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<p>Dear Mahmoud,</p>
<p>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.<br>
</p>
<p>best, Kim<br>
</p>
<br>
<div class="moz-cite-prefix">On 04/19/2018 09:57 AM, Mahmoud
Yaseen Suaifan wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CAOkUS5F3UbZsmtfP-mDENsSN3xu3guPxRoQDL82EuHWjhrue7A@mail.gmail.com">
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charset=utf-8">
<div dir="auto">Sorry for all again.
<div dir="auto">I don't know why the attachment was not
uploaded. Here you are again.</div>
<div dir="auto">Thanks.</div>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr">On Thu, Apr 19, 2018, 16:54 Mahmoud Yaseen
Suaifan <<a href="mailto:mahma7@gmail.com"
moz-do-not-send="true">mahma7@gmail.com</a>> wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div dir="auto">Hello Dr. Peter,
<div dir="auto"><br>
<div dir="auto">Sorry that i haven't done it from the
beginning.</div>
<div dir="auto">Please find attached a zip file contains
the whole story. The main confusing idea that i can't
find reasonable justifications for results.</div>
<div dir="auto"><br>
</div>
<div dir="auto">Thanks in advance.</div>
<div dir="auto"><br>
</div>
<div dir="auto">Best regards,</div>
<div dir="auto">Mahmoud</div>
<div dir="auto"><br>
</div>
</div>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr">On Thu, Apr 19, 2018, 15:05 Peter Kjær
Willendrup <<a href="mailto:pkwi@fysik.dtu.dk"
target="_blank" rel="noreferrer"
moz-do-not-send="true">pkwi@fysik.dtu.dk</a>>
wrote:<br>
</div>
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.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div
style="word-wrap:break-word;line-break:after-white-space">
Hello again Mahmoud,
<div><br>
</div>
<div>Please send a copy of your instrument file along,
otherwise I can do nothing but guesswork. </div>
<div><br>
</div>
<div>Best,</div>
<div><br>
</div>
<div>Peter</div>
<div><br>
<div><br>
<blockquote type="cite">
<div>On 18 Apr 2018, at 02:51 , Mahmoud Yaseen
Suaifan <<a href="mailto:mahma7@gmail.com"
rel="noreferrer noreferrer" target="_blank"
moz-do-not-send="true">mahma7@gmail.com</a>>
wrote:</div>
<br
class="m_-6095297309211403739m_-7737197393323434482Apple-interchange-newline">
<div>
<div dir="auto">Thank you Dr. peter.
<div dir="auto">Your replies are always
appreciated & of course helpful.</div>
<div dir="auto">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"><br>
</div>
<div dir="auto">Best regards,</div>
<div dir="auto">Mahmoud</div>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr">On Wed, Apr 18, 2018, 03:37
Peter Kjær Willendrup <<a
href="mailto:pkwi@fysik.dtu.dk"
rel="noreferrer noreferrer"
target="_blank" moz-do-not-send="true">pkwi@fysik.dtu.dk</a>>
wrote:<br>
</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">Dear
Mahmoud,
<div><br>
<div>
<blockquote type="cite">
<div>On 16 Apr 2018, at 06:05 ,
Mahmoud Yaseen Suaifan <<a
href="mailto:mahma7@gmail.com"
rel="noreferrer noreferrer
noreferrer" target="_blank"
moz-do-not-send="true">mahma7@gmail.com</a>>
wrote:</div>
<br
class="m_-6095297309211403739m_-7737197393323434482m_2944751217251282736Apple-interchange-newline">
<div>
<div dir="auto">
<div dir="auto">I have simple
question please: In McStas,
what is the default
propagation medium of
neutrons? </div>
<div dir="auto">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">A<span
style="font-family:sans-serif">ny
help regarding this issue
is appreciated.</span><span
style="font-family:sans-serif"> </span></div>
</div>
</div>
</blockquote>
<br>
</div>
<div>
<div>In between the “components” in
a McStas simulation, the neutrons
are indeed propagated in vacuum. </div>
<div><br>
</div>
</div>
<div>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><br>
</div>
<div>(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><br>
</div>
<div>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"
rel="noreferrer noreferrer
noreferrer" target="_blank"
moz-do-not-send="true">http://mcstas.org/download/components/samples/Incoherent.html</a>)
component - with your best estimate
of the needed cross-sections in the
medium.</div>
<div><br>
</div>
<div><br>
</div>
<div>Best and hope this helps,</div>
</div>
<div><br>
</div>
<div>Peter</div>
</div>
</blockquote>
</div>
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