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Question Weird effect while tracking photon in a "spaghetti" calorimeter 

Keywords: tracking geometry error
Forum: Event and Track Management
Date: 05 May, 2005
From: Emanuele Leonardi <Emanuele Leonardi>

I am simulating a cylindrical "spaghetti" calorimeter using a
half-cylinder of lead filled with thightly packed 1mm-diameter
toroidal scintillating fibers (there are ~38000 of those). The whole
system is wrapped into a 1mm aluminum pocket.

Shooting ~1GeV photons I sometimes get the following effect:

*********************************************************************************************************
* G4Track Information:   Particle = gamma,   Track ID = 1,   Parent ID = 0
*********************************************************************************************************

Step#      X         Y         Z        KineE    dEStep   StepLeng  TrakLeng    Volume     Process
    0   -985 mum  -746 mum  -101 m       1 GeV     0 eV      0 fm      0 fm        World    initStep
    1   -985 mum    66 cm  -72.3 m       1 GeV     0 eV   28.4 m    28.4 m         World  Transportation
    2   -985 mum  66.1 cm  -72.2 m       1 GeV     0 eV    4.3 cm   28.5 m        Pocket  Transportation
    3   -985 mum  66.1 cm  -72.2 m       1 GeV     0 eV   4.98 mm   28.5 m      Detector  Transportation
    4   -985 mum  66.7 cm    -72 m       0 eV      0 eV   24.1 cm   28.7 m         Fiber        conv

*********************************************************************************************************
* G4Track Information:   Particle = e+,   Track ID = 3,   Parent ID = 1
*********************************************************************************************************

Step#      X         Y         Z        KineE    dEStep   StepLeng  TrakLeng    Volume     Process
    0   -985 mum  66.7 cm    -72 m     253 MeV     0 eV      0 fm      0 fm        World    initStep
    1   12.7 cm      5 m     125 m     253 MeV0.0135 eV    197 m     197 m    OutOfWorld  Transportation

*********************************************************************************************************
* G4Track Information:   Particle = e-,   Track ID = 2,   Parent ID = 1
*********************************************************************************************************

Step#      X         Y         Z        KineE    dEStep   StepLeng  TrakLeng    Volume     Process
    0   -985 mum  66.7 cm    -72 m     746 MeV     0 eV      0 fm      0 fm        World    initStep
    1  -4.14 cm      5 m     111 m     746 MeV0.0137 eV    183 m     183 m    OutOfWorld  Transportation

As you can see, the photon is tracked through aluminum (Pocket) and
lead (Detector) with no interaction. Then it reaches the scintillating
fiber (Fiber) and converts to a e+/e- pair.

And here comes the surprise: when the photon converts,
position (-985 mum,66.7 cm,-72 m) is inside a fiber, but when e+ and e-
are generated, the same position is in the World (which is filled with
a 10^-7 mbar vacuum) so that both particles travel down to the world
boundary and are lost.

What kind of error in my geometry definition can give this weird effect?

Please note that by increasing fiber diameter and spacing by a factor
10 (i.e. 1cm diameter and 1.35cm spacing) while leaving lead brick
dimensions unmodified I could not reproduce the effect.

If any G4 guru wishes to give a look to the effect, you can check
my code out from the CERN CVS repository at

http://isscvs.cern.ch/cgi-bin/cvsweb.cgi/SpaCal/?cvsroot=spacal

I use the following macro file to get the described effect:

/tracking/storeTrajectory 1
/tracking/verbose 1
/hits/verbose 1
/digi/Verbose 10
/gun/particle gamma
/gun/energy 1. GeV
/gun/position 0. 0. -10069.3 cm
/gun/direction 0. 67. 2881.
/run/beamOn

Thanks

         Emanuele Leonardi

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1 None: Re: Weird effect while tracking photon in a "spaghetti" calorimeter   (Vladimir Grichine - 09 May, 2005)
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