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Forum: Event and Track Management
Date: 19 Aug, 2004
From: Huaimin Liu <>

Dear Geant4 experts,

In the tracking part of section 5.1.1 in the User Guide V6.2,
the algorithm to handle one step is given. I think some points
sholud be clarified.

Item 3: The geometry navigator calculates the Safety. If the 
        minimum physical-step-length from all processses is
        shorter than Safety, it will be selected as the next
        step length. In this case, no geometrical calculations
        will be performed.||
  It's easy to be understood logically. But for next item
Item 4: If the mimimum physical-step-length is longer than Safety,
        the distance to the next boundary is re-calculated.||
  According to the definition, Safety is the distance to the next
  boundary. Why Safety is re-calculated? How? Is the re-calculated
  Safety is different from the previous one? Why?

I notice the difference of expresions between the current 
documentation and previous ones (say V5.0). In the old documentation,
Item 4 says if the minimum physical-step-length is longer than
Safety, the current point of the particle is "backed up" to its
previous step point when the previous step is not limited by
geometrical boundary, then the Safety is re-calculated. Does it mean
the previous step will be altered by this action? At least to me,
it's hard to understand the concept.

One more question. In Geant4, transportantion is regarded as a 
physical process. What is the physical-step-length it produces?
Is it the length from the current point to the world boundary?

Thank you very much.

Huaimin Liu 
IHEP, Beijing

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