[Halld-pid] light guide simulation

[Beni Zihlmann]

Hi Sasha,
the interpretation might be not be so easy with this limited data.
Also I have to say  that this simulation was done with
a perfect polished surface. So only attenuation in the material
is effectively contributing to the losses. I have now added code
that handles the surface roughness and losses can happen
now also at the surfaces. The only drawback with this is that
I do not have yet good parameters that I can trust that represent
a typical surface of reasonably good polished lucite light guides.
Currently I used the following settings:
specular peak 0.95
specular lobe  0.045
back scattering 0.005
sigma alpha 0.1 (surface roughness)
If you have some sources/literature that might suggest different
better values for these parameters please let me know.

Yes I can make a geometry with a bend in the rectangular part
and use your geometry values. I will do that. I will also generate
a list of results for very different lengths of light guides and bend
radii as suggested by Mark to get a better feeling where the losses
are.

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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