FLUKA: LAM-BIAS
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LAM-BIAS
Used to bias the decay length of unstable particles, the inelastic nuclear
interaction length of hadrons, photons, electrons and muons and the
direction of decay secondaries
The meaning of WHAT(1)
...WHAT(6)
depends on the value of SDUM
.
SDUM
= DCDRBIAS and SDUM
= DCY-DIRE are used to activate and define decay
direction biasing; SDUM
= GDECAY selects decay length biasing and inelastic
nuclear interaction biasing; and if SDUM
= blank, decay life biasing and
inelastic nuclear interaction biasing are selected.
Other LAM-BIAS cards with SDUM
= DECPRI, DECALL, INEPRI, INEALL allow to
restrict biasing to primary particles or to extend it also to further
generations.
for SDUM
= DCY-DIRE:
The decay secondary product direction is biased in a direction indicated by
the user by means of a unit vector of components U, V, W (see Notes 4
and 5):
WHAT(1)
= U (x-direction cosine) of the preferred direction to be used in
decay direction biasing
Default
: 0.0
WHAT(2)
= V (y-direction cosine) of the preferred direction to be used in
decay direction biasing
Default
: 0.0)
WHAT(3)
= W (z-direction cosine) of the preferred direction to be used in
decay direction biasing
Default
: 1.0
WHAT(4)
> 0.0: lambda for decay direction biasing. The degree of biasing
decreases with increasing lambda (see Note 5).
= 0.0: a user provided routine (UDCDRL, see 13}) will be called at
each decay event, to provide both direction and lambda for decay
direction biasing
< 0.0 : resets to default (lambda = 0.25)
Default
= 0.25
WHAT(5)
, WHAT(6)
: not used
for SDUM
= DCDRBIAS:
WHAT(1)
> 0.0: decay direction biasing is activated (see Notes 4 and 5)
= 0.0: ignored
< 0.0: decay direction biasing is switched off
WHAT(2)
, WHAT(3)
: not used
WHAT(4)
= lower bound of the particle id-numbers (or corresponding name)
for which decay direction biasing is to be applied
("From particle WHAT(4)
...").
Default
= 1.0.
WHAT(5)
= upper bound of the particle id-numbers (or corresponding name)
for which decay direction biasing is to be applied
("...to particle WHAT(5)
...").
Default
= WHAT(4)
if WHAT(4)
> 0, 64 otherwise.
WHAT(6)
= step length in assigning numbers. ("...in steps of WHAT(6)
").
Default
= 1.0.
for all other SDUM
's:
WHAT(1)
: biasing parameter for decay length or life, applying only to
unstable particles (with particle numbers >= 8). Its meaning
differs depending on the value of SDUM
, as explained in the
following.
for SDUM
= GDECAY:
WHAT(1)
< 0.0 : the mean DECAY LENGTH (in cm) of the particle in the
LABORATORY frame is set = |WHAT(1)
| if smaller than
the physical decay length (otherwise it is left
unchanged). At the decay point sampled according to
the biased probability, Russian Roulette (i.e.
random choice) decides whether the particle actually
will survive or not after creation of the decay
products. The latter are created in any case and
their weight adjusted taking into account the ratio
between biased and physical survival probability.
> 0.0 : the mean DECAY LENGTH (in cm) of the particle in the
LABORATORY frame is set = WHAT(1)
if smaller than
the physical decay length (otherwise it is left
unchanged). Let P_u = unbiased probability and
P_b = biased probability: at the decay point sampled
according to P_b, the particle always survives
with a reduced weight W(1 - P_u/P_b), where W is the
current weight of the particle before the decay. Its
daughters are given a weight W P_u/P_b (as in
case WHAT(1)
< 0.0).
= 0.0 : ignored
for SDUM
= blank:
-1 < WHAT(1)
< 0. : the mean LIFE of the particle in its REST frame
is REDUCED by a factor = |WHAT(1)
|. At the decay
point sampled according to the biased
probability, Russian Roulette (i.e. random
choice) decides whether the particle actually
will survive or not after creation of the decay
products. The latter are created in any case and
their weight adjusted taking into account the
ratio between biased and physical survival
probability.
0 < WHAT(1)
< +1. : the mean LIFE of the particle in the REST frame
is REDUCED by a factor = WHAT(1)
. At the decay
point sampled according to the biased
probability, the particle always survives with
a reduced weight. Its daughters are given the
same weight.
|WHAT(1)
| > 1 : a possible previously given biasing parameter
is reset to the default value (no biasing)
WHAT(1)
= 0 : ignored
WHAT(2)
: biasing factor for hadronic inelastic interactions
-1 < WHAT(2)
< 0. : the hadronic inelastic interaction length of the
particle is reduced by a multiplying factor |WHAT(2)
|.
At the interaction point sampled according to
the biased probability, Russian Roulette (i.e.
random choice) decides whether the particle actually
will survive or not after creation of the
secondaries products. The latter are created in
any case and their weight adjusted taking into
account the ratio between biased and physical
survival probability.
0. < WHAT(2)
< 1. : the hadronic inelastic interaction length of the
particle is reduced by a multiplying factor WHAT(2)
,
At the interaction point sampled according to
the biased probability, the particle always
survives with a reduced weight. The secondaries
are created in any case and their weight
adjusted taking into account the ratio between
biased and physical survival probability.
