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

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 A set of "release notes" is available at

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: Thursday, 19-Feb-2015 11:02:02 CET
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