Stepper accuracy: parameters involved¶

• ExN02MagneticField.cc
  • Inside the class constructor we can find the following accuracy parameters:
    • delta one step: 1.0e-6 nm
    • delta intersection: 1.0e-9 nm
    • minimum step size: 1.0e-2 mm
    • epsilon min and epsilon max appear commented out, which implies that their default values might be used (see below).
    • the same goes for miss distance.

• Default values
  • These can be found at geometry/magneticfield/src/G4FieldManager.cc
    • This file provides the implementation of the G4FieldManager class. Setters and getters for deltas and epsilons are defined.
    • Looking at the constructors, we can observe the default values in use:
      • fDefault_Delta_One_Step_Value: 0.01 mm
      • fDefault_Delta_Intersection_Val: 0.001 mm
      • fEpsilonMinDefault: 5.0e-5 mm (units are missing, but these slides show G4's default units)
      • fEpsilonMaxDefault: 0.001 mm (units missing for this one as well)
  • geometry/magneticfield/src/G4ChordFinder.cc
    • Implementation of class G4ChordFinder (where the miss distance is set)
    • Again, the constructors specify the default value for this parameter:
      • fDefaultDeltaChord: 0.25 mm
  • stepMax is set to 100 mm, by default, during the detector construction (see source:N02to/src/ExN02DetectorConstruction.cc@0ab505d2#L206).
    • G4UserLimits class uses DBL_MAX as default value, but this is never used in our code.

• Further details
  • These slides briefly explain the purpose of the epsilon max parameter:
    The MaximumEpsilonStep parameter is the relative accuracy for the endpoint of 'ordinary' integration steps, those which do not intersect a volume boundary. Limits estimated error |Delta x| of endpoint of each physics step (of length len): |Delta x| < Epsilon * len
  • The other set of slides mentioned above contain the following default units, which might eventually come in handy:
    • Length: mm
    • Time: ns
    • Energy: MeV
    • Angle: Radian
    • Temperature: Kelvin
  • All units are defined at source/externals/clhep/include/CLHEP/Units/SystemOfUnits.h (which also specifies the basic (default) units)
• References
  • Slides showing default units: http://www.tifr.res.in/~gobinda/inosim/geant_ino2013.pdf
  • Short explanation of epsilon max parameter: http://geant4.lngs.infn.it/corso_infn/slides/field.pdf
  • John's talk on propagation in the field in Geant4: http://geant4.in2p3.fr/2007/prog/JohnApostolakis/EMField.pdf