The very early implementations of the method for neutron instruments used home-made computer programs (see e.g. papers by J.R.D. Copley, D.F.R. Mildner, J.M. Carpenter, J. Cook), more general packages have been designed, providing models for most parts of the simulations. These present existing packages are: NISP [See+00], ResTrax [SK97], McStas [Wil+14; LN99; WFL04; Wil+14; Mcs], Vitess [Wec+00; Vit], IDEAS [LW02] and IB (Instrument Builder) [Ibw]. Supplementing the Monte Carlo based methods, various analytic phase-space simulation methods exist, including Neutron Acceptance Diagram Shading (NADS) [Nad]. Their usage usually covers all types of neutron spectrometers, most of the time through a user-friendly graphical interface, without requiring programming skills.
The neutron ray-tracing Monte-Carlo method has been used widely for e.g. guide studies [Cop93; Far+02; Sch+04], instrument optimization and design [ZLa04; Lie05]. Most of the time, the conclusions and general behavior of such studies may be obtained using the classical analytic approaches, but accurate estimates for the flux, the resolutions, and generally the optimum parameter set, benefit advantageously from MC methods.
Recently, the concept of virtual experiments, i.e. full simulations of a complete neutron experiment, has been suggested as a major asset for neutron ray-tracing simulations. The goal is that simulations should be of benefit to not only instrument builders, but also to users for training, experiment planning, diagnostics, and data analysis.
In the late 90’ies at Risø National Laboratory, simulation tools were urgently needed, not only to better utilize existing instruments (e.g. RITA-1 and RITA-2 [Mas+95; Cla+98; Lef+00]), but also to plan completely new instruments for new sources (e.g. the Spallation Neutron Source, SNS [Sns] and the planned European Spallation Source, ESS [Ess]). Writing programs in C or FORTRAN for each of the different cases involves a huge effort, with debugging presenting particularly difficult problems. A higher level tool specially designed for simulating neutron instruments was needed. As there was no existing simulation software that would fulfill our needs, the McStas project was initiated. In addition, the ILL required an efficient and general simulation package in order to achieve renewal of its instruments and guides. A significant contribution to both the component library and the McStas kernel itself was early performed at the ILL and included in the package. ILL later became a part of the core McStas team. Similarly, the PSI has applied McStas extensively for instrument design and upgrades, provided important component additions and contributed several systematic comparative studies of the European instrument Monte Carlo codes. Hence, PSI has also become a part of the core McStas team. Since year 2001 Risø was no longer a neutron source, and the authors from that site have moved on to positions at University of Copenhagen (NBI) and Technical University of Denmark (DTU Physics), hence these two partners have joined the core McStas team. Finally, trough general emphasis on use of McStas as a tool for simulating the ESS instruments and virtual data, plus the partial secondment of one DTU-based McStas author, the ESS Data Management and Software Centre (ESS DMSC) is now contributing to the projecte.