[mcstas-users] Neutron propagation medium

[Kim Lefmann]

> Dear Mahmoud,
>
> I looked at your results. The simulations are correct, but your 
> calculations are not. The point is that the slit at 4.17 meter acts as 
> a virtual source. Hence, you need to use the 1/r^2 law from that point 
> on. This roughly agrees with the 3 simulation results being 1: 1/4: 1/9.
>
> best, Kim
>
>
> On 04/19/2018 09:57 AM, Mahmoud Yaseen Suaifan wrote:
>> 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 
>> <mailto: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 <mailto: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 <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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>>
>>
>>
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