McStas logo McStas - A neutron ray-trace simulation package ILL; PSI; ESS Niels Bohr Institute DTU Physics NEXMAP

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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


June 10th, 2015: McStas-2.2a supplemental ZIP archives for Mac OS X.

At least one user has experienced problems installing on OS X Yosemite from the tgz archive available here.

The problem was investigated and was caused by corruption during automatic 'double click' unarchiving of the tgz file. The solution was to unarchive using a tar command in the terminal:

tar xzf McStas-2.2a-10.10_Yosemite.tgz

If you experience this type of problem, either untar manually as shown above or try the ZIP version of the relevant archive - available here . Content wise these archives are exactly the same as the tgz version.


June 1st, 2015: Updated Ubuntu live-dvd with McStas available.

An updated iso image based on Ubuntu 14.04 and including McStas 2.2a, 2.1, 2.0 and 1.12c has been uploaded to our Ubuntu dvd archive. The iso also included the latest release of iFit and Mantid.


May 29th, 2015: Fresh built McStas 2.2a released

Dear all,

The bug reported earlier today related to the initialization of our random number generator was in fact not as severe as first anticipated:

Using a seed of 0 - which is the default in McStas and means using the clock as random number seed, instead implicitly indeed set the seed to 0 always. This meant that to generate two simulations with statistically different output, one would have had to set the seed explicitly to two different values in the two cases.

In any case, the bug has now been fixed in the newly released McStas 2.2a, which is available for all platforms at http://downloads.mcstas.org/mcstas-2.2a.

It further seems we have an issue with our "umbrella"-metapackage (called mcstas-suite) for both Debian and RPM systems. The solution is to explicitly install the mcstas-suite-perl and/or mcstas-suite-python packages.

Best regards,

Peter Willendrup


May 29th, 2015: Random number generator bug in McStas 2.2.

Dear all,

We have unfortunately discovered a serious bug in random number generation in the newly released McStas 2.2.

A patched replacement-release will be released either today or early next week. Sorry for the inconvenience this may have caused you.

Best regards,

Peter Willendrup


May 27th, 2015: McStas 2.2 released!

Here it is: McStas 2.2 is released and ready for download via http://downloads.mcstas.org/mcstas-2.2

Selected, important highligts from the release are:

  • General:
    • The new component/instrument keyword DEPENDENCY allows to automatically require / compile instruments with external libraries.
    • The Mantid-IDF generation through our Perl mcdisplay tool has been optimized.
    • Bugfix: The use of WHEN with EXTEND code only considered the first statement (report by M. Appel)
  • Infrastructure:
  • Components:
    • A new model for the ESS butterfly moderator - see documentation for the ESS_moderator.comp. Developed by Peter Willendrup in close collaboration with Troels Schoenfeldt DTU Nutech/ESS.
      ** A separate document with recommended settings for this component will be released later, also a version backported to McStas 1.12c. **
    • The Isotropic_Sqw component was broken in McStas 2.1. The expected behaviour has been restored. Users of this component should upgrade to McStas 2.2 asap. Sorry for the inconvenience.
  • Instruments:
    • ILL_IN13 backscattering instrument contributed by Emmanuel Farhi
    • ILL_IN4 tof instrument contributed by Emmanuel Farhi
  • Datafiles:
    • Improved water Sqw data files contributed by Emmanuel Farhi (- see also JNST 52 (2015) 844 )
    • Added CaF2 crystal, improved Al2O3 sapphire transmission
  • Tools:
    • McStas 2.2 is the first release to include a first version of our new mcgui-py tool, based on Python, QT and Scintilla. We think it is a very nice start at this long-awaited replacement of mcgui (Perl) and want you to try it out even though a few edges may still be rough here and there. For installation instructions, see the README's included with the installation packages.
    • McStas 2.2 further includes a simple mcplot plotter for Unix platforms (Linux, OS X) based solely on shell-script + Matlab/iFit/Octave, no need for Perl or Python with this plotter, called mcplot-matlab
Greetings from the McStas team - hope you will enjoy this new release! :-) - Full list of changes is available in the CHANGELOG

February 19th, 2015: Bugfixed-release of the ESS_moderator.comp

Jan Saroun from NPI in Czech Republic (and author of RESTRAX/SIMRES) has spotted an unfortunate error in the ESS_moderator.comp component, relating to the 2014 "pancake" case.

The bug is a missing distance-factor in a solid-angle computation and will lead to a too high brilliance emitted from the source.

Unfortunately was never spotted in our tests since these happen with an instrument where this distance is set to 1 m... (and 1 x 1 = 1... )

The updated source packge for use with McStas 1.12c and 2.x is available for download at http://mcstas.org/download/share/ESS_moderator_February_2015.tgz. A set of "release notes" is available at http://mcstas.org/download/share/ESS_moderator_February_2015.tgz.html.

Sorry for any inconvenience this may cause.


February 10th, 2015: Supplemental documentation for ESS moderators in McStas

A number of McStas users have expressed that the documentation for the ESS_moderator component distributed with McStas is not completely clear. To remedy this, Ken Andersen and I have (with the help of data from Troels Schönfeldt) produced a supplemental PDF document that will hopefully help.

Please note that at this point, only TDR-like and pancake moderators are described in McStas. An updated set of moderator descriptions will be released with McStas 2.2 during the spring of 2015.


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


Last Modified: Wednesday, 10-Jun-2015 21:01:01 CEST
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