FLUKA: LOW-BIAS
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LOW-BIAS
requests non-analogue absorption and/or an energy cutoff during
low-energy neutron transport on a region by region basis
See also PART-THR, LOW-NEUT
WHAT(1)
> 0.0 : group cutoff (neutrons in energy groups with number
>= WHAT(1)
are not transported).
This value can be overridden in the user routine UBSSET
(argument IGCUTO in the calling list, see 13})
Default
= 0.0 (no cutoff)
WHAT(2)
> 0.0 : group limit for non-analogue absorption (neutrons in
energy groups >= WHAT(2)
undergo non-analogue absorption)
This value can be overridden in the user routine UBSSET
(argument IGNONA in the calling list, see 13})
Non-analogue absorption is applied to the NMGP-WHAT(2)
+1
groups with energies equal or lower than those of
group WHAT(2)
if WHAT(2)
is not > NMGP, otherwise it
isn't applied to any group (NMGP is the number of
neutron groups in the cross section library used:
it is = 260 in the standard FLUKA neutron library)
Default
: if option DEFAULTS is used with SDUM
= CALORIMEtry,
ICARUS, NEUTRONS or PRECISIOn, the default is = NMGP+1
(usually 261), meaning that non-analogue absorption is
not applied at all.
If DEFAULTS is missing, or is present with any other
SDUM
value, the default is the number of the first thermal
group (usually 230).
WHAT(3)
> 0.0 : non-analogue SURVIVAL probability. Must be =< 1.
This value can be overridden in the user routine UBSSET
(argument PNONAN in the calling list, see 13})
Default
: if option DEFAULTS is used with SDUM
= EET/TRANsmut,
HADROTHErapy, NEW-DEFAults or SHIELDINg, the default
is = 0.95.
If DEFAULTS is missing, or is present with any other
SDUM
value, the default is 0.85.
WHAT(4)
= lower bound of the region indices (or corresponding name) in
which the indicated neutron cutoff and/or survival parameters
apply
("From region WHAT(4)
...")
Default
= 2.0.
WHAT(5)
= upper bound of the region indices (or corresponding name) in
which the indicated neutron cutoff and/or survival parameters
apply
("...to region WHAT(5)
...")
Default
= WHAT(4)
WHAT(6)
= step length in assigning indices. ("...in steps of
WHAT(6)
").
Default
= 1.
SDUM
: not used
Default
(option LOW-BIAS not given): the physical survival probability
is used for all groups excepting thermal ones, which are assigned
a probability of 0.85. However, if option DEFAULTS has been
issued with SDUM
= EET/TRANsmut, HADROTHErapy, NEW-DEFAults or
SHIELDINg, this default value is changed to 0.95.
If SDUM
= CALORIMEtry, ICARUS, NEUTRONS or PRECISIOn, the default
is physical survival probability for all groups, including thermal.
Notes:
1) The groups are numbered in DECREASING energy order (see 10}
for a detailed description). Setting a group cutoff larger
than the last group number (e.g. 261 when using a 260-group
cross section set) results in all neutrons been transported,
i.e. no cutoff is applied.
2) Similarly, if WHAT(2)
is set larger than the last group number,
non-analogue neutron absorption isn't applied to any group (this is
recommended for calorimetry studies and all cases where fluctuations
and correlations are important).
3) The survival probability is defined as 1 - (Sigma_abs/Sigma_T)
where Sigma_abs is the inverse of the absorption mean free
path and Sigma_T the inverse of the mean free path for
absorption plus scattering (total macroscopic cross section).
The LOW-BIAS option allows the user to control neutron
transport by imposing an artificial survival probability
and corrects the particle weight taking into account the
ratio between physical and biased survival probability.
4) In some programs (e.g., MORSE) the survival probability is always
forced to be = 1. In FLUKA, if the LOW-BIAS option is not
chosen, the physical survival probability is used for all
non-thermal groups, and the default 0.85 is used for the
thermal groups. (This exception is to avoid endless thermal
neutron scattering in materials with low thermal neutron
absorption cross section). To get the physical survival
probability applied to ALL groups, as needed for fully analogue
calculations, the user must use LOW-BIAS with WHAT(2)
larger
than the last group number.
5) In selecting a forced survival probability for the thermal
neutron groups, the user should have an idea of the order of
magnitude of the actual physical probability. The latter can
take very different values: for instance it can range between
a few per cent for thermal neutrons in Boron-10 to about
80-90% in Lead and 99% in Carbon. The choice will be often for
small values of survival probability in the thermal groups in
order to limit the length of histories, but not if thermal
neutron effects are of particular interest.
6) Concerning the other energy groups, if there is interest in
low-energy neutron effects, the survival probability for
energy groups above thermals in non-hydrogenated materials
should be set at least = 0.9, otherwise practically no
neutron would survive enough collisions to be slowed down. In
hydrogenated materials, a slightly lower value could be
acceptable. Setting less than 80% is likely to lead to
erroneous results in most cases.
7) Use of a survival probability equal or smaller than the
physical one is likely to introduce important weight
fluctuations among different individual particles depending
on the number of collisions undergone. To limit the size of
such fluctuations, which could slow down statistical
convergence, it is recommended to define a weight window by
means of options WW-THRESh, WW-FACTOr and WW-PROFIle.
Example (number based):
LOW-BIAS 60.0 47.0 0.95 5.0 19.0 0.0
LOW-BIAS 261.0 230.0 0.82 7.0 15.0 4.0
The same example, name based:
LOW-BIAS 60.0 47.0 0.95 FifthReg Nineteen 0.0
LOW-BIAS 261.0 230.0 0.82 RegSeven Fifteen 4.0
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