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McStas - A neutron ray-trace simulation package

McStas is a general tool for simulating neutron scattering instruments and experiments. It is actively supported by DTU Physics, NBI KU, ESS, PSI and ILL

Simulated scattering from a hollow-cylinder vanadium sample.

The plot shows the intensity of scattered neutrons (red is highest intensity). The sample is at the center of the sphere with the neutron beam coming from the left. Clearly seen is the shadowing effect of the sample causing a lower intensity opposite the beam. Also seen is the effect of the non-symmetric geometry of the sample, causing lower intensity directly above and to the side of the sample.

Recent news

October 6th, 2016: Use Microsofts "Windows subsystem for Linux" for running McStas under Windows 10!

Partnering with Ubuntu Linux, Microsoft has released the (experimental) feature "Windows subsystem for Linux" (WSL), also known as "Bash on Ubuntu on Windows". Read more here and here.

It turns out that it is relatively straightforward to get McStas running under this new feature - thereby overcoming some of the annoyances of McStas on the Windows platform.

To try it out, please follow the below steps

  1. Install a copy of Windows 10 64 bit
  2. Update it beyond "Windows 10 Version 1607" - aka the "anniversary build"
  3. Enable "developer mode" in settings
  4. Use "turn features on or off" to enable "Bash in windows"
  5. Start bash via the start icon / cmd+esc
  6. Install McStas as usual on a Debian systems. Also remember that you can easily install openmpi etc.
  7. Install and start the Xming X-server
  8. Turn back to your bash and execute
    • export DISPLAY=localhost:0.0
    • mcgui & or mcgui-py & (or any of the usual McStas commands)

As the feature is considered experimental, you might still find some rough edges, but so far it seems functional and looks very promising!

A few extra tips:

  • An easy way to access your data simulated under Linux is to run from within your Windows home folder, accessible under Linux at /mnt/c/Users/<your_username>.
  • Add Xming to your windows Startup items via "windows+r" key and shell:startup
  • Add the line export DISPLAY=localhost:0.0 to your Linux .bashrc file

September 21st, 2016: McStas 2.3 on macOS Sierra

If you want to install McStas on the new macOS Sierra, here is a recipe:

  1. Install xcode commandline tools by issuing the command xcode-select --install in a terminal
  2. Install Xquartz from
  3. In your System Preferences, Security & Privacy tab, select to allow installation from Mac App Store and identified developers
  4. Download McStas 2.3 labled for use on El Capitan from and unpack. Open the resulting McStas-2.3-10.11_El_Capitan folder.
  5. Right-click/Ctrl-click McStas-2.3-10.11_El_Capitan.pkg packge and choose Open. (Allow installation when prompted to install package form unidentified developer.)
  6. If you want to use our Python tools
    • Download and install Anaconda Python 2.7 from Contiuum
    • In a terminal, go to your Anaconda binary folder $HOME/anaconda/bin and issue the commands
      • ln -s qmake-qt4 qmake
      • ln -s moc-qt4 moc
    • Relocate your McStas-2.3-10.11_El_Capitan folder where it was downloaded/unpacked and open the extras
    • Right-click/Ctrl-click the BuildMe.command script and choose Open. (Allow execution when prompted.)
    • Follow instructions in the terminal that opens.
If you upgraded from an earlier Mac OS X release, your installed McStas 2.3 is expected to "survive" that upgrade.

Please report any issue you may find to

September 15th, 2016: Updated component library for McStas 2.3 released, including ESS_butterfly.comp

A updated component library has been pushed to the deb and rpm repositories, plus to the relevant platform folders at If you are on Windows or Mac OS, please simply locate the

folder in the download site, find the relevant mcstas-comps-2.3 package and install it on top of your existing McStas 2.3.

Highlights are

  • An new implementation of the ESS moderator component called
    and test instrument
    The moderator is of course also available as a "DIY update archive" for people running older versions of McStas, and can be downloaded from a new update folder at the download site or from the share. For more information on the updated ESS moderator, its use and benchmark data, please consult The website also contain (currently draft) documents written with Ken Andersen and Luca Zanini, explaining the reasoning behind changing from the BF2 design to the BF1 design, neutronical calculations and benchmarks. Please note that these are still in DRAFT status, are expected to be in their final shape around 2016-09-22.
  • Bug-fixed MCPL i/o components and library - see for more details.
  • ... plus other minor things, see the GitHub log for mcstas-comps for more information.

