading or writing information from/to data files

• Convenient conversion factors between relevant units, etc.

The McStas meta-language was designed to be readable, with a verbose syntax and explicit mentioning of otherwise implicit information. The recommended way to get started with the meta-language is to start by looking at the examples supplied with McStas, modifying them as necessary for the application at hand.