[Halld-pid] light guide simulation

[Tim Whitlatch]

Hi  All,

If the bend is in the rectangular portion, it can be pretty small (about 
5° assuming 25 cm to shield)

Tim

Beni Zihlmann wrote:
> 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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