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Question Unified model ground surfaces 

Forum: Processes Involving Optical Photons
Date: 07 Apr, 2009
From: Andrzej Zuranski <Andrzej Zuranski>

Hi,

I'm simulating a setup of a TOF detector using scintillation/cerenkov light. I attach the PMTs to the end of a light guide and specify a surface there with the air gap and properties. The relevant part of code is:

  const G4int n=2;
  G4double Ephot[n]={1.9*eV,6.2*eV}; //650-200nm

  Connection = new G4OpticalSurface("Connection");
  Connection->SetModel(unified);
  Connection->SetFinish(ground);
  Connection->SetSigmaAlpha(0.2);
  Connection->SetType(dielectric_dielectric);

  G4MaterialPropertiesTable* ConnectionPT = new G4MaterialPropertiesTable();
  G4double RindexCon[n]={1.,1.};
  G4double SpecularLobe[n]={0.2,0.2};
  G4double SpecularSpike[n]={0.3,0.3};
  G4double BackScatter[n]={0.1,0.1};
  ConnectionPT->AddProperty("RINDEX",Ephot,RindexCon,n);
  ConnectionPT->AddProperty("SPECULARLOBECONSTANT",Ephot,SpecularLobe,n);
  ConnectionPT->AddProperty("SPECULARSPIKECONSTANT",Ephot,SpecularSpike,n);
  ConnectionPT->AddProperty("BACKSCATTERCONSTANT",Ephot,BackScatter,n);

I first tried with a polished surface (no surface defined) and was looking at the boundary process status for this surface in the Stepping Action requiring that the PreStepPoint is in the light guide and the PostStepPoint is in the PMT window and the Step Status is fGeomBoundary for Pre and Post. Suprisingly some of the statuses for the polished surface were Fresnel Reflection and Total Internal Reflection. Is this right that the PreStepPoint and PostStepPoint always are on the opposite sides of the surface even though the photon is reflected, so it stays in the same volume?

Than I tried with a ground surface, the numbers above are purely invented except the refractive index, so the question arises how to estimate those values to simulate the physics right for a given surface that looks perfectly polished at a glance? Or we shall hope that at the end of the day those values don't matter that much.

So after making the surface as above I checked again the boundary status for step points on the opposite sides of the surface and some of them changed to Spike, Lobe, BackScatter or Lambertian but still majority of them were saying Fresnel Refraction. Mysterious reflections described above has disappered in this case. Shouldn't we expect for the ground surface that there are no longer Fresnel refractions, but only the above four types? Or everything is right and expected? For information I'm using geant4.9.1.p02, I havent looked at the G4OpBoundaryProcess.cc code in details but maybe you know more about it.

Best Regards,

Andrzej Zuranski

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1 Feedback: Re: Unified model ground surfaces   (Gumplinger Peter - 08 Apr, 2009)
(_ Feedback: Re: Unified model ground surfaces   (Gumplinger Peter - 08 Apr, 2009)
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