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Re: Group velocity for Cherenkov photon propagation in G3/G4 [message #11462 is a reply to message #11459] Wed, 26 January 2011 17:41 Go to previous messageGo to previous message
Jochen Schwiening is currently offline  Jochen Schwiening
Messages: 84
Registered: June 2009
Location: GSI, Darmstadt
continuous participant
From: *gsi.de
Hi Mohammad,

thanks for the suggestion.

I used TrackLength() and TrackTime() in PndDrc to calculate
the photon velocity in the bar at several times in the
Cherenkov photon's life. What I see is that the apparent
velocity does not behave as I expect. Here's an example for
two photons in Geant3 in the same event as they pass their
30ns mark:

time: 30.1524, path: 532.029, velocity: 17.6447, energy: 4.16593, wavelength: 296.971
time: 30.0086, path: 537.002, velocity: 17.8949, energy: 1.5531, wavelength: 796.575

This is probably because the TrackTime() of the photon includes
the time of flight of the primary particle. If I manually
calculate the time of flight of the primary particle and
subtract it I get velocity values around 19cm/ns, closer to
the expected values.

To illustrate the point I plotted (in excel, sorry) the observed
velocity vs wavelength for a few hundred photons in G3 and in
G4 as well as the expected group velocity. Please see the
attached figure.

Would you have a suggestion how I can access in PndDrc.cxx
the TOF of the particle and subtract it?
And is TrackLength() a "clean" quantity, which contains only
the path inside the bar? Or do I need to make a correction to
get the path that corresponds to the corrected TrackTime()
value I need to use?

Thanks,
Jochen

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