Development is also moving fast on our tools, so we are further planning to release a McStas 2.4 within forseeable future.

Enjoy! :)
Peter Willendrup

April 6th, 2016: McStas 2.3 released!

Here it is: McStas 2.3 is released and ready for download via

Selected, important highligts from the release are listed below. The full list of changes is also available HERE.

  • General:
    • As of McStas 2.3, we support a new binary event file format: "MCPL" (Monte Carlo Particle List), developed by Thomas Kittelmann, ESS. The format aims to allow easier interchange of particle event data between monte carlo codes, and read/write functionality has been implemented for MCNP, Geant4 and McStas.
    • Interoperability with Mantid has been improved, as OFF-based detectors can be used to define detector geometries in Mantid-oriented simulation runs. See for more documentation on the use.
  • Infrastructure:
    • To build McStas from GitHub, you will now need a CMake version >= 3.0
    • At the ILL, Emmanuel Farhi has set up a nice web-service called cif2hkl, which allows to generate Laz/Lau structure data files from CIF files. See
    • A new e-Learning infrastructure for neutron scattering has been set up at . Among other things, it features a web-simulator powered by McStas. Once you have signed up, you will have access to many interesting things, among these this "close to complete" list of McStas instruments that can be run on the web:
  • Components:
    • New component SASview_model.comp by Jakob Garde and Torben Nielsen allows to use SASview scattering kernels from within McStas. See also the test instrument templateSasView.instr
    • New components MCPL_input and MCPL_output by Erik Knudsen. Used to read and write MCPL binary event files. See corresponding test instruments below.
    • New component Refractor.comp by Emmanuel Farhi and Bob Cubitt, ILL. Implements a single bulk material shape that can be used as a prism or lens.
    • PerfectCrystal.comp by Markus Appel, ILL / FAU Erlangen-Nuernberg. Perfect crystal component, primarily for use as monochromator and analyzer in backscattering spectrometers. Reflections are simulated using a Darwin, Ewald or Gaussian profile. Doppler movement of the monochromator is supported.
    • MultiDiskChopper.comp by Markus Appel, ILL / FAU Erlangen-Nuernberg. Models a disk chopper with a freely configurable slit pattern. For simple applications, use the DiskChopper component instead.
    • Updated version of Sample_nxs from Mirko Boin (HZB), including more nxs material definitions.
    • Bugfix to ESS_moderator from Jan Saroun (RESTRAX/Simres): missing focusing/direction cosine inserted. Please note that this component does not implement "engineering reality" and currently uses a coordinate system centered on the moderator assembly. An updated moderator component which references the "Moderator focus coordinate system" will be released later during the spring of 2016.
  • Instruments:
    • New test instrument templateSasView.instr, documenting use of SASview_model.comp
    • New test instruments Test_MCPL_input.instr and Test_MCPL_output.instr, documenting use of the MCPL components
    • New test instrument Test_PowderN_Res.instr, demonstrates difference of using 'banana, theta' and 'banana, divergence' in Monitor_nD - and can be used to show influence of sample size on detected line with in diffractometers.
    • New test instrument LLB_6T2.instr by Xavier Fabreges, LLB. Model of the LLB 6T2 thermal single crystal diffractometer.
    • New ILL_IN13.instr by E. Farhi and A. Dennison, ILL. Models the IN13 Thermal neutron backscattering spectrometer.
    • New SAFARI_MPISI.instr by Deon Marais, Necsa. Models the SAFARI instrument 'Materials Probe for Internal Strain Investigations'
    • New SAFARI_PITSI.instr by Deon Marais, Necsa. Models the SAFARI instrument 'Powder Instrument for Transition in Structure Investigations'
IMPORTANT: Please ensure to use our "migration scripts" for McStas 2.0 and 1.12c if you want these to co-exist with 2.3. See

March 7th, 2016: Most mcstas-2.2a instruments available at

If you take a look at this page, you will see that it resembles the examples part of a McDoc documentation page - with one exception: The Run simulation links on the right side of the table points to virtual instruments that may be run through the new McStas web frontend.

