[mcstas-users] Action of a WHEN condition

Andrew Jackson andrew.jackson at esss.se
Tue Sep 5 14:20:24 CEST 2017

Dear Jörg

I would place the components downstream of the bispectral switch in relation to the moderator or an Arm starting there (which might have the correct rotation of the whole beamline wrt the moderator) rather than the previous component - in reality they will be fixed, so you are then just inserting or not the switch (which has a rotation of its own). I do something similar to enable different collimation lengths in a SANS instrument - I place each guide piece with respect to an Arm that is at the beginning of the collimation rather than with respect to previous components, a combination of WHEN statements allows me to select which pieces are in the beam.

This does mean you need to pay a bit more attention to lengths of components etc, but probably better matches reality.

Andrew Jackson

Instrument Scientist - Small Angle Scattering

Deputy Head of Neutron Instruments Division

European Spallation Source, P.O Box 176, SE-221 00 Lund, Sweden

Adjunct Associate Professor (Ajungerad Lektor)
Physical Chemistry, Lund University, P.O. Box 124, SE-221 00, Lund, Sweden

Mobile: +46 72 179 2015

E-mail: andrew.jackson at esss.se


From: mcstas-users [mcstas-users-bounces at mcstas.org] on behalf of "Voigt, Jörg" [j.voigt at fz-juelich.de]
Sent: Tuesday, September 05, 2017 2:06 PM
To: mcstas-users at mcstas.org
Subject: [mcstas-users] Action of a WHEN condition

Dear McStas users,

I want to model a bispectral switch, where a bender is used to reflect the cold neutrons into the neutron guide, while this bender can be removed to open the way for the thermal neutrons from the ESS butterfly moderator. As the bender deflects the beam, I need to rotate the respective component by the deflection angle, so I have a ROTATED statement. What happens to the coordinate system of the subsequent components, if I use a WHEN statement to ignore the bender in case I want to use neutrons only from the thermal moderator? Will it be rotated?

Best regards

Dr. Jörg Voigt
  Jülich Centre for Neutron Science JCNS und Peter Grünberg Institut PGI
  JCNS-2, PGI-4: Streumethoden
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j.voigt at fz-juelich.de

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