= 0.0 : ignored
WHAT(2)
>= 1.0 : a possible previously set biasing factor is
reset to the default value of 1 (no biasing).
WHAT(3)
: If > 2.0 : number or name of the material to which the inelastic
biasing factor has to be applied.
< 0.0 : resets to the default a previously assigned value
= 0.0 : ignored if a value has been previously assigned to
a specific material, otherwise all materials (default)
0.0 < WHAT(3)
=< 2.0 : all materials.
WHAT(4)
= lower bound of the particle id-numbers (or corresponding name)
for which decay or inelastic interaction biasing is to be applied
("From particle WHAT(4)
...").
Default
= 1.0.
WHAT(5)
= upper bound of the particle id-numbers (or corresponding name)
for which decay or inelastic interaction biasing is to be applied
("...to particle WHAT(5)
...").
Default
= WHAT(4)
if WHAT(4)
> 0, 64 otherwise.
WHAT(6)
= step length in assigning numbers. ("...in steps of WHAT(6)
").
Default
= 1.0.
for SDUM
= DECPRI, DECALL, INEPRI, INEALL:
SDUM
= DECPRI: decay biasing, as requested by another LAM-BIAS card with
SDUM
= GDECAY or blank, must be applied only to primary
particles.
= DECALL: decay biasing, as requested by another LAM-BIAS card with
SDUM
= GDECAY or blank, must be applied to all
generations (default).
= INEPRI: inelastic hadronic interaction biasing, as requested by
another LAM-BIAS card with SDUM
= blank, must be applied
only to primary particles.
= INEALL: inelastic hadronic interaction biasing, as requested by
another LAM-BIAS card with SDUM
= blank, must be applied
to all generations (default)
Default
(option LAM-BIAS not given): no decay length, decay direction, or
inelastic interaction biasing
Notes:
1) Option LAM-BIAS can be used for three different kinds of biasing:
a) biasing of the particle decay length (or life),
b) biasing of the direction of the decay secondaries, and
c) biasing of the inelastic hadronic interaction length.
2) Depending on the SDUM
value, two different kinds of biasing are
applied to the particle decay length (or life).
In both cases, the particle is transported to a distance
sampled from an imposed (biased) exponential distribution:
If WHAT(1)
is positive, decay products are created, but the
particle survives with its weight and the weight of its
daughters is adjusted according to the ratio between the biased
and the physical survival probability at the sampled
distance. If WHAT(1)
is negative, decay is performed and the
weight of the daughters is set according to the biasing, but the
survival of the primary particle is decided by Russian
Roulette according to the biasing. Again, the weights are adjusted
taking the bias into account.
3) The laboratory decay length corresponding to the selected
mean decay life is obtained by multiplication by BETA*GAMMA*c.
4) Decay direction biasing is activated by a LAM-BIAS card
with SDUM
= DCDRBIAS. The direction of decay secondaries is
sampled preferentially close to the direction specified by
the user by means of a second LAM-BIAS card with SDUM
= DCY-DIRE.
5) The biasing function for the decay direction is of the form
exp{-[1-cos(theta)]/lambda}
where theta is the polar angle between the sampled direction and the
preferential direction (transformed to the centre of mass reference
system). The degree of biasing is largest for small positive values
of lambda (producing direction biasing strongly peaked along the
direction of interest) and decreases with increasing lambda. Values
of lambda >= 1.0 result essentially in no biasing.
6) Biasing of hadronic inelastic interaction length can be done either
in one single material (indicated by WHAT(3)
) or in all materials
(default). No other possibility is foreseen for the moment.
7) When choosing the Russian Roulette alternative, it is
suggested to set also a weight window (cards WW-FACTOr and
WW-THRESh) in order to avoid too large weight fluctuations.
8) Reduction factors excessively large can result in an abnormal
increase of the number of secondaries to be loaded on the stack,
especially at high primary energies. In such cases, FLUKA issues
a message that the secondary could not be loaded because of a
lack of space. The weight adjustment is modified accordingly
(therefore the results are not affected) but if the number of
messages exceeds a certain limit, the run is terminated.
9) Biasing of the hadronic inelastic interaction length can be
applied also to photons, electrons and positrons (provided option
PHOTONUC is also requested with the relevant parameters) and muons
(provided option MUPHOTON is also requested); actually, it is often
a good idea to do this in order to increase the probability of
photonuclear interaction.
10) For photons, a typical reduction factor of the hadronic inelastic
interaction length is the order of 0.01-0.05 for a shower initiated
by 1 GeV photons or electrons, and of 0.1-0.5 for one at 10 TeV.
For electrons and positrons, a typical reduction factor is about
5.E-4
Examples (number based):
LAM-BIAS -3.E+3 1. 1. 13. 16. 0.GDECAY
LAM-BIAS 0.0 0.02 11. 7. 0. 0.INEPRI
The same examples, name based:
LAM-BIAS -3.E+3 1. 1. PION+ KAON- 0.GDECAY
LAM-BIAS 0.0 0.02 11. PHOTON 0. 0.INEPRI
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