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None strange warning in Fresnel reflection/refraction simulation 

Forum: Documentation and Examples
Date: 20 Sep, 2006
From: Aaron <Aaron>

I am trying to observe Fresnel reflection/refraction in GEANT4.

I have a very simple simulation setup.  I have two boxes sitting
directly on top of one another; they meet at a horizontal plane
interface.  Each box has specified indices of refraction, and they are
both specified for a range of energies.  I generate monoenergetic
optical photons in the top box and aim them at the horizontal plane at
a designated angle.  I only have Fresnel reflection/refraction (and
Transport) turned on, so the photons either reflect or refract when
they reach the horizontal boundary.

Normally, this seems to work just fine.  But occasionally, GEANT4 gives
me the following warning message:

WARNING - G4Navigator::ComputeStep()
          Track stuck, not moving for 10 steps
          in volume -pTop- at point (0,0,0)
          direction: (0.9974334734,-0.07159934398,0).
          Potential geometry or navigation problem !
          Trying pushing it of 9e-10 mm ...

(The name of my top box is "pTop", and all photons are setup to hit the
horizontal interface at the origin (0,0,0).)

As far as I can tell, the above error appears only when the indices of
refraction for the top and bottom materials (at the same energy) are
sufficiently close.  I cannot quantify "sufficiently close" any better,
since it seems to depend on the values of the indices of refraction
themselves.

As an example, consider the following energies and indices of
refraction for the top and bottom boxes:

   const G4int NENERGIES=1;
   G4double xrayEnergy[NENERGIES]={1.0*keV};

   G4double indOfRefrTop[NENERGIES]={1.5};
   G4double indOfRefrBot[NENERGIES]={1.4};

If I try to run this simulation with 1keV photons, I get the
previosly-mentioned error.  But, if I simply change the value of
indOfRefrBot from "1.4" to "1.0", the errors go away entirely.

Does anyone have any ideas what's going on here?

Thanks,

Aaron


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1 Question: Re: strange warning in Fresnel reflection/refraction simulation   (Aaron - 20 Sep, 2006)
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