[mcstas-users] Monitor_nD question (V3.5.23)

[Peter Kjær Willendrup]

Hi Uli,

Since McStas 3, Monitor_nD measures user variables via the so-called particle USERVARS, that become “part of the particle”.
(This consequently needs better documentation it seems, sorry about that!)

In your instrument file, add a USERVARS block below DECLARE and before INITIALIZE, something like this:

INSTRUMENT my_instrument()

DECLARE %{
%}

USERVARS %{
int xtalindex; // A double is of course also OK
%}

INITIALIZE %{
%}

Best and hope this helps?

Peter


On 24 Mar 2025, at 21.24, Wildgruber, Christoph U. via mcstas-users <mcstas-users at mcstas.org> wrote:

Hi,

I am trying to follow the recipe in the Monitor_nD component description.
The purpose is to track the value of ‘xtalindex’ together with the particle index ‘n’ on an event basis
to evaluate our multi-crystal analyzer setup:


How to monitor any instrument/component variable into a Monitor_nD
Suppose you want to monitor a variable 'age' which you assign somwhere in
the instrument:
COMPONENT MyMonitor = Monitor_nD(
xwidth = 0.1, yheight = 0.1,
user1="age", username1="Age of the Captain [years]",
options="user1, auto")
AT ...


to keep track of a variable I am setting with “EXTEND”.
…
COMPONENT Analyzer_14 = Monochromator_flat_CUW(zwidth = Alength, yheight = Awidth_14,
                                                                                                   mosaich = mos, mosaicv = mos,
                                                                                                   r0 = 0.99, DM = 3.355)
AT (0, 0, 0) RELATIVE Mono_Arm_14
ROTATED (0, analyzer_rot_14, 0) RELATIVE Mono_Arm_14
EXTEND
%{
     xtalindex = 14.0;
%}
…


…and use ‘xtalindex’ the following way:

COMPONENT Collimator_out = Monitor_nD(user1 = ”xtalindex”, username1 = “Crystal index”,
                                                                  options = "user1,n,x,y,z,vx,vy,vz,t,list all neutrons verbose",
                                                                   xmin = 0.0, xmax = 1.0, ymin = -0.2, ymax = 0.2,
                                                                   filename="collimator_out.evt", restore_neutron=1)
AT (0, 0, 0.2) RELATIVE Monitor_Arm

I do get an extra column in the output file but the value is always 0, regardless how I
define and initialize xtalindex. I must be doing something fundamentally wrong.

Any suggestions are greatly appreciated!

Thanks,

Uli

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Peter Kjær Willendrup
Forskningsingeniør, Specialkonsulent

DTU Physics

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Technical University of Denmark


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Department of Physics
Fysikvej
Building 307
DK-2800 Kongens Lyngby

Main office at
ESS DMSC
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Direct +45 2125 4612
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