[Halld-pid] light guide simulation

[Beni Zihlmann]

Hi All,
taking Tim's suggestion and having a 5 degree bend first over a little 
longer
distance and then adding the tapered section will give a better result:
with a bend of 5 degree about 13cm long and a 15 cm tapered section
and a 7cm long round section I see a loss of about 24%.:
Loss in bending section: 12.0075%
Loss in tapered section: 9.38166%
Loss in round section    : 3.29412%
Loss TOTAL               : 23.8274%

This looks rather promising. As also Sasha proposed doing the bend
first in the rectangular section seems to be better as the bend angle
can be shallower.

cheers,
Beni


> Hi Benny,
>
> Taking into account this and your previous results, it's
> interesting and somewhat puzzling that light loss stays
> within 25-30% range no matter where the bend is and
> how long the light guide sections are (within reasonable
> limits). Statistical error for ~500 events entering a light guide
> is about 4.5%. So, all results are almost withing 1 sigma
> of each other. Even if 3% difference between reasonable
> designs remains at higher statistics, we are still talking
> about 72%-75% of light getting through. This is not like
> choosing between, say, 60% and 80% designs. If light loss
> is so similar then, I guess, we'll go with a bent rectangular
> section simply because it is somewhat easier to bend
> than the tapered one.
>
> Sasha
>
> On Friday, March 25, 2011, 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_Desi
>>>>>> gn
>>>>>>
>>>>>> 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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