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Question Reflection/Absorption problems in Optical Surfaces 

Forum: Geometry
Date: 18 Nov, 2005
From: andreas battaglia <andreas.battaglia@roma1.infn.it>

Hi,
I have some problems in optical photons' reflection/absorption on optical surfaces.

I'm trying to simulate a polystyrene (n=1.6) scintillator wrapped by a tyvek (n=1.54)
reflector.
The tyvek should reflect most of the optical photons and I supposed a situation in
which there is a thin air layer between the wrapping and the scintillator.

From "User's Guide: For Application Developers" (5.2.5 Optical Photon Processes),
from the article "A. Levin and C. Moisan, A More Physical Approach to Model the 
Surface Treatment of Scintillation Counters and its Implementation into DETECT, 
TRIUMF Preprint TRI-PP-96-64, Oct. 1996" and from the forum mails:

Geometry.
 418. Surface Properties:how to model a diffusive (98%) and absorbing (2%)
      surface   by Maria Lucia Magliozzi, 7/18/05
 163. reflective surface problem  by Kenneth Carrell, 3/31/04
 155. Optical Surface properties  by Vlasios Vasileiou, 3/17/04
  37. Optical Surfaces  by Pedro Silva, 7/05/02
  16. Surface finish  by Rui Bugalho de Moura, 2/01/02

I think that the following should be the right code:

      
      ///Optical Surface between Scintillator and Wrapping

      G4OpticalSurface* OpScintSurface = new G4OpticalSurface("ScintSurface");
      G4double sigma_alpha = 0.1;
      OpScintSurface -> SetSigmaAlpha(sigma_alpha);
      OpScintSurface->SetType(dielectric_dielectric);
      OpScintSurface->SetFinish(groundbackpainted);
      OpScintSurface->SetModel(unified);
      G4LogicalBorderSurface* ScintSurface = new G4LogicalBorderSurface("ScintSurface", 
           physiScintillator,physiTyvek,OpScintSurface);


      const G4int nEntries = 17;
      G4double PhotonEnergy[nEntries] =
         { 4.94864*eV, 4.12387*eV, 3.53474*eV, 3.0929*eV, 
           2.74924*eV, 2.47432*eV, 2.24938*eV, 2.06193*eV, 
           1.90332*eV, 1.76737*eV, 1.64955*eV, 1.54645*eV, 
           1.45548*eV, 1.37462*eV, 1.30227*eV, 1.23716*eV, 1.01*eV };  
      G4double SpecularLobe[nEntries]= 
        { 0.1, 0.1, 0.1, 0.1, 0.1, 0.1, 0.1, 0.1, 0.1, 
          0.1, 0.1, 0.1, 0.1, 0.1, 0.1, 0.1, 0.1 };
      G4double SpecularSpike[nEntries] = 
        { 0.1, 0.1, 0.1, 0.1, 0.1, 0.1, 0.1, 0.1, 0.1, 
          0.1, 0.1, 0.1, 0.1, 0.1, 0.1, 0.1, 0.1 };
      G4double Backscatter[nEntries] = 
        { 0.1, 0.1, 0.1, 0.1, 0.1, 0.1, 0.1, 0.1, 0.1,
          0.1, 0.1, 0.1, 0.1, 0.1, 0.1, 0.1, 0.1 };      
      G4double Reflectivity[nEntries] = 
        { 0.74,  0.8,  0.89,  0.94,  0.95, 0.96,  0.965, 0.97, 
          0.965, 0.97, 0.965, 0.965, 0.96, 0.955, 0.95,  0.955 ,  0.955};
      //G4double Efficiency[nEntries] = 
      //  { 0.1, 0.1, 0.1, 0.1, 0.1, 0.1, 0.1, 0.1,  
      //    0.1, 0.1, 0.1, 0.1, 0.1, 0.1, 0.1 };
      G4double RefractiveIndexAirLayer[nEntries] = 
        { 1, 1, 1, 1, 1, 1, 1, 1,  
          1, 1, 1, 1, 1, 1, 1 };

      G4MaterialPropertiesTable* myST1 = new G4MaterialPropertiesTable();  
      myST1->AddProperty("RINDEX",                PhotonEnergy, RefractiveIndexAirLayer, nEntries);
      myST1->AddProperty("SPECULARLOBECONSTANT",  PhotonEnergy, SpecularLobe,    nEntries);
      myST1->AddProperty("SPECULARSPIKECONSTANT", PhotonEnergy, SpecularSpike,   nEntries);
      myST1->AddProperty("BACKSCATTERCONSTANT",   PhotonEnergy, Backscatter,     nEntries);
      myST1->AddProperty("REFLECTIVITY", PhotonEnergy, Reflectivity, nEntries);
      //myST1->AddProperty("EFFICIENCY",   PhotonEnergy, Efficiency,   nEntries);

      OpScintSurface->SetMaterialPropertiesTable(myST1);


(Beyond, in the volumes definitation I gave the refractive index and the absorbtion spectrum of both 
 the scintillator and the tyvek).

Now, for 1 event I get 920 optical photons of which 17 are absorbed in the tyvek volume and 767 are absorbed 
by the optical surface between the scintillator and the wrapping.
So, about 85% of optical photons are absorbed (not reflected!!!) by the reflector.

What's wrong? Why doesn't the wrapping reflect?

Thank you very much.

Andreas Battaglia

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1 None: Re: Reflection/Absorption problems in Optical Surfaces   (Liang Zhan - 20 Nov, 2005)
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