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7.8} Multiple Coulomb scattering

 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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