geant4

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         Geant4 - an Object-Oriented Toolkit for Simulation in HEP
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                                   WLS
                                ----------

  This application simulates the propagation of photons inside a Wave Length
Shifting (WLS) fiber.


1- Geometry Definition

  The default geometry is as follow:

- A perfect, bare, PMMA fiber: 0.5mm radius, 2m length at center(0,0,0)
  of the World.
- A circular MPPC with 0.5mm radius at the +z end of the fiber
- World and coupling materials are G4_AIR
- Photons will always refracted out to coupling material before
  reaching MPPC
- There are many flexible parameters that the user could specify.
  They are under the /WLS directory of help.


2- Material Choices

  There are several materials that the user can use for the fiber core,
  world and coupling.
  
They are:

- Vacuum (G4_Galactic)
- Air (G4_AIR)
- PMMA, n = 1.60
- Pethylene, n = 1.49
- FPethylene, n = 1.42
- Polystyrene, n = 1.60
- Silicone, n = 1.46


3- Photon Source

  This program uses the General Particle Source (G4GeneralParticleSource) 
  provided by GEANT4 for
  generating photons. The energy of the photon must be within 2.00 eV
  to 3.47 eV.  For detail instruction on how to use the General Particle
  Source, please visit their home page at:

  http://reat.space.qinetiq.com/gps/


4- Hit

  A hit is registered when the photon is absorbed on the MPPC surface.
  Information stored in hit includes the local coordinate of the location
  the photon is absorbed on the MPPC, the global coordinate where the
  photon left the fiber and the transit time of the photon.


5- Stepping Action

  The stepping action keeps track of the number of bounces a photon has
  gone through.  In order to prevent infinite loop and extremely skewed
  rays taking up computing time, there is a limit of the number of
  bounces that a photon can go through before it is artificially killed.
  The default limit is 100,000.  The user can set his/her own limit using
  the /stepping/setBounceLimit command.  A value of 0 will turn off the
  limit.  All photons artificially killed will have murderee flag turned
  on in their UserTrackInformation.


6- Visualization

  To visualize a photon's trajectory, simply use vis.mac macro in
  interactive mode or in your own macro.


7- main()

 - execute wls in 'batch' mode from macro files
 - you can enter an optional integer seed for batch mode 
         % wls wls.in (optional: enter an integer seed here)
                 
 - wls in 'interactive mode' with visualization
         % wls
         ....
         Idle> /control/execute vis.mac
         Idle> /run/beamOn 1
         ....
         Idle> exit