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Feedback Re: transmission probabily in optical surface 

Forum: Processes Involving Optical Photons
Date: 17 Jul, 2003
From: Gumplinger Peter <Gumplinger Peter>

Hi,

Yes, I agree with the relation: p=exp(-g/l).

G4OpAbsorption::GetMeanFreePath returns the MeanFreePath stored with: AddProperty("ABSLENGTH".....

in the G4MaterialPropertiesTable. The actual step length is calculated in:

G4VDiscreteProcess::PostStepGetPhysicalInteractionLength

as:

currentInteractionLength = GetMeanFreePath(..... value = theNumberOfInteractionLengthLeft * currentInteractionLength;

and theNumberOfInteractionLengthLeft is initialized in:

G4VProcess::ResetNumberOfInteractionLengthLeft() to:

theNumberOfInteractionLengthLeft = -log( G4UniformRand() );

in the usual way.

I can only imagine that you've got something wrong with the units in which you specified 'ABSLENGTH':

  G4double Absorption[nEntries] =
           {0.005*cm,

If you have access to the Geant4 source, you can print out the length returned in G4OpAbsorption::GetMeanFreePath. This length should be of the order of 0.005cm and the Geant4 steps taken should almost always be limited by G4OpAbsorption when your slab is 1cm thick.

Turn /tracking/verbose 1 and look at the steps you get. The output is something like this:

Step#      X         Y         Z        KineE    dEStep   StepLeng  TrakLeng    Volume     Process
    1 21.928 cm3.7893 cm12.404 cm2.2685 eV     0 eV25.467 cm25.467 cm   Tank OpAbsorption

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