[mcstas-users] Absolute interaction position calculation

[Peter Kjær Willendrup]

Hi again Rob,

This one was also overlooked, sorry... :-(

On Tue Aug 12 10:40:26 CEST 2025 Robert Dalgliesh - STFC UKRI robert.dalgliesh at stfc.ac.uk wrote:

Is there a straightforward way to calculate the absolute interaction position of a neutron within a component after you have used PROP_Z0?


I am thinking you mean after SCATTER? (PROP_Z0 just puts everything at the z=0 plane of the component? Anyway, the procedure is about the same

I would like to know the interaction point in both local (component) coordinates and in the world system so I can then work out the interaction position relative to other components.
Having dug around in the documentation I couldn’t see anything obvious.

Yes, there is indeed:

1) Add USERVARS for measuring local and global coordinates to your instrument file:

USERVARS %{
  double xloc;
  double yloc;
  double zloc;
  double xglob;
  double yglob;
  double zglob;
%}

2) Add an EXTEND block on your sample comp:

COMPONENT Sample = Something(…)
AT (0,0,0) RELATIVE SamplePos
EXTEND %{
  // Measure neutron coordinates in sample frame
  // (can be made to depend on e.g. if (SCATTERED) )
  xloc=x;
  yloc=y;
  zloc=z;
%}

3) Add an Arm located at the Sample position, but ROTATED into the global frame:

COMPONENT SemiGlobal = Arm()
AT (0,0,0) RELATIVE SamplePos
ROTATED (0,0,0) ABSOLUTE
EXTEND %{
  // Take comp absolute comps...
  coords_get(POS_A_COMP_INDEX(INDEX_CURRENT_COMP), &xglob, &yglob, &zglob);
  // … and add the relative coordinates of the neutron in the semi-global frame
  xglob+=x;
  yglob+=y;
  zglob+=z;
%}

(There are surely other ways to do that calculation also, but this is the simplest one in my mind.)

4) Measure on the vars using Monitor_nD(user1=“xglob”…) or Flex_monitor_1/2/3D(user1=“xglob”…)

All the best and happy easter,
Peter


Peter Kjær Willendrup
Forskningsingeniør, Specialkonsulent

DTU Physics

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Technical University of Denmark


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Department of Physics
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