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Question Cerenkov radiation production below threshold 

Forum: Processes Involving Optical Photons
Date: 31 Aug, 2012
From: <esther>

Hi,

I'm simulating 10^6 positrons with energies extracted from F-18 emission spectrum, which produce Cherenkov radiation in water. I plotted the number of Cherenkov photons produced by each of these positrons as a function of positrons energies. Unexpectedly, what I found is that Cherenkov emission is nonzero even below threshold (which is equal to 263 keV for beta particles in water). If I take the average number of photons for every energy value, it turns out to be very close to zero, but I would like to know how I can justify this behavior, because I would have expected a number of photons identically equal to zero below threshold. I think it could be related to the fact that (as I read from G4PhysicsRefManual):

"Photons emitted with an energy beyond a certain value are immediately re-absorbed by the material; this is the window of transparency of the radiator. "

Is this right? How does Geant threat the energy values users give as input (in my case they are the positrons energies from which geant calculates velocity beta=v/c)? Does it create a mean beta=v/c value with a distribution deltaBeta which has tails below threshold that produce Cherenkov photons which are re-absorbed?

To be sure I wasn't counting optical photons produced from other processes, I even inactivated scintillation, so I'm pretty sure I'm considering only Cherenkov contribution.

In the plot here after you can see what my problem is.

Thanks in advance for any helpful answer.

Esther

   Attachment:
      http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2012/08/31/09.05-926-cherenkovN_vs_E.png

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1 Feedback: Re: Cerenkov radiation production below threshold   (Gumplinger Peter - 31 Aug, 2012)
(_ More: Re: Cerenkov radiation production below threshold   (esther - 09 Sep, 2012)
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