MCSTHRESh

Defines some of the accuracy requirements for Multiple Coulomb Scattering (MCS) of heavy charged particles (hadrons and muons).

See also MULSOPT

     WHAT(1) >= 0.0 : detailed multiple Coulomb scattering for primary
                      charged hadrons and muons down to the minimum energy
                      allowed by Molière's theory
              < 0.0 : detailed multiple Coulomb scattering for primary
                      charged hadrons and muons down to a kinetic energy
                      equal to |WHAT(1)| (GeV)
              Default = 1.0 if DEFAULTS is present with SDUM = CALORIMEtry,
                      HADROTHErapy, ICARUS or PRECISIOn.
                      If SDUM = EET/TRANsmut, the default is = -0.01 (transport
                      of primaries with multiple Coulomb scattering down to
                      10 MeV).
                      With any other SDUM value, or if DEFAULTS is missing, the
                      default is = -0.02 (transport of secondaries with multiple
                      Coulomb scattering down to 20 MeV).

     WHAT(2) >= 0.0 : detailed multiple Coulomb scattering for secondary
                      charged hadrons and muons down to the minimum energy
                      allowed by Molière's theory
              < 0.0 : detailed multiple Coulomb scattering for secondary
                      charged hadrons and muons down to a kinetic energy
                      equal to |WHAT(2)| (GeV)
              Default = 1.0 if DEFAULTS is present with SDUM = CALORIMEtry,
                      HADROTHErapy, ICARUS or PRECISIOn.
                      If SDUM = EET/TRANsmut, NEW-DEFAults or SHIELDINg, the
                      default is = -0.02 (transport of secondaries with multiple
                      Coulomb scattering down to 20 MeV).
                      With any other SDUM value, or if DEFAULTS is missing, the
                      default is = -1.0 (transport of secondaries with multiple
                      Coulomb scattering down to 1 GeV).

     WHAT(3), WHAT(4), WHAT(5), WHAT(6) : not used

     SDUM :    not used

     Default (option MCSTHRES not given): the defaults depend on option DEFAULTS
             as explained above and in Note 6.

Notes:

1) The MCSTHRES option is not often used, since option DEFAULTS ensures the MCS parameter setting most appropriate for a wide range of problems. In most cases, it is suggested to have multiple Coulomb scattering fully activated for both primary and secondary particles over the whole energy range. This corresponds to using WHAT(1) >= 0.0 and WHAT(2) >= 0.0 (or at least WHAT(2) < 0.0 with an absolute value much smaller than beam energy).

2) WHAT(1) < 0.0 with |WHAT(1)| not much smaller than primary energy should generally be avoided. The reason is twofold: a) tracking accuracy would be spoiled for no substantial gain in speed b) FLUKA tracking without MCS does not take into account the variation of nuclear interaction cross section with energy.

3) However, there are some cases where it can be useful to set WHAT(1) and/or WHAT(2) to a negative number with absolute value LARGER than beam energy. In this case no MCS is performed at all, but tracking and maximum energy loss per step are controlled anyway by the most sophisticated transport algorithm available (see FLUKAFIX, STEPSIZE). Complete suppression of multiple scattering can be useful in some particular cases, for instance when replacing a gas of extremely low density by a gas of the same composition but of much larger density in order to increase the frequency of inelastic interactions (of course, the results must then be scaled by the density ratio). In such a case, one should also select the biased density so that no re-interaction of secondaries can take place. An alternative way to switch off completely multiple Coulomb scattering of hadrons and muons is to use MULSOPT with WHAT(2) >= 3.0 (MULSOPT, however, can deal also with electrons and positrons, while MCSTHRES can't; on the other hand, MULSOPT does not allow to distinguish between primary and secondary particles).

4) In order to get the most accurate treatment of Multiple Coulomb Scattering, a step optimisation and higher order corrections can be requested by option MULSOPT (but with an important increase in CPU time).

5) In pure electromagnetic or low-energy neutron problems, MCSTHRES does not need to be given and has no effect.

6) Here are the MCS settings corresponding to available DEFAULTS SDUM options:

               CALORIMEtry, HADROTHErapy, ICARUS, PRECISIOn: Multiple scattering
                  threshold at minimum allowed energy both for primary and
                  secondary charged particles
               EET/TRANsmutation: MCS threshold = 10 MeV for primaries and
                  20 MeV for secondaries
               NEW-DEFAults, SHIELDINg: 20 MeV threshold for both primaries and
                  secondaries
               Any other SDUM value, or DEFAULTS missing: 20 MeV for primaries
                  and 1 GeV for secondaries

Example:

 *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8
 BEAM           120.0       0.0       0.0       0.0       0.0      1.0 PION+
 MCSTHRES         1.0     -0.01       0.0       0.0       0.0      0.0
 *         In this example, the primary beam consists of 120 GeV/c pi+
 *         mesons which are transported by simulating accurately multiple
 *         Coulomb scattering at all energies. For the secondary hadrons
 *         generated, MCS is performed instead only until they reach 10 MeV.

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