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Hi All,<br>
<br>
If the bend is in the rectangular portion, it can be pretty small
(about 5° assuming 25 cm to shield)<br>
<br>
Tim<br>
<br>
Beni Zihlmann wrote:
<blockquote cite="mid4D8D0C34.6040401@jlab.org" type="cite">Hi Sasha,
<br>
I modified the code so that I can change the geometry on the fly and I
can
<br>
modify the lengths for each section and where the bend should be either
in
<br>
the rectangular section before the tapering or in the round section
after the
<br>
tapering.
<br>
Attached is a file containing the light loss calculations with the
dimensions you
<br>
gave me. You did not quote a value for the bend so I chose a bend of 10
degree.
<br>
You can see that there is an optimal length for the tapered section. I
get the
<br>
best results for a tapered section with a length of about 15 to 20 cm.
<br>
<br>
cheers,
<br>
Beni
<br>
<blockquote type="cite">Beni,
<br>
<br>
Thanks for clarifying. I didn't realize at first that the listing
<br>
of photon loss percentage in your table on wiki starts
<br>
with the second light guide section, not the first one. My fault.
<br>
<br>
So, can I conclude from your results that, from the point
<br>
of view of light loss, the design with smaller bend and longer
<br>
tapered section is just a little bit (24.5% vs 28%) but better
<br>
nevertheless than the one with larger bend and shorter section?
<br>
<br>
Also, would it be possible to simulate the light guides which we
<br>
already have: 12cm rectangular, 7cm tapered, 10cm round
<br>
sections, with a bend in the rectangular section? I guess
<br>
a portion of a cylinder volume should describe such bend
<br>
section well enough.
<br>
<br>
Sasha
<br>
<br>
<br>
On Wednesday, March 23, 2011, Beni Zihlmann wrote:
<br>
<blockquote type="cite">Hi Alexander,
<br>
I think you miss interpreted which numbers belong to which section.
<br>
the first number in the list belongs to the beginning of the first
<br>
strait rectangular section which is 3cm long. This number is about 500.
<br>
I generated 1000 photons in the scintillator about 60cm from this point
<br>
where the photons were generated on a strait path at random positions
<br>
along the width of the paddle and randomly in 2pi (2pi== theta 0 to 90
<br>
degree
<br>
and phi 0 to 360 degree).
<br>
The second number which is slightly lower than the first is the number
<br>
of photons after this 3cm strait section and is the number of photons
<br>
at the beginning of the tapered section. The third number is the
<br>
photons after the tapered section and at the beginning of the round
<br>
bending section. This is were about 15% of the photons got lost namely
<br>
in the tapered section.
<br>
<br>
I hope that explains your concerns.
<br>
<br>
cheers,
<br>
Beni
<br>
<br>
<blockquote type="cite">On Tuesday, March 22, 2011, Beni Zihlmann
wrote:
<br>
<blockquote type="cite">Hi All,
<br>
I have simulated a light guide using GEANT4 and looked at how much
<br>
light we lose.
<br>
You can find the results at the following link.
<br>
<a class="moz-txt-link-freetext" href="http://www.jlab.org/Hall-D/software/wiki/index.php/Light_Guide_Design">http://www.jlab.org/Hall-D/software/wiki/index.php/Light_Guide_Design</a>
<br>
<br>
Because it is very hard to model a bend in the tapered section where
<br>
by "tapered"
<br>
I mean the part that converts a rectangular shape into a round shape
<br>
I left this
<br>
part strait. I introduced a bend afterwards in the round section.
<br>
That is much easier
<br>
to model. It turns out that most light gets lost in the tapered
<br>
section anyway even
<br>
without a bend. Secondly as expected a larger bend causes more light
<br>
to be lost.
<br>
In general we can expect a light loss of about 25% to 30% through the
<br>
full light guide
<br>
including interfaces also to the PMT.
<br>
<br>
any thoughts and ideas are welcome,
<br>
</blockquote>
Beni,
<br>
<br>
An interesting result. What I'm puzzled about is that more than
<br>
half of all photons (15% out of 25%) are lost in the first very short
<br>
rectangular section of the light guide. Obviously, this is not due to
<br>
photon absorption in lucite because this section is only about
<br>
one-tenth of the total length. I doubt that this is due to
<br>
rectangular shape of this section either. TOF paddle has the same
<br>
shape and is about 100 times longer, which means that amount of light
<br>
from hits at the far end of TOF paddle would be equal to
<br>
(1-0.15)^100=nothing if such shape is a culprit.
<br>
<br>
I suspect that the reason for this large loss is 2pi solid angle of
<br>
photons generated near the light-guide entrance. A significant
<br>
fraction of these photons will have incident angle higher than
<br>
the angle of total internal reflection. Many of them will escape
<br>
when they hit light guide wall for the first time. That, of course,
<br>
is likely to happen in its first section for large-angle photons.
<br>
<br>
Loss of light in light guides is most important for the smallest
<br>
signals coming from hits at the far end of a TOF paddle. My
<br>
feeling is that mostly photons which are nearly parallel to
<br>
the paddle will reach the light guide at the opposite end.
<br>
So, my suggestion would be to run your Monte Carlo
<br>
with photons at 0 (or nearly 0) angle instead of 2pi soild angle
<br>
to see how the conclusion about light loss in the light guide
<br>
depends on the assumption about initial angular distribution
<br>
of the generated photons.
<br>
<br>
Sasha
<br>
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