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.
Notes: 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).
WHAT(1) < 0.0 with |WHAT(1)| not much smaller than primary energy
should generally be avoided. The reason is twofold:
i) tracking accuracy would be spoiled for no substantial
gain in speed, and
ii) FLUKA tracking without MCS does not take into account
the variation of nuclear interaction cross section
with energy.
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).
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).
In pure electromagnetic or low-energy neutron problems,
MCSTHRES does not need to be given and has no effect.
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 (the comment lines shown are allowed input lines):
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+...
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.
