|Message: Re: Physics Process Flags||Not Logged In (login)|
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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/ Dennis
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