| Re: Group velocity for Cherenkov photon propagation in G3/G4 [message #11465 is a reply to message #11462] |
Thu, 27 January 2011 11:33   |
Jochen Schwiening
Messages: 85
Registered: June 2009
Location: GSI, Darmstadt
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continuous participant |
From: *gsi.de
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Hi Mohammad,
thanks for the suggestion.
I tried this on G3 and G4. The track entering and exiting are both
well-defined quantities for Cherenkov photons in Geant 3 where
the TrackLength and TrackTime are defined as I intuitively
expect: the clock (time and path) starts when the photon
is produced and stops when the photon exits the bar at the
backward end of the bar, which I selected as my stop point.
In Geant 4, however, the photons exit and re-enter on every
internal reflection, which means that a photon which propagates
inside the bar for 400cm only reports a TrackLength on the mm
level and a TrackTime on the picosecond scale, corresponding
to the last bounce before exiting the bar at the backward end.
Therefore, the calculated velocity is not based on the same
paths as the G3 numbers.
Having said that, the distribution of velocity vs. wavelength
looks somewhat different now but still strange, as you see
from the attached plot. I still don't see any reason why the
velocity for a given photon in Geant (3 or 4) is not a single
number but is spread out so much - unless the path or time
quantities are not the true/correct numbers.
Any ideas?
Thanks,
Jochen
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