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Keywords: physics process flags
Forum: Physics List
Re: Question Physics Process Flags (Daniel McDevitt)
Date: 27 Apr, 2005
From: Dennis H. Wright <Dennis H. Wright>

Hi Daniel,

   The flags are necessary because certain Geant4 processes involving 
charged particles must be invoked in a specific order.  The values of 
the flags indicate the order - the lower the flag value, the later 
the process is invoked. Negative flag values indicate that part of the
process is not invoked.  This feature is illustrated by multiple 
scattering.  The Geant4 multiple scattering process requires range 
information for the particle.  The range depends on two other processes, 
ionization and bremsstrahlung, so they must be invoked before multiple 
scattering and hence they have higher numbers than multiple scattering.
Any other process must have a higher number than that of bremsstrahlung,
which is why the positron annihilation process is registered as number 
4 in example N02.  
   The position of the flags in the call to AddProcess refers to the 
different actions that a process can take.  There are three such 
actions, called at-rest, along-step and post-step, which may be used 
separately or in combination.  The second argument in the AddProcess 
call is for the at-rest part of the process, the third argument is for 
the along-step part and the fourth is for the post-step part.
  Looking at the positron call:

  pManager->AddProcess(new G4eplusAnnihilation(), 0,-1, 4);

we see that the at-rest action is invoked (0), the along-step action
is not invoked (-1), and the post step action is invoked (4), so that
annihilation can occur at rest or in flight.  Note that 0 is a valid 
flag value, but it is usually reserved for the transportation or 
parameterized shower processes.  Since the transportation process 
does not have an at-rest action, it is OK to use it for the positron
annihilation at rest.

   Unfortunately this information has not yet been gathered in one
place, but we hope to do this soon as we are in the midst of a web
page re-organization.   
   There is a partial discussion of this in the Application Developers
Guide, Section 2.5.3, and if you want to go to the source, the meaning 
of the flags is described in comments in the code for 
G4ProcessManager.hh in geant4/source/processes/management/


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