CERN homepage INFN homepage
FLUKA: 7.9} Step length Previous Index Next

7.9} Step length

 Another aspect of the condensed history approximation is that charged particle
 transport is performed in steps. The finite fraction of the particle energy
 which is lost and deposited in matter in each step is an approximation for the
 sum of innumerable tiny amounts of energy lost by the particle in elastic and
 inelastic collisions.

 In early Monte Carlo programs results could depend critically on the size of
 the step, mainly due to the inaccurate determination of the path length
 correction (ratio between the length of the actual wiggling path of the
 particle and that of the straight step connecting the two endpoints). For a
 more complete discussion, see [Aar93a,Fas01]. The multiple scattering algorithm
 used by FLUKA [Fer91a] provides a robust independence of the results from the
 step size, but for problems where a special accuracy is requested, or when
 magnetic fields are present, it is possible for the user to override the
 default step length. Two options control the maximum fractional energy loss per
 step: EMFFIX for electrons and positrons, and FLUKAFIX for muons and charged
 hadrons. The second one is seldom used, however, except in problems of very
 large dimensions typical of cosmic ray research. Option STEPSIZE is used
 instead to limit the absolute length of the step, independent of the energy
 lost. Contrary to EMFFIX and FLUKAFIX, it works also in vacuum. While its use
 is highly recommended in problems with magnetic fields, to ensure that steps be
 smaller than the dimensions of the current regions and of those that border it,
 when no magnetic fields are present this option should better be avoided, as it
 would imply no obvious advantage and could even downgrade performance.

Previous Index Next