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Question Optical Photons: alternative ways of defining wrapping material? 

Forum: Geometry
Date: 27 Nov, 2008
From: Estela Suarez <Estela Suarez>

Hello,

I am simulating a plastic scintillator bar wrapped in a reflector (aluminum foil) with an air gap between the foil and the scintillator and placed on top of a photomultiplier (PMT). I define the volume as a box of scintillator inside of a slightly larger box of air, which is again inside of a slightly larger box of aluminum. They are located in such way that in the lower surface the three boxes end together (so that the scintillator is in direct contact with the PMT window and no air or aluminum is in between). (I hope this is the correct approach).

I need to implement the scintillator roughness and the reflectivity of the aluminum so that I can accurately simulate the tracking of the optical photons produced in the scintillator when an X-ray photon interacts in it.

Looking at several posts from this Forum and at the examples N06 and LXe I have seen two possibilities of implementing the optical characteristics of my geometry, and I am not sure why they give completely different results.

The options are: 1) Define a single dielectric-metal surface with groundbackpainted finish, where I give the refractive index of the air and the reflective index of the aluminum as parameters:

  G4OpticalSurface* OpAirSurface = new G4OpticalSurface("AirSurface");
  G4double sigma_alpha = 0.035;
  OpAirSurface -> SetType(dielectric_metal);
  OpAirSurface -> SetModel(unified);
  OpAirSurface -> SetFinish(groundbackpainted);	//reflector wrapping with air in between
  OpAirSurface -> SetSigmaAlpha(sigma_alpha);

  G4double ScintAirSpecularlobe[nEntries] = {1.0, 1.0, 1.0};	
  G4double ScintAirSpecularspike[nEntries] = {0.0, 0.0, 0.0};	
  G4double ScintAirBackscatter[nEntries] = {0.0, 0.0, 0.0};
  G4double ScintAirRindex[nEntries] = {1.0, 1.0, 1.0};	//refractive index of air between scintillator and wrapping
  G4double ScintAirReflec[nEntries] = {0.98, 0.98, 0.98};  //reflective index of wrapping

  G4MaterialPropertiesTable* AirGapSurfProp = new G4MaterialPropertiesTable();

  AirGapSurfProp -> AddProperty("RINDEX",PhotonEnergy,ScintAirRindex,nEntries);
  AirGapSurfProp -> AddProperty("SPECULARLOBECONSTANT",PhotonEnergy,ScintAirSpecularlobe,nEntries);
  AirGapSurfProp -> AddProperty("SPECULARSPIKECONSTANT",PhotonEnergy,ScintAirSpecularspike,nEntries);
  AirGapSurfProp -> AddProperty("BACKSCATTERCONSTANT",PhotonEnergy,ScintAirBackscatter,nEntries);
  AirGapSurfProp -> AddProperty("REFLECTIVITY",PhotonEnergy,ScintAirReflec,nEntries);

  OpAirSurface -> SetMaterialPropertiesTable(AirGapSurfProp);
  G4LogicalBorderSurface* AirSurface = 
  	new G4LogicalBorderSurface("AirSurface", sci_box_phys, AirGap_phys,OpAirSurface);

2) Defining the two interfaces (scintillator-air and air-aluminum) separately as dielectric-dielectric with ground finish and dielectric-metal with polished finish, as follows:

  //--------  Scintillator Air Skin: Unified model
  G4OpticalSurface* OpAirSurface = new G4OpticalSurface("Sci-AirSurface");

  G4double sigma_alpha = 0.035;

  OpAirSurface -> SetType(dielectric_dielectric);
  OpAirSurface -> SetModel(unified);
  OpAirSurface -> SetFinish(ground);
  OpAirSurface -> SetSigmaAlpha(sigma_alpha);

  G4double ScintAirSpecularlobe[nEntries] = {1.0, 1.0, 1.0};
  G4double ScintAirSpecularspike[nEntries] = {0.0, 0.0, 0.0};
  G4double ScintAirBackscatter[nEntries] = {0.0, 0.0, 0.0};
  G4double ScintAirRindex[nEntries] = {1.0, 1.0, 1.0};

  G4MaterialPropertiesTable* AirGapSurfProp = new G4MaterialPropertiesTable();

  AirGapSurfProp -> AddProperty("RINDEX",PhotonEnergy,ScintAirRindex,nEntries);
  AirGapSurfProp -> AddProperty("SPECULARLOBECONSTANT",PhotonEnergy,ScintAirSpecularlobe,nEntries);
  AirGapSurfProp -> AddProperty("SPECULARSPIKECONSTANT",PhotonEnergy,ScintAirSpecularspike,nEntries);
  AirGapSurfProp -> AddProperty("BACKSCATTERCONSTANT",PhotonEnergy,ScintAirBackscatter,nEntries);

  OpAirSurface -> SetMaterialPropertiesTable(AirGapSurfProp);
  G4LogicalBorderSurface* AirSurface = 
  	new G4LogicalBorderSurface("Sci-AirSurface", sci_box_phys, AirGap_phys, OpAirSurface);

//-------

  // Air-Aluminum  
  G4OpticalSurface* OpSciWrapSurface = new G4OpticalSurface("Air-AluSurface");  
  OpSciWrapSurface -> SetType(dielectric_metal);
  OpSciWrapSurface -> SetFinish(polished);
  OpSciWrapSurface -> SetModel(glisur);

  G4double reflect[nEntries] = {0.98, 0.98, 0.98};

  G4MaterialPropertiesTable *SciWrapProperty = new G4MaterialPropertiesTable();
  SciWrapProperty -> AddProperty("REFLECTIVITY", PhotonEnergy, reflect, nEntries);

  OpSciWrapSurface -> SetMaterialPropertiesTable(SciWrapProperty);

  G4LogicalBorderSurface* SciWrapSurface = 
  	new G4LogicalBorderSurface("Air-AluSurface", AirGap_phys,alufoil_phys,OpSciWrapSurface);

I checked both possibilities: with option 1) I collect in the PMT only 20% if the optical photons produced, weather with option 2) I collect 40%. What is making such a large difference between them? Aren't they conceptually the same?

Another question concerning option 1): since the surface definition contains already the air and aluminum characteristics, is it necessary to define the physical volumes of air and aluminum at all? Isn't it like a double definition of the same thing? Could I just define the scintillator in the world volume without anything else?

Lastly, a general question: if I define the MaterialPropertiesTable of aluminum and air including their respective refractive indexes, do they play a role at all in the optical photon tracking, or is the tracking only taking the MaterialPropertiesTable of the surfaces into account?

Thanks a lot for any clarification you may provide.

Best regards, Estela.

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1 Feedback: Re: Optical Photons: alternative ways of defining wrapping material?   (Gumplinger Peter - 27 Nov, 2008)
(_ Feedback: Re: Optical Photons: alternative ways of defining wrapping material?   (Estela Suarez - 28 Nov, 2008)
1 Feedback: Re: Optical Photons: alternative ways of defining wrapping material?   (Wolfgang Lukas - 10 Dec, 2008)
2 Feedback: Re: Optical Photons: alternative ways of defining wrapping material?   (Gumplinger Peter - 10 Dec, 2008)
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