FLUKA: PAIRBREM
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PAIRBREM
controls simulation of pair production and bremsstrahlung by
high-energy muons, charged hadrons and light ions (up to alpha's)
See also MUPHOTON
WHAT(1)
= 0.0 : ignored
= 1.0 : pair production by muons and charged hadrons is
activated
= 2.0 : bremsstrahlung by muons and charged hadrons is
activated
= 3.0 : pair production and bremsstrahlung are both
activated
= -1.0 : pair production is inhibited
= -2.0 : bremsstrahlung is inhibited
= -3.0 : pair production and bremsstrahlung are both
inhibited
Default
= 3.0 (both pair production and bremsstrahlung are
activated).
However, if DEFAULTS is present with SDUM
= EET/TRANsmut,
EM-CASCAde, NEUTRONS or SHIELDINg, the default is = -3.0
(pair production and bremsstrahlung inhibited)
WHAT(2)
>= 0.0 : e+, e- kinetic energy threshold (in GeV) for
explicit pair production. A value of 0.0 is meaningful
(it does mean zero energy) and is even recommended.
< 0.0 : no explicit pair production (overrides a possible
non-negative value set in a previous PAIRBREM
card), but the energy loss due to pair production is
taken into account in a continuous approximation.
Default
= -1.0 (no explicit pair production).
However, if DEFAULTS is not present, or is present
with SDUM
= CALORIMEtry, ICARUS, NEW-DEFAults or
PRECISIOn, the default is 0.0 (explicit pair production
with zero threshold).
WHAT(3)
> 0.0 : photon energy threshold (GeV) for explicit
bremsstrahlung production
=< 0.0 : no explicit bremsstrahlung production is simulated
(it overrides a possible positive value set in a
previous PAIRBREM card), but the energy loss due to
bremsstrahlung is taken into account in a
continuous approximation
Default
= -1.0 (no explicit bremsstrahlung production)
However, if DEFAULTS is not present, or is present
with SDUM
= NEW-DEFAults, the default is 0.001 (explicit
bremsstrahlung production with threshold 1 MeV).
If DEFAULTS has SDUM
= CALORIMEtry, ICARUS or PRECISIOn,
the default is 0.0003 (explicit bremsstrahlung production
with threshold 300 keV).
WHAT(4)
= lower bound of the indices of the materials in which the
indications given by WHAT(1)
...WHAT(3)
apply
("From material WHAT(4)
...")
Default
= 3.0
WHAT(5)
= upper bound of the indices of the materials in which the
indications given by WHAT(1)
...WHAT(3)
apply
("... to material WHAT(5)
...")
Default
= WHAT(4)
WHAT(6)
= step length in assigning indices
("...in steps of WHAT(6)
")
Default
= 1.0
SDUM
= reserved for program development
Default
(option PAIRBREM not given): if DEFAULTS is not present, or is
present with SDUM
= NEW-DEFAults, both pair production and
bremsstrahlung are activated in all materials, with explicit
generation of secondaries of energy >= 0 and >= 1 MeV, respectively.
If DEFAULTS is present with SDUM
= CALORIMEtry, ICARUS or
PRECISIOn, both pair production and bremsstrahlung are activated in
all materials, with explicit generation of secondaries of energy
>= 0 and >= 300 keV, respectively.
If DEFAULTS is present with SDUM
= EET/TRANsmut, EM-CASCAde,
NEUTRONS or SHIELDINg, pair production and bremsstrahlung are
inhibited.
In any other case, both pair production and bremsstrahlung are
activated in all materials, without explicit generation of
secondaries (continuous loss approximation)
Notes:
1) Initialisation of bremsstrahlung and pair production by muons,
charged hadrons and light ions (up to alpha's) is very demanding
in computer time. On the other hand, these effects must be taken
into account for a correct simulation at high energy. It is
suggested to inhibit them in the phase of input preparation and
debugging, but to activate them again in production runs (in
long runs the time taken by initialisation is of course a
smaller fraction of total time). In pure electron-photon
problems and in low-energy hadron problems, the effects
should be inhibited (WHAT(3)
= -3.0), unless this is already
ensured by the chosen DEFAULTS option.
2) When setting a threshold for pair and bremsstrahlung
production by muons, charged hadrons and light ions (up to alphas),
the following considerations should be taken into account:
- photon production threshold (WHAT(3)
) must be of course not
lower than the photon transport cutoff as set by EMFCUT
or by the chosen default. (In general it will be reasonable to
set it equal to it)
- on the contrary, the electron and positron production threshold
(WHAT(2)
) should in general be set = 0.0, whatever the
electron transport cutoff, unless PHOTON transport cutoff
is set higher than 511 keV. If the positron is produced
with an energy lower than electron transport cutoff, it
will be forced to annihilate at the point of production, but
the two 511 keV annihilation photons will be generated correctly.
3) If option PAIRBREM is not activated, by default FLUKA
treats both bremsstrahlung and pair production by muons
and charged hadrons as continuous energy losses (i.e., without
generating secondaries and depositing their energy at the
point of production). This will reproduce correctly the
average ranges but not the straggling and the dose
distributions. A similar approach is found in A. Van
Ginneken's code CASIM [Van75,Van86,Qia87].
4) Virtual photonuclear interactions by high-energy muons are
controlled by option MUPHOTON.
5) Here are the settings for pair and bremsstrahlung production by
high-energy muons and charged hadrons, corresponding to available
DEFAULTS options:
6) CALORIMEtry, ICARUS, PRECISIOn: pair production is activated in all
materials, with explicit generation of secondaries of any energy;
bremsstrahlung is also activated in all materials, with explicit
generation of photons having energy >= 300 keV
NEW-DEFAults, or DEFAULTS missing: pair production is activated in
all materials, with explicit generation of secondaries of any
energy; bremsstrahlung is also activated in all materials, with
explicit generation of photons having energy >= 1 MeV.
Any other SDUM
value: both pair and bremsstrahlung production are
activated in all materials, without explicit generation of
secondaries (continuous loss approximation)
Example 1:
PAIRBREM 3.0 0.0 0.0012 4.0 10.0 3.0
Example 2:
PAIRBREM 3.0 -1.0 -1.0 6.0 7.0
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