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Question Incorrect velocity of optical photons in a scintillating medium 

Forum: Processes Involving Optical Photons
Date: 20 Mar, 2013
From: Christoph Aberle <Christoph Aberle>

Hello all, 

when shooting optical photons through scintillator I get wrong photon velocities if I use a wavelength-dependent index of refraction. I'm using 
Geant4.9.6 and the particle gun to insert optical photons with a fixed wavelength in a fixed direction to check the speed. If I use a constant index 
of refraction the speed of the optical photons match exactly my expectations (c_vacuum/n). 

Below are some details if you are interested in this. 
Here is the code I use to register the refractive index numbers: 

const G4int NUMENTRIES = 17;
G4double ppckov[NUMENTRIES] = {6.27105e-06*MeV,5.99528e-06*MeV,5.88034e-06*MeV,5.716e-06*MeV,5.43736e-06*MeV,5.14118e-06*MeV,4.78062e-06*MeV,4.34087e-06*MeV,4.15e-06*MeV,3.91184e-06*MeV,3.65546e-06*MeV,3.40206e-06*MeV,3.12481e-06*MeV,2.81639e-06*MeV,2.6125e-06*MeV,2.09863e-06*MeV,1.54975e-06*MeV};
G4double rindex[NUMENTRIES] = {1.773,1.751,1.72,1.684,1.637,1.586,1.547,1.514,1.501,1.487,1.473,1.468,1.462,1.455,1.448,1.438,1.43};

The photons energy I used to check the speed is 3.12481*eV where the refractive index is 1.462. The optical photon is released in z-direction and the global 
time (step->GetTrack()->GetGlobalTime()) is recorded once the photon hits a stainless steel surface which is 6.5m away. I checked that there are no offsets 
in the global time by changing the start position of the optical photon. The speed I get is 6.5m/33.194ns which corresponds to an index of refraction of 
1.531 and not 1.462. If I enter a constant index of refraction (replace all 17 entries in rindex with 1.462) I get exactly the expected velocity of 6.5m/31.699ns. 

Any ideas why the wavelength-dependent refractive index doesn't give the expected speed of light? If more information/code is needed let me know. 

Thank you!

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1 Disagree: Re: Incorrect velocity of optical photons in a scintillating medium   (Gumplinger Peter - 20 Mar, 2013)
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