11  Special-purpose components

The chapter deals with components that are not easily included in any of the other chapters because of their special nature, but which are still part of the McStas system.

One part of these components deals with splitting simulations into two (or more) stages. For example, a guide system is often not changed much, and a long simulation of neutron rays “surviving” through the guide system could be reused for several simulations of the instrument back-end, speeding up the simulations by (typically) one or two orders of magnitude. As of McStas 3.x, the recommended components for this are MCPL_input and MCPL_output, which store and read back neutron rays using the portable, compressed, multi-code MCPL event format (with backends for McStas/McXtrace as well as several other Monte Carlo transport codes – MCNP, PHITS, Geant4, Vitess, SIMRES). The older, code-specific event components (Virtual_input/Virtual_output and similar MCNP/Tripoli4/Vitess-specific readers/writers) perform the same role but are now in the obsolete component category (see the Obsolete chapter) and are kept only for backward compatibility; new instrument work should prefer MCPL.

Progress_bar is a simulation utility that displays the simulation status, but assumes the form of a component.

Components for simulating instrument resolution functions (Res_sample, TOF_Res_sample and Res_monitor) are documented in the Samples and Monitors chapters respectively, matching their current component-library category (they predate the present category split and were historically documented here).