-------------------------------- The concept of multiple scattering is an approximation to physical reality (condensed history approximation [Ber63], where charged particles undergo a very large number of single collisions with the atomic electrons, too many to be simulated in detail except in very special cases. All the theoretical treatments which have been developed are valid only within certain limits, and none of them gives rules on how to handle material boundaries and magnetic fields. FLUKA uses an original approach [Fer91a, based on Molière's theory [Mol47,Mol48,Bet53,Mol55], which gives very good results for all charged particles in all circumstances (even in backscattering problems), preserving various angular and space correlations and freeing the user from the need to control the particle step length. Although the default treatment is always more than satisfactory, the user has the possibility to request various kinds of optimisation, for both electrons/positrons and heavy charged particles. This can be done by means of option MULSOPT, which offers also the possibility to switch off completely multiple scattering in selected materials. The latter is a technique used when simulating particle interactions in gases of very low density such as are contained in accelerator vacuum chambers: the simulation is done for a gas of much larger density and the results are scaled to the actual low density: but scaling is meaningful only if no scattering takes place. Another very important feature of option MULSOPT is single scattering, which can be requested in various degrees at boundary crossing, when the limits of Molière's theory are not satisfied, and even all the time (but the latter possibility is to be used only for problems of very low energy, because it is very demanding in CPU time). There is also another option connected with multiple scattering, which however concerns only heavy charged particles such as hadrons and muons: MCSTHRES allows to set a threshold below which multiple Coulomb scattering is not performed. However, the CPU time saved is minimal and the option is not frequently used.
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