As usual, you must have an account at to execute these simulations.

March 3rd, 2016: Announcement of, e-learning platform for neutron scattering

Dear all,

We are happy to officially announce the availability of An e-learning platform for neutron scattering.

The platform was developed with support from NMI3-II (EU-FP7) and features an introductory course in neutron scattering and muon spin spectroscopy. The courses contain a varied selection of interactive learning material using three main technologies

  1. A "WIKIbook" built on Mediawiki with various extensions, e.g. for producing mathematical expressions. It is used as the main source of textbook-material
  2. A Learning Management System (LMS) built on Moodle which , provides exercises, quizzes and evaluation
  3. A web simulator for the McStas neutron instrument simulation program, allowing users to perform virtual neutron scattering experiments on predefined instruments
The platform will be further developed and the learning material expanded in connection with SINE2020 (EU-H2020).

To access the e-learning portal, go to, choose "Get an account" and fill in your personal information.

We hope that you will enjoy this new resource and encourage you to try it out!

On behalf of the team,

Peter Willendrup

For background information on the platform, the teaching material and didactical considerations, please read the related article from Neutron News Volume 24, Issue 1, 2013 - DOI:10.1080/10448632.2013.751795.

- - - - - - -


The main funding for the project is provided by the European Union's 7th Framework Programme for research, technological development and demonstration under the NMI3-II Grant number 283883 and from the European Union's Horizon 2020 research and innovation programme under grant agreement No 654000.

We are also grateful for the financial support recieved from the University of Copenhagen via the "Good Education" and "Education 2016" programmes and from Interreg IV via the OMIC programme.


The people behind the infrastructure are

Project leader: Linda Udby, Associate Professor, Niels Bohr Institute, University of Copenhagen

Platform development leader: Peter Willendrup, Senior Research Engineer, Technical University of Denmark

Course content and didactical developers: Julie Hougaard, PhD student, Niels Bohr Institute, University of Copenhagen & Jesper Bruun, Assistant Professor, Department of Science Education, University of Copenhagen

Content management and development: Pia J. Ray, PhD student & Linda Udby, Associate Professor, Niels Bohr Institute, University of Copenhagen

Software engineer: Jakob Garde, Software developer, Technical University of Denmark

Running server administrator: Jörg Pulz, Technical University of Munich

Learning material:

Contents was contributed by

University of Copenhagen: Kim Lefmann, Linda Udby, Pia Jensen Ray, Julie Hougaard, Lise Arleth, Kell Mortensen, Bente Lebech, Jesper Bruun

Institut Laue-Langevin, ILL: Andrew Wildes, Alain Filhol, Helmut Schober

Technische Universität München: Jörg Pulz, Jürgen Neuhaus

European Spallation Source, ESS: Markus Strobl

February 16th, 2016: Patch for Tk::wrap errors from mcgui

If you ever encounter an error like
Tk::Error###: This shouldn't happen at /usr/share/perl/5.18/Text/ line 84.
 Tk callback for .toplevel
 Text::Wrap::wrap at /usr/share/perl/5.18/Text/ line 84
 main::run_dialog_create at /usr/local/bin/mcgui line 752
 main::run_dialog at /usr/local/bin/mcgui line 824
 main::my_system at /usr/local/bin/mcgui line 985
 main::menu_run_simulation at /usr/local/bin/mcgui line 1320
 main::__ANON__ at /usr/local/bin/mcgui line 1752
 Tk callback for .frame1.button
 Tk::__ANON__ at /usr/lib/perl5/ line 251
 Tk::Button::butUp at /usr/lib/perl5/Tk/ line 175
 (command bound to event)
- then the fix is to replace run_dialog_create in your local mcgui by follwing the instructions described here.

Previous news items: 2015, 2014, 2013, 2012, 2011, 2010, 2009, 2008, 2007, 2006, 2005, 2004, 2003,2002, 2001, 2000, 1999, 1998.

Last Modified: Friday, 07-Oct-2016 11:38:02 CEST
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