[mcstas-users] Neutron propagation medium

[Mahmoud Yaseen Suaifan]

Sorry for all again.
I don't know why the attachment was not uploaded. Here you are again.
Thanks.

On Thu, Apr 19, 2018, 16:54 Mahmoud Yaseen Suaifan <mahma7 at gmail.com> wrote:

> 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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>>
>>
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