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Question multiple optical photon bouncing  

Forum: Processes Involving Optical Photons
Date: 28 Mar, 2016
From: Balint Radics <Balint Radics>

Hello,

I hope this is the right forum to ask this question. We are wondering why our optical photons are bouncing so much between optical surfaces. Here is the long explanation.

We are using Geant4.9.5 with latest patch. We are simulating optical photon propagation from liquid argon to a wavelength shifter and a reflector surface (layers: LAr/WLS/Reflector) in such a way that

LAr-WLS optical surface is unified,ground,dielectric_dielectric WLS-Reflector optical surface is unified,ground,dielectric_metal

(we call the Reflector volume as WLSSupport, so in the illustration below it will is called like that)

and we also add optical surfaces for the opposite ordering (Reflector/WLS/LAr) such that in case the optical photon is progressing backwards after the reflection the proper surface ordering is present.

We find that in many cases the optical photon is bouncing a lot (see below) between the thin surface boundaries before either getting back to the large liquid argon volume or being absorbed somewhere. This bouncing seems to be independent of the surface quality (ground or polished). We thought initially that the reason was that the refraction indices of LAr and WLS allowed this enhanced bouncing but even if we put the LAr and WLS material properties to have the same refraction indices the bouncing still occurs. Otherwise, the LAr refraction index is around 1.2-1.5 in the range of photon energies 2-10 eV. The WLS material refraction index is 1.63 in the same range.

The bouncing is illustrated by the following "/tracking/verbose 1" output for one particular photon at the end of this message. The track lengths is increasing after each subsequent and seems to reach several meters length (the output is in units of mm and reaches values of order 1e+03 mm) which is difficult to believe to be the case in our system. We believe in reality this extremely long bouncing physically is not happening in our materials, in particular because the WLS material is quite porous so there is no real physical surface boundary between the LAr and WLS but rather the WLS is just sitting on the reflector in grains or so...but we have put this optical surface like that hoping that we can tune the optical model parameters properly. Maybe we are screwing something up...

We set the SigmaAlpha parameter to 0 or to 0.1 rad, but this does not seem to effect the existence of this bouncing behavior either.

Which optical surface parameter could enhance this effect so much?

Thanks for help, Balint

*********************************************************************************************************
* G4Track Information:   Particle = opticalphoton,   Track ID = 692,   Parent ID = 659
*********************************************************************************************************

