[mcstas-users] Neutron propagation medium

[Mahmoud Yaseen Suaifan]

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> wrote:

> Dear Mahmoud,
>
> On 16 Apr 2018, at 06:05 , Mahmoud Yaseen Suaifan <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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