biases the downscattering probability during low energy neutron
transport on a region by region basis
(see also LOW-NEUT, LOW-BIAS)
WHAT(1) > 0.0 : group limit for biased downscattering probability.
Neutrons in energy groups IG >= WHAT(1) are given
downscattering importances = WHAT(2)**(IG-WHAT(1)).
This value can be overridden in the user routine UBSSET
(argument IGDWSC in the calling list, see 13})
Default = 0.0 (no biased downscattering)
WHAT(2) = biasing factor for down-scattering from group IG-1 into group IG
This value can be overridden in the user routine UBSSET
(argument FDOWSC in the calling list, see 13})
Default = 1.5
WHAT(3) : not used
WHAT(4) = lower bound of the region indices to which downscattering
biasing is to be applied ("From region WHAT(4)...").
Default: = 2.0
WHAT(5) = upper bound of the region indices to which downscattering
biasing is to be applied ("...to region WHAT(5)...")
Default: = WHAT(4)WHAT(6) = step length in assigning indices. ("...in steps of
WHAT(6)").
Default: 1.0
SDUM : not used
Default (option LOW-DOWN not given): no downscatter biasing
Note: this option can be useful only in very particular problems,
for instance to calculate the response of instruments
based on moderation (Bonner spheres, rem-counters).
Very powerful but also very dangerous, can lead to errors
of orders of magnitude if not used by experts.
U S E W I T H T H E M A X I M U M C A R E !!!!
P A R T I C U L A R L Y F O R H Y D R O G E N A -
T E D M A T E R I A L S !!!!
The groups are numbered in DECREASING energy order (see 10}
for a detailed description).
When this option is used, the natural probabilities of scatter
from group I to group J, P(I-->J), are altered by an importance
factor V(J). Selection of the outgoing group L is made from
a biased distribution function P(I-->J)*V(J) with an associated
weight correction.
Example:
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+...
LOW-DOWN 38.0 1.1 0.0 4.0 25.0 0.0
* In all regions from 4 to 25, neutron downscattering is biased from
* group 38 to last. Assuming we are using the standard FLUKA library with
* 72 groups, that means all energies below 273.24 keV. In group 38, the
* downscattering relative importance is set equal to 1.1, in group 39 to
* 1.1**2 = 1.21, in group 40 to 1.1**3 = 1.331 etc.
