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In Physics Reference Manual, item Computing the energy loss :
Stepsize Limit Due to Continuous Energy Loss
"Continuous energy loss imposes a limit on the stepsize because of the energy dependence of the cross sections. It is generally assumed in MC programs that the particle cross sections are approximately constant along a step, i.e. the step size should be small enough that the change in cross section, from the beginning of the step to the end, is also small. In principle one must use very small steps in order to insure an accurate simulation, however the computing time increases as the stepsize decreases. A good compromise is to limit the stepsize by not allowing the stopping range of the particle to decrease by more than 20 % during the step. This condition works well for particles with kinetic energies , but for lower energies it gives very short stepsizes.
To cure this problem a lower limit on the stepsize was introduced. There is a natural choice for this limit: the stepsize cannot be smaller than the range cut parameter of the program. The stepsize limit varies smoothly with decreasing energy from the value given by the condition to the lowest possible value range cut. These are the default step limitation parameters; they can be overwritten using the UI command ``/process/eLoss/StepFunction 0.2 1 mm'', for example."
This mechanism is handled automatically by G4VEnergyLossProcess.
See also the plot in : http://wwwasd.web.cern.ch/wwwasd/geant4/reports/gallery/electromagnetic/stepping/stepping.html
The 2 parameters of the function which control smoothly the limitation of the step size can be set with the command /process/eLoss/StepFunction
See examples/extended/electromagnetic/TestEm1, for instance the macro run01.mac
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