---------------- 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.
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