[mcstas-users] A few questions regarding mcstas

[Peter Kjær Willendrup]

Dear Daniel,


On 22 Aug 2017, at 21:56 , sdannyp at campus.technion.ac.il<mailto:sdannyp at campus.technion.ac.il> wrote:


I am new to Mcstas, and I am trying to simulate a double axis diffractometer. I am running into a few difficulties with the simulation:

1) I want to define a group of collimators and monitor detectors that are positioned radially around the sample, each having it's own scanning angle. This results in only the first collimator interacting with the neutron ray, as any neutron that misses gets ABSORB'ed. How do I circumvent this?

From what I understand you are trying to achieve, you should probably add two GROUP’s of components (see the manual around section 4.4 and forward), something like:

COMPONENT Colli1 = Collimator(…)
AT (…)
GROUP Collimators

COMPONENT Colli2 = Collimator(…)
AT (…)
GROUP Collimators
.
.
.

COMPONENT Moni1 = PSD_monitor(…)
AT (…)
GROUP Collimators

COMPONENT Moni2 = PSD_monitor(…)
AT (…)
GROUP Collimators

COMPONENT Moni3 = PSD_monitor(…)
AT (…)
GROUP Collimators
.
.
.

COMPONENT MoniN = PSD_monitor(…)
AT (…)
GROUP Collimators

- Also have a look in the examples folder of your McStas installation, there is a number of instruments using the GROUP keyword for components working “in parallel”.

My reason to think that this will help is that GROUP more or less means if ( ABSORBed || ! SCATTERED ) try_the_next_comp;

An important detail is that the members of the GROUP are placed next to each other in the instr file, and one makes sure that all neutrons are eventually SCATTERED (meaning that it can in some cases be good to add an Arm() EXTEND’ed by a SCATTER; at the end of the group.


2) I would like to compare the results of the simulation with the theoretical Caglioti resolution function, that depends on the divergence angles of the collimators and the mosaic spread. As I've noticed, Mcstas uses a triangular transmission function for the collimators and not a gaussian one. Will the result be different than the Caglioti curve, and is this approximation reasonable for real collimators?

In my understanding, a triangular transmission function is closer to measurement reality than the Caglioti gaussian. :-)

3) Are there any plans for a Mcstas workshop in the near future, and if so, where can I get more information?

Nothing planned as of yet, but I am sure there will be one during first half of 2018. Will keep you in the loop.


Best and good luck simulating,

Peter


Peter Kjær Willendrup
Senior Research Engineer, Special Advisor

DTU Physics

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Technical University of Denmark



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Department of Physics
Fysikvej

Building 307

DK-2800 Kongens Lyngby

Direct +45 2125 4612

Mobil +45 2125 4612
Fax +45 4593 2399


pkwi at fysik.dtu.dk<mailto:pkwi at fysik.dtu.dk>

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