Home » PANDA » PandaRoot » Monte Carlo Engines » Group velocity for Cherenkov photon propagation in G3/G4
Group velocity for Cherenkov photon propagation in G3/G4 [message #11457] |
Wed, 26 January 2011 11:46 |
Jochen Schwiening
Messages: 85 Registered: June 2009 Location: GSI, Darmstadt
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continuous participant |
From: *gsi.de
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Hi folks,
I am looking at the time of propagation of Cherenkov photons
inside the DIRC radiator bars and see some slightly odd values.
This prompts the question how the velocity of the photon is
calculated in our framework. Does our VMC properly calculate
the group velocity of the Cherenkov photons from the material
index tables? Does that happen correctly in both G3 and G4?
Do any of you know and maybe know how to check the velocity
of the photon of a given wavelength, for instance in our
PndDrc.cxx code?
Thanks,
Jochen
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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 |
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 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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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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Re: Group velocity for Cherenkov photon propagation in G3/G4 [message #11477 is a reply to message #11457] |
Fri, 28 January 2011 21:33 |
Jochen Schwiening
Messages: 85 Registered: June 2009 Location: GSI, Darmstadt
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continuous participant |
From: *unitymediagroup.de
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Hi Oliver,
I like your suggestion to check individual photons without
reflections and other potential complications, Stefano had
suggested it as well.
To do this I used a photon gun placed inside the fused silica
radiator bar, close to the forward end of the bar, and shot
the photon along the Z axis of the bar towards the readout end,
recording the production time and bar exit time as well as
the path.
I created 10 photons with a fixed wavelength and varied the
wavelength in 14 runs from 200nm to 700nm. I see that all
10 photons always have the exact same wavelength, path, time,
and velocity.
The attached plot shows that the calculated velocity in Geant 4
is pretty much right on top of the calculated group velocity.
Unfortunately I cannot use the photon gun in Geant 3 because
the photon gun does not work for us in Geant 3 (still the
floating point exception crash when we select a 50000050 photon
as primary particle) but at least for Geant 4 your test and my
check suggests that the observed scatter of photon velocities
is not due to a problem with the group velocity calculation
in Geant (4) but due to incorrectly assigned paths or times
from TrackLength() and/or TrackTime().
We still need to make sure that the photon velocity is correct
in Geant 3 since using Geant 4 to generate event samples is
prohibitively slow (more than a factor 10 per-event time). Any
ideas for that would be very welcome since generating our DIRC
reconstruction look-up tables in G4 and applying it to G3 tracks
without being able to cross-check G3 with G3 and G4 with G4 is
not great. (Plus, since Peter says that the velocity is wrong in
Geant 3, that only the phase velocity is used, it's important
that we verify and correct this mistake.)
Have a great weekend,
Jochen
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