|Message: Re: Scintillation rise time||Not Logged In (login)|
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Response to question...
Ok so Martin and I discussed this offline and the scintillation rise and fall time are defined in terms of 1/e lifetime instead of half-life. I am a nuclear phsyicist so dumbly assumed that everyone everywhere worked in half-lives, I was wrong!
However, with regard to rise. In the new Geant4 beta, if the new scintillation (with rise) method is invoked, then the equation being sampled is:
I = exp(-t/tauD)*(1-exp(-t/tauR))
where tauD and tauR are rise and decay respectively. (1-exp(-t/tauR)) governs the rise. The confusion arises in the definition of rise time. Some quote as 10% to 90% time and some define it as the time taken to reach (1-1/e). At present the tauR parameter in the code is for the latter. This is demonstrated by:
1-1/e = 1-exp(-t/tauR)
1/e = exp(-t/tauR)
1 = t/tauR
i.e. time taken to reach 1-1/e is tauR
Now, having understood that the code does things in this elegant (thank you Martin) way we can now work out what needs to be input if we are presented with the 10% to 90% rise case.
I = 1-exp(-t/tauR)
-t/tauR = ln(1-I)
t = -tauR*ln(1-I)
Need to subtract t for I = 0.9 and 0.1
t2 - t1 = -tauR*ln(1-0.9) - - tauR*ln(1-0.1)
where t2-t1 is the rise time
Rise = tauR(ln0.9 - ln0.1)
Rise = tauR(ln9)
tauR = Rise/ln9
So if you have a 10% to 90% "Rise" and you want to input the correct tauR into the new scintillation algorithm you need to use tauR = Rise/ln9.
I hope this helps and it would be great if this could be put in the documentation or G4scintillation.cc file somewhere.
Many thanks, especially to Peter and Martin for their help in clearing this whole thing up.
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