[Halld-pid] light guide simulation

[Beni Zihlmann]

Hi Sasha,
I modified the code so that I can change the geometry on the fly and I can
modify the lengths for each section and where the bend should be either in
the rectangular section before the tapering or in the round section 
after the
tapering.
Attached is a file containing the light loss calculations with the 
dimensions you
gave me. You did not quote a value for the bend so I chose a bend of 10 
degree.
You can see that there is an optimal length for the tapered section. I 
get the
best results for a tapered section with a length of about 15 to 20 cm.

cheers,
Beni
> 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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