Step#    X(mm)    Y(mm)    Z(mm) KinE(MeV)  dE(MeV) StepLeng TrackLeng  NextVolume ProcName
    0     -243     -312      214  2.94e-06        0        0         0         WLS initStep
    1     -243     -312      214  2.94e-06        0   0.0667    0.0667  WLSSupport Transportation
    2     -243     -312      214  2.94e-06        0        0    0.0667         WLS Transportation
    3     -243     -312      214  2.94e-06        0    0.135     0.202      LArCol Transportation
    4     -346     -192      233  2.94e-06        0      159       159         WLS Transportation
    5     -346     -192      233  2.94e-06        0    0.135       159  WLSSupport Transportation
    6     -346     -192      233  2.94e-06        0        0       159         WLS Transportation
    7     -346     -192      233  2.94e-06        0    0.135       159      LArCol Transportation
    8     -393    -41.5      251  2.94e-06        0      159       318         WLS Transportation
    9     -393    -41.4      251  2.94e-06        0    0.135       319  WLSSupport Transportation
   10     -393    -41.4      251  2.94e-06        0        0       319         WLS Transportation
   11     -393    -41.3      251  2.94e-06        0    0.135       319      LArCol Transportation
   12     -378      116      269  2.94e-06        0      159       478         WLS Transportation
   13     -378      116      269  2.94e-06        0    0.135       478  WLSSupport Transportation
   14     -378      116      269  2.94e-06        0        0       478         WLS Transportation
   15     -378      116      269  2.94e-06        0    0.135       478      LArCol Transportation
   16     -302      255      287  2.94e-06        0      159       637         WLS Transportation
   17     -302      255      287  2.94e-06        0    0.135       637  WLSSupport Transportation
   18     -302      255      287  2.94e-06        0        0       637         WLS Transportation
   19     -302      255      287  2.94e-06        0    0.135       637      LArCol Transportation
   20     -178      353      306  2.94e-06        0      159       796         WLS Transportation
   21     -178      353      306  2.94e-06        0    0.135       796  WLSSupport Transportation
   22     -178      353      306  2.94e-06        0        0       796         WLS Transportation
   23     -178      353      306  2.94e-06        0    0.135       796      LArCol Transportation
   24    -25.7      395      324  2.94e-06        0      159       955         WLS Transportation
   25    -25.6      395      324  2.94e-06        0    0.135       956  WLSSupport Transportation
   26    -25.6      395      324  2.94e-06        0        0       956         WLS Transportation
   27    -25.5      395      324  2.94e-06        0    0.135       956      LArCol Transportation
   28      131      373      342  2.94e-06        0      159  1.11e+03         WLS Transportation
   29      131      373      342  2.94e-06        0    0.135  1.11e+03  WLSSupport Transportation
   30      131      373      342  2.94e-06        0        0  1.11e+03         WLS Transportation
   31      131      373      342  2.94e-06        0    0.135  1.12e+03      LArCol Transportation
   32      267      292      360  2.94e-06        0      159  1.27e+03         WLS Transportation
   33      267      292      360  2.94e-06        0    0.135  1.27e+03  WLSSupport Transportation
   34      267      292      360  2.94e-06        0        0  1.27e+03         WLS Transportation
   35      267      292      360  2.94e-06        0    0.135  1.27e+03      LArCol Transportation
   36      303      242      367  2.94e-06        0     61.6  1.34e+03         WLS Transportation
   37      303      242      367  2.94e-06        0   0.0352  1.34e+03  WLSSupport Transportation
   38      303      242      367  2.94e-06        0        0  1.34e+03         WLS Transportation
   39      303      242      367  2.94e-06        0   0.0352  1.34e+03      LArCol Transportation
   40     -142     -369      455  2.94e-06        0      761   2.1e+03         WLS Transportation
   41     -142     -369      455  2.94e-06        0    0.028   2.1e+03  WLSSupport Transportation
  42     -142     -369      455  2.94e-06        0        0   2.1e+03         WLS Transportation
   43     -142     -369      455  2.94e-06        0    0.028   2.1e+03      LArCol Transportation
   44    -62.2      391      543  2.94e-06        0      769  2.87e+03         WLS Transportation
   45    -62.2      391      543  2.94e-06        0    0.028  2.87e+03  WLSSupport Transportation
   46    -62.2      391      543  2.94e-06        0        0  2.87e+03         WLS Transportation
   47    -62.2      391      543  2.94e-06        0    0.028  2.87e+03      LArCol Transportation
   48      259     -328      633  2.94e-06        0      792  3.66e+03 topSideReflector Transportation
   49      259     -328      633  2.94e-06        0        0  3.66e+03      LArCol Transportation
   50      212     -308      643  2.94e-06        0     51.7  3.71e+03      GArCol Transportation
   51      212     -308      643  2.94e-06        0        0  3.71e+03      LArCol Transportation
   52     -393    -48.3      513  2.94e-06        0      671  4.38e+03         WLS Transportation
   53     -393    -48.3      513  2.94e-06        0   0.0317  4.38e+03  WLSSupport Transportation
   54     -393    -48.3      513  2.94e-06        0        0  4.38e+03         WLS Transportation
   55     -393    -48.2      513  2.94e-06        0   0.0317  4.38e+03      LArCol Transportation
   56      151      365      377  2.94e-06        0      697  5.08e+03         WLS Transportation
   57      151      365      377  2.94e-06        0   0.0317  5.08e+03  WLSSupport Transportation
   58      151      365      377  2.94e-06        0        0  5.08e+03         WLS Transportation
   59      151      365      377  2.94e-06        0   0.0317  5.08e+03      LArCol Transportation
   60      244     -312      241  2.94e-06        0      697  5.77e+03         WLS Transportation
   61      244     -312      241  2.94e-06        0   0.0317  5.77e+03  WLSSupport Transportation
   62      244     -312      241  2.94e-06        0        0  5.77e+03         WLS Transportation
   63      244     -312      241  2.94e-06        0   0.0317  5.77e+03      LArCol Transportation
   64     -391    -58.7      105  2.94e-06        0      697  6.47e+03         WLS Transportation
   65     -391    -58.6      105  2.94e-06        0   0.0317  6.47e+03  WLSSupport Transportation
   66     -391    -58.6      105  2.94e-06        0        0  6.47e+03         WLS Transportation
   67     -391    -58.6      105  2.94e-06        0   0.0317  6.47e+03      LArCol Transportation
   68      142      369      -31  2.94e-06        0      697  7.17e+03         WLS Transportation
   69      142      369      -31  2.94e-06        0   0.0317  7.17e+03  WLSSupport Transportation

*********************************************************************************************************

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