[Halld-pid] light guide simulation

[Alexander Ostrovidov]

Beni,

Thanks for clarifying. I didn't realize at first that the listing
of photon loss percentage in your table on wiki starts
with the second light guide section, not the first one. My fault.

So, can I conclude from your results that, from the point
of view of light loss, the design with smaller bend and longer
tapered section is just a little bit (24.5% vs 28%) but better 
nevertheless than the one with larger bend and shorter section?

Also, would it be possible to simulate the light guides which we 
already have: 12cm rectangular, 7cm tapered, 10cm round
sections, with a bend in the rectangular section? I guess 
a portion of a cylinder volume should describe such bend 
section well enough.

Sasha


On Wednesday, March 23, 2011, Beni Zihlmann wrote:
> Hi Alexander,
> I think you miss interpreted which numbers belong to which section.
> the first number in the list belongs to the beginning of the first
> strait rectangular section which is 3cm long. This number is about 500.
> I generated 1000 photons in the scintillator about 60cm from this point
> where the photons were generated on a strait path at random positions
> along the width of the paddle and randomly in 2pi (2pi== theta 0 to 90
> degree
> and phi 0 to 360 degree).
> The second number which is slightly lower than the first is the number
> of photons after this 3cm strait section and is the number of photons
> at the beginning of the tapered section. The third number is the
> photons after the tapered section and at the beginning of the round
> bending section. This is were about 15% of the photons got lost namely
> in the tapered section.
> 
> I hope that explains your concerns.
> 
> cheers,
> Beni
> 
> > On Tuesday, March 22, 2011, Beni Zihlmann wrote:
> >> Hi All,
> >> I have simulated a light guide using GEANT4 and looked at how much
> >> light we lose.
> >> You can find the results at the following link.
> >> http://www.jlab.org/Hall-D/software/wiki/index.php/Light_Guide_Design
> >> 
> >> Because it is very hard to model a bend in the tapered section where
> >> by "tapered"
> >> I mean the part that converts a rectangular shape into a round shape
> >> I left this
> >> part strait. I introduced a bend afterwards in the round section.
> >> That is much easier
> >> to model. It turns out that most light gets lost in the tapered
> >> section anyway even
> >> without a bend. Secondly as expected a larger bend causes more light
> >> to be lost.
> >> In general we can expect a light loss of about 25% to 30% through the
> >> full light guide
> >> including interfaces also to the PMT.
> >> 
> >> any thoughts and ideas are welcome,
> > 
> > Beni,
> > 
> > An interesting result. What I'm puzzled about is that more than
> > half of all photons (15% out of 25%)  are lost in the first very short
> > rectangular section of the light guide. Obviously, this is not due to
> > photon absorption in lucite because this section is only about
> > one-tenth of the total length. I doubt that this is due  to
> > rectangular shape of this section either. TOF paddle has the  same
> > shape and is about 100 times longer, which means that amount of light
> > from hits at the far end of TOF paddle would be equal to
> > (1-0.15)^100=nothing if such shape is a culprit.
> > 
> > I suspect that the reason for this large loss is  2pi solid angle of
> > photons generated near the light-guide entrance. A significant
> > fraction of these photons will have incident angle higher than
> > the angle of total internal reflection. Many of them will escape
> > when they hit light guide wall for the first time. That, of course,
> > is likely to happen in its first section for large-angle photons.
> > 
> > Loss of light in light guides is most important for the smallest
> > signals coming from hits at the far end of a TOF paddle. My
> > feeling is that mostly photons which are nearly parallel to
> > the paddle will reach the light guide at the opposite end.
> > So, my suggestion would be to run your Monte Carlo
> > with photons at 0 (or nearly 0) angle instead of 2pi soild angle
> > to see how the conclusion about light loss in the light guide
> > depends on the assumption about initial angular distribution
> > of the generated photons.
> > 
> > Sasha
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