Last version:
FLUKA 2024.1.0, April 30th 2024
(last respin 2024.1.0)
flair-2.3-0e 06-May-2024


-- New Fluka Major Release
( 06.05.2024 )

FLUKA 2024.1.0 is available.
Flair-2.3-0epy3 adapted

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sets the minimum and maximum step size on a region-by-region basis for transport of all charged particles (hadrons, muons and electrons)


     WHAT(1) >= 0.0 : minimum step size in cm (overrides, if larger, region
                      by region the overall minimum step implicitly set by
                      WHAT(2) of MGNFIELD)
              < 0.0 : whatever happens, the location of boundary intersection
                      will be found with an uncertainty =< |WHAT(1)|. Of
                      course, for a given boundary, this value should be
                      applied to both regions defining the boundary.
            Default:  no default

     WHAT(2) = maximum step size in cm
               Default = 100000. cm for a region without magnetic field
                       = 10. cm for a region with magnetic field
                       = twice the minimum step if the latter has been
                         defined larger than the maximum (Not implemented

     WHAT(3) = lower bound of the region indices in which the indicated
               step size is to be applied ("From region WHAT(3)...").
               Default: = 2.0

     WHAT(4) = upper bound of the region indices in which the indicated
               step size is to be applied (" region WHAT(4)...")
               Default: = WHAT(3)

     WHAT(5) = step length in assigning indices. (" steps of
               Default: 1.0

     WHAT(6), SDUM : not used

     Default (option STEPSIZE not given): the above defaults apply in
             all regions (10 cm with magnetic field, 1.E5 cm without)


  • 1) This option differs from EMFFIX and FLUKAFIX for the following main reasons:
  • it is given by region rather than by material
  • therefore, it is effective also in vacuum regions
  • the maximum step is determined in an absolute way (i.e., in cm)
    rather than in terms of maximum energy loss
  • it allows to set not only a maximum but also a minimum step length. This may be necessary in order to avoid the forcing of extremely small steps when a low-energy charged particle spirals in a magnetic field. Option MGNFIELD offers a similar possibility, but not tuned by region.

  • 2) Option STEPSIZE may be essential in and around regions of very small dimensions or in regions where a magnetic field is present (and is rarely required otherwise).

  • 3) The maximum step size for a given region can be decided from the following considerations:
  • in a region with magnetic field it should not be larger than the minimum dimension of the region itself and of its neighbor regions. Obviously, it should also be larger than the minimum step possibly imposed by MGNFIELD or by STEPSIZE itself.
  • in a non-vacuum region, it should not be larger than about one-third of its minimum dimension, in order to allow the multiple-scattering algorithm to work in optimal conditions.


 ASSIGNMAT        2.0      15.0      30.0       5.0       1.0       0.0
 * A magnetic field is present in vacuum regions 15, 20, 25 and 30.
 MGNFIELD        20.0       0.2      0.10       0.0       0.0       0.0
 STEPSIZE       -0.05       0.0      20.0      25.0       0.0       0.0
 STEPSIZE         0.3       0.0      15.0       0.0       0.0       0.0
 * The maximum deviation angle due to magnetic field in any step is set
 * = 20 degrees, and boundary crossings must be identified with an error
 * not larger than 2 mm. If the max. angle constraint forces the step to
 * be shorter than 10 cm, the step will be set = 10 cm in region 20, 25, 30,
 * but will be set = 3 mm in region 15 (WHAT(1) of STEPSIZE overrides the
 * general setting due to WHAT(3) of MGNFIELD). Whatever the size of the
 * step, however, the accuracy of the boundary crossing position must be
 * equal or better than 0.5 mm in regions 20 and 25 (probably contiguous,
 * since the same accuracy must be set for regions on either side of a
 * boundary). The value of the magnetic field will be provided at each
 * step by the user routine MAGFLD.

The same example, name based:

 ASSIGNMAT     VACUUM  FIRSTREG  FOURTREG       5.0       1.0       0.0
 MGNFIELD        20.0       0.2      0.10       0.0       0.0       0.0
 STEPSIZE       -0.05       0.0  SECONREG  THIRDREG       0.0       0.0
 STEPSIZE         0.3       0.0  FIRSTREG       0.0       0.0       0.0

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