LOW–DOWN¶

Summary

Biases the downscattering probability during low-energy neutron transport on a region by region basis

See also LOW-BIAS, LOW–NEUT

WHAT(1) > 0.0:: group limit for biased downscattering probability.

Neutrons in energy groups IG ≥WHAT(1) are given downscattering importances = WHAT(2)\(^{\texttt{IG-WHAT(1)}}\).

This value can be overridden in user routine UBSSET by assigning a value to variable IGDWSC.
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 user routine UBSSET by assigning a value to variable FDOWSC.
Default: = 1.5
WHAT(3) :: not used
WHAT(4) =: lower bound of the region indices (or corresponding name) in which downscattering biasing is to be applied

(“From region ``WHAT(4)``…”)
Default: = 2.0
WHAT(5) =: upper bound of the region indices (or corresponding name) in 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

Notes

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, it can lead to errors of orders of magnitude if not used by experts.

Summary

!!!! Use with the maximum care !!!!

!!!! especially with hydrogenated materials !!!!

The groups are numbered in decreasing energy order (see Low-energy neutrons in FLUKA for a detailed description).
When this option is used, the natural probabilities of scatter from group I to group J, P(I \(\rightarrow`J), are altered by an importance factor ``V(J)`\). Selection of the outgoing group J is made from a biased distribution function P(I\(\rightarrow\)J)\(\cdot\)V(J) with an associated weight correction.

Example (number based):

*...+....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
*  260 groups, that means all energies below 7.985162 MeV. 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.

The same example, name based:

*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+...
LOW-DOWN        38.0       1.1       0.0   RegFour  The25Reg       0.0