<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
</head>
<body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">
Hello again Mahmoud,
<div class=""><br class="">
</div>
<div class="">Please send a copy of your instrument file along, otherwise I can do nothing but guesswork. </div>
<div class=""><br class="">
</div>
<div class="">Best,</div>
<div class=""><br class="">
</div>
<div class="">Peter</div>
<div class=""><br class="">
<div><br class="">
<blockquote type="cite" class="">
<div class="">On 18 Apr 2018, at 02:51 , Mahmoud Yaseen Suaifan <<a href="mailto:mahma7@gmail.com" class="">mahma7@gmail.com</a>> wrote:</div>
<br class="Apple-interchange-newline">
<div class="">
<div dir="auto" class="">Thank you Dr. peter.
<div dir="auto" class="">Your replies are always appreciated & of course helpful.</div>
<div dir="auto" class="">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.</div>
<div dir="auto" class=""><br class="">
</div>
<div dir="auto" class="">Best regards,</div>
<div dir="auto" class="">Mahmoud</div>
</div>
<br class="">
<div class="gmail_quote">
<div dir="ltr" class="">On Wed, Apr 18, 2018, 03:37 Peter Kjær Willendrup <<a href="mailto:pkwi@fysik.dtu.dk" class="">pkwi@fysik.dtu.dk</a>> wrote:<br class="">
</div>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div style="word-wrap:break-word;line-break:after-white-space" class="">Dear Mahmoud,
<div class=""><br class="">
<div class="">
<blockquote type="cite" class="">
<div class="">On 16 Apr 2018, at 06:05 , Mahmoud Yaseen Suaifan <<a href="mailto:mahma7@gmail.com" target="_blank" rel="noreferrer" class="">mahma7@gmail.com</a>> wrote:</div>
<br class="m_2944751217251282736Apple-interchange-newline">
<div class="">
<div dir="auto" class="">
<div dir="auto" class="">I have simple question please: In McStas, what is the default propagation medium of neutrons? </div>
<div dir="auto" class="">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?</div>
<div dir="auto" class="">A<span style="font-family:sans-serif" class="">ny help regarding this issue is appreciated.</span><span style="font-family:sans-serif" class=""> </span></div>
</div>
</div>
</blockquote>
<br class="">
</div>
<div class="">
<div class="">In between the “components” in a McStas simulation, the neutrons are indeed propagated in vacuum. </div>
<div class=""><br class="">
</div>
</div>
<div class="">Not having access to your instrument file, my best guess is that what you are observing is an effect of divergence in your beam. </div>
<div class=""><br class="">
</div>
<div class="">(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.)</div>
<div class=""><br class="">
</div>
<div class="">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 (<a href="http://mcstas.org/download/components/samples/Incoherent.html" target="_blank" rel="noreferrer" class="">http://mcstas.org/download/components/samples/Incoherent.html</a>)
component - with your best estimate of the needed cross-sections in the medium.</div>
<div class=""><br class="">
</div>
<div class=""><br class="">
</div>
<div class="">Best and hope this helps,</div>
</div>
<div class=""><br class="">
</div>
<div class="">Peter</div>
</div>
</blockquote>
</div>
_______________________________________________<br class="">
mcstas-users mailing list<br class="">
<a href="mailto:mcstas-users@mcstas.org" class="">mcstas-users@mcstas.org</a><br class="">
https://mailman2.mcstas.org/mailman/listinfo/mcstas-users<br class="">
</div>
</blockquote>
</div>
<br class="">
</div>
</body>
</html>