LOW-DOWN

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)**(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 (or corresponding name) to
               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) 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

Notes:

1) 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 !!!!
            E S P E C I A L L Y   W I T H   H Y D R O G E N A T E D
                             M A T E R I A L S !!!!

2) The groups are numbered in DECREASING energy order (see (10) for a detailed description).

3) 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 (number based):

 *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8
 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....+....8
 LOW-DOWN        38.0       1.1       0.0   RegFour  The25Reg       0.0

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