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Question Geant4 handle optical process vs. optical process in reality 

Forum: Processes Involving Optical Photons
Date: 09 Aug, 2013
From: Josh Xie <Josh Xie>

Hello people:

Since two of my projects involving scintillating/fluorescence process, understand how Geant4 handles optical process is crucial for me. 

After digging around internet, I found Geant4 doesn't require you to define excitation spectrum. 

From my experience and knowledge, this is how I understand Geant4 handles optical process (Could you please tell me if I'm wrong in any point? Thanks!):

1. For scintillating process: any particle that can deposit energy into scintillating material will trigger scintillating process, and the emission photon distribution will follow emission spectrum we defined. 
This suggests that, the number of photons got generated will only depend on the total energy of the particle in every step deposited in the material. Somehow I believe there is a threshold for the minimum energy deposition to trigger the process, which correspond to some constant of the material. 
Since "opticalphoton" will not deposit any energy until it dies, it will not trigger scintillating process before it dies. 

2. For WLS process: any incoming "optical photon" will be absorbed in respect of 
WLS-Absorption-Length(which defines the average travelling length of a photon before it gets absorbed- is this same as Absorption-Length?). Once a photon got absorbed, it will emit a photon with different energy in respect of the distribution of emission spectrum we defined. 
Therefore, this suggests that any optical photon that overcomes the minimum threshold for WLS will be shifted into different wavelength regardless what the absorption spectrum is. 

Following above conclusions, however, there is one thing I don't understand:
Is WLS process decent for the process that involves one higher energy photon (which is still Geant4's "opticalphoton") causes excitation of multiple lower energy photons? e.g. Europium sulfate fluorescing red under ultraviolet light. 
This situation could not be explained/simulated by above two cases. 

I will not complete my project if I couldn't understand this optical process fully. Any advise/answers will be very helpful. Thank you a lot in advance!

~Josh

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1 Feedback: Re: Geant4 handle optical process vs. optical process in reality   (Gumplinger Peter - 16 Aug, 2013)
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