[Halld-pid] light guide simulation

[Alexander Ostrovidov]

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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> >> 
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