Last version:
FLUKA 2023.3.3, January 31st 2024
(last respin 2023.3.3)
flair-2.3-0d 13-Sep-2023

News:

-- Fluka Release
( 31.01.2024 )

FLUKA 2023.3.3 has been released.
Next FLUKA Course
The 23rd FLUKA course
will be held at the Lanzhou University, China, on June 1-8, 2024

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# LOW-DOWN

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

```     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
```