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Idea Re: Setting a User Limit and G4EmCalculator 

Forum: Run Management
Re: Question Setting a User Limit and G4EmCalculator (David)
Date: 06 Sep, 2012
From: Vladimir Ivanchenko <Vladimir Ivanchenko>

Hello,

Unfortunately, you should have problem to compute any physical value when you do not yet build geometry. Geant4 kernel performs initialisation in the following order: 
      1) list of particles
      2) geometry
      3) physics

So,  one cannot know range of a particle in DetectorConstruction class during volume construction. G4EmCalculator may work when all EM processes are instantiated and initialized. 

CSDA range is rarely used, so by default it cannot be computed properly. One needs to enable construction of CSDA tables using UI command:

/process/eLoss/CSDARange true
/run/initialise

In $G4INSTALL/examples/extended/electromagnetic/TestEm0 we show how to use EM calculator in RunAction at the end of run. At this moment one can compute CSDA ranges and later type obtained values into your UserLimits.

Please, note that usage of step limits not necessary increase accuracy of your simulation but definitely add extra CPU. This feature should be applied with care and only if you are sure that without extra step limitation Geant4 does not provide desired accuracy.

VI   

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