[mcstas-users] Neutron propagation medium

[Mahmoud Yaseen Suaifan]

Hello Dr. Peter,

Sorry that i haven't done it from the beginning.
Please find attached a zip file contains the whole story. The main
confusing idea that i can't find reasonable justifications for results.

Thanks in advance.

Best regards,
Mahmoud


On Thu, Apr 19, 2018, 15:05 Peter Kjær Willendrup <pkwi at fysik.dtu.dk> wrote:

> 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>
> 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>
> 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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