[mcstas-users] Fwd: Neutron Beam tracking - MCSTAS

[Peter Kjær Willendrup]

Dear Maria,

Answers to the specifics below...

On 05 Feb 2014, at 09:50 , María Freiría López <m.freiria87 at gmail.com<mailto:m.freiria87 at gmail.com>> wrote:

The obtained results concerning neutron spectrum at the 2 stations differ from each other. Eventhough the neutrons in both tubes come from the cold source, based on the results, we think that the neutrons have a different nature, due to the heterogeneous properties along the cold source. A point to be taken into account is the fill level  of the cold source (D2 - liquid deuterium aprox. 25K). Some experts suggest that the major part of the neutron flux analyzed at SANS-1 comes from the lower part of the cold source and in the case of PGAA the neutrons have their origin in the upper part of the tank (near the fill level). This would explain the different results at the 2 stations:

<Experiment_Scheme_2.jpg>

In order to proof this theory, we started to work with MCSTAS.

I got the instruments and components defined in MCSTAS which describe the geometry of the tube structure. But, as a beginner, I need to ask for some help:

I need to know the origin of the neutrons which enter the tubes NL-4a and b. In other words: from which part of the cold source do most of the neutrons come in each case (NL-4a and NL-4b):

The most easy way to do this is using PreMonitor_nD and Monitor_nD:

1) Right after the source, put

COMPONENT SrcExit1 = PreMontior_nD(monitor_comp=AfterTube1)
AT (0,0,1e-6) RELATIVE Source

2) After your first pieces of neutron optics, put a corresponding Monitor_nD

COMPONENT AfterTube1 = Monitor_nD(options=“auto x, y, premonitor”, filename=“Aftertube1”)
AT (0,0,1) RELATIVE Optics

Perhaps also consult the more elaborate explanation of this available in the component manual.

<Experiment_Scheme_3.jpg>

Is it true that the neutron spectrum is different at the 2 experimental stations because the beam has its origin in different parts (the lower and the upper part) of the cold source? Or is the distribution almost homogeneous so that the beam properties in both tubes should be the same?

As a starting point, the source is assumed to have uniform brilliance across the surface - but such an effect could be modelled by putting several sources (with independent brilliance definition) and doing a Monte Carlo choice between them (e.g. via the WHEN keyword - see e.g. LL_H53_D16.instr in your installation.).

Further, the spectra after the neutron optics will of course be slightly different, as the source position (and therefore incoming divergence) may be slightly different in the two cases - which then through the reflectivity parametrisation of the two pieces of optics perform slightly different.

In order to answer these questions, the suggestion made is to implement a kind of "marker" at the beginning of SR-1 divided in "n" cells. This would make it possible to know which of these neutrons "make their way" to SANS-1 and which others to PGAA.

As I understand what you want to do, this is exactly what the premonitor will do for you. Alternatively you can add some extra c-code (EXTEND keyword) and add your own variables for monitoring the processes - also see LL_H53_D16.instr here)

Is it viable to create such a "marker"? Or do you have any other suggestions for solving this problem?

As I said, as a beginner I am not conscious of the possibilities that MCSTAS offers, that is why I was directed to you by Peter Link, Neutron Optics department at the FRM II.

I hope I could explain my problem in a more or less understandable way :)

I think so! :-)

Hope my tips help you - feel free to write back for further hints.

Best regards,

Peter Willendrup





Peter Kjær Willendrup
Senior Research Engineer, Special Consultant

DTU Physics



Technical University of Denmark



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