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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....+...
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

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