requests non-analogue absorption and/or an energy cut-off during
low-energy neutron transport on a region by region basis
(see also PART-THR, LOW-NEUT)
WHAT(1) > 0.0 : group cut-off (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 cut-off)
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 = 72 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 73), 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 NMGP (usually 72), i.e. the
number of the last group (usually a thermal group).
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 in which the indicated
neutron cut-off and/or survival parameters apply
("From region WHAT(4)...")
Default = 2.0.
WHAT(5) = upper bound of the region indices in which the indicated
neutron cut-off 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.
Note: the groups are numbered in DECREASING energy order (see 10}
for a detailed description). Setting a group cut-off larger
than the last group number (e.g. 73 when using a 72-group
cross section set) results in all neutrons been transported,
i.e. no cut-off is applied.
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).
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.
In some programs like 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 group. (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.
In selecting a forced survival probability for the thermal
neutron group, 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 group in
order to limit the length of histories, but not if thermal
neutron effects are of particular interest.
Concerning the other energy groups, if there is interest in
low-energy neutron effects, the survival probability for
energy groups above thermal 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.
Use of a survival probability equal or larger 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 and WW-FACTOr.
Example:
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+...
LOW-BIAS 60.0 47.0 0.95 5.0 19.0 0.0
LOW-BIAS 73.0 72.0 0.82 7.0 15.0 4.0
* Note that the second LOW-BIAS card overrides the settings of the first one
* concerning regions 7, 11 and 15. Therefore, we will have an energy cutoff
* equal to the upper edge of the 60th group (61.442 eV in the standard FLUKA
* neutron library) in regions 5,6,8,9,10,12,13,14,16,17,18 and 19. In these
* same regions, analogue neutron absorption is requested down to an energy
* equal to the upper edge of group 47 (15.034 keV in the standard library),
* and biased absorption, with a fixed probability of 95%, at lower energies.
* In regions 7, 11 and 15, no cutoff is applied (supposing we are using the
* standard 72-group library), and non-analogue absorption is requested only
* for group 72 (the thermal group in our case), with a probability of 82%.
