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Hi Georges, Indeed this forum is not much for this generic question, but anyway. Processes return a proposed step length (and not a cross-section) and are themselves responsible for making the related sampling of the exponential law, using their own mean free path. This sampling is actually done in the process base class G4VProcess. You can get the mean free path of a process calling its "G4double GetCurrentInteractionLength() const (method from G4VProcess)". If "-1" is returned, the process is not a true physical one (eg: transportation). I would believe that if you call these methods at the end of a step (in a user stepping action) you would still get the mean free paths for the step that just occurred. However, there is no easy way to get the proposed step lengths themselves. If this just for "curiosity" purpose, a /tracking/verbose 10 would dump everything about what processes do, but if you do not that information for your application, it is more complicated. The comparison between step length proposed by processes is made in the G4SteppingManager : this is its responsibility of selecting the process that returns the smallest step length. Cheers, Marc PS: message re-sent: because of web mail server change, not sure the first attempt will succeed... On 02/07/2014 02:47 PM, George Dedes wrote: > *** Discussion title: Fast Simulation, Transportation & Others > > Hi, > > I put this question here, as it is generic and doesn't seem to fit > anywhere else than a category that has "& Others": > > For the decision of each step size, each process proposes a step and the > lowest one decides the invoked interaction and the actual step size. I > guess that each process returns either a macroscopic cross section or > directly a mean free path, from which then a step length is sampled > according to the exponential decay law. Then all step lengths should be > compared and take the lowest. I would like to know in which class(es), > the sampling and the comparison of the proposed steps is done. > > Thanks in advance, George > > PS: I see that printing the proposed step from > G4Navigator::ComputeStep(), gives already the final proposed step, which > coincides with the step length that I get from my SteppingAction > > ------------------------------------------------------------- > Visit this GEANT4 at hypernews.slac.stanford.edu message (to reply or unsubscribe) at: > http://hypernews.slac.stanford.edu/HyperNews/geant4/get/fastsim/72.html