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Forum: Physics List
Re: Question physics processes order
Date: 17 Dec, 2008
From: Vladimir Ivanchenko <Vladimir Ivanchenko>

Hello,

There is recommended order of Geant4 processes which one can observe looking into examples and physics_lists directories. The reasons are following:

Ordinary processes like ionisaton, bremsstrahlung, Compton scattering before each particle step defines step limit based on values of cross sections and random number generation. It is optimal to select minimal step limitation from these processes and only after ask for step limits from the multiple scattering process and transportation, because established step limit allows to perform more fast computation of geometry limit. For example, if a particle is in the middle of a volume, Compton process limit is 1.2 mm and it is known that minimal distance to a geometry boundary is above 10 cm, there is no need o perform precise geometry computations. To this end the order of processes is useful but not mandatory.

When DoIt methods start, then it is mandatory first define the step of a particle before any other computation, so transportation must be first. For changed particles it is mandatory to apply multiple scattering as a next process, because multiple scattering computes "true step length", which is always larger than the "geometrical step length", because the charge particle trajectory is not a staight line but some integral of scatterings.

Integer numbers are used to the explicite ordering. If a number is negative, then the process is inactive for given DoIt.

VI

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1 Question: Re: physics processes order   (bgastro@pmo.ac.cn - 22 Dec, 2008)
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