[mcstas-users] Neutron propagation medium

[Peter Kjær Willendrup]

Hello again Mahmoud,

Please send a copy of your instrument file along, otherwise I can do nothing but guesswork.

Best,

Peter


On 18 Apr 2018, at 02:51 , Mahmoud Yaseen Suaifan <mahma7 at gmail.com<mailto:mahma7 at gmail.com>> wrote:

Thank you Dr. peter.
Your replies are always appreciated & of course helpful.
Actually, i'm aware of the beam divergence, that's why i always use larger detector window than expected beam size. The confusing thing is that i'm following the same methodology while handling output flux (counts/sec normalized over beam area). After 2m simulation results matches calculation, but after 6m the simulation almost half of calc. Not that much complicated script. Straight forward (source + slit aperture + det) that's all. I guessed because of air ( as i know every 1 m of air reduces flux by 7% right?) But you confirmed it's vacuum. I'll think about it again.

Best regards,
Mahmoud

On Wed, Apr 18, 2018, 03:37 Peter Kjær Willendrup <pkwi at fysik.dtu.dk<mailto:pkwi at fysik.dtu.dk>> wrote:
Dear Mahmoud,

On 16 Apr 2018, at 06:05 , Mahmoud Yaseen Suaifan <mahma7 at gmail.com<mailto:mahma7 at gmail.com>> wrote:

I have simple question please: In McStas, what is the default propagation medium of neutrons?
And if such concept is there, how can we handle it (for exsmple, changing mediums like Vacuum, Air, or Helium ...etc ) to study effects or to get more accurate outputs? Since i noticed from some results that the intensity after few meters from aperture was reduced by some factor as if it is propagating in Air (Air scattering is affecting the results ?? ), ( my thought that the medium is vacuum). Shall i take into consideration some correction factor to modify my data?
Any help regarding this issue is appreciated.

In between the “components” in a McStas simulation, the neutrons are indeed propagated in vacuum.

Not having access to your instrument file, my best guess is that what you are observing is an effect of divergence in your beam.

(With a divergent beam and a fixed, limited monitor area being placed at increasing distance from an aperture, the highest divergence neutrons will eventually get lost due to distance collimation.)

If you on the other hand wanted to approximate effects of scattering or attenuation by e.g. air, I would suggest that you add a “sample component”, e.g. by means of the Incoherent (http://mcstas.org/download/components/samples/Incoherent.html) component - with your best estimate of the needed cross-sections in the medium.


Best and hope this helps,

Peter
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