Last version:
FLUKA 2024.1.0, April 30th 2024
(last respin 2024.1.0)
flair-2.3-0e 06-May-2024


-- New Fluka Major Release
( 06.05.2024 )

FLUKA 2024.1.0 is available.
Flair-2.3-0epy3 adapted

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defines extra factors dependent on energy group ("profiles"), to modify the basic setting of the low-energy neutron weight-windows in selected sets of regions, or the low-energy neutron importances in each region


     WHAT(1) = weight-window extra factor for the profile defined by
               WHAT(6), concerning the energy groups defined by WHAT(3),
               WHAT(4) and WHAT(5) (both the top and bottom window levels
               will be multiplied by WHAT(1))
             = 0.0 : ignored
             < 0.0 : resets to default (extra factor = 1.0)
               Default = 1.0 (windows are not modified)

     WHAT(2) = importance extra factor. See Note 3).
             = 0.0 : ignored
             < 0.0 : resets to default (extra factor = 1.0)
               Default = 1.0 (importances are not modified)

     WHAT(3) = lower bound of the group number for which the extra factor
               WHAT(1) or WHAT(2) is requested ("From group WHAT(3)...")
               Default = 1.0 (the group of highest energy)

     WHAT(4) = upper bound of the group numbers for which the extra
               factor WHAT(1) or WHAT(2) is requested (" group
               Default = WHAT(3)

     WHAT(5) = step length in assigning group numbers (" steps of
               Default = 1.0

     WHAT(6) = profile number defined by WHAT(1), WHAT(3-5) (up to 5
               different profiles are allowed).
               Default: profile number 1.

     SDUM :    not used

     Default (option WW-PROFIle not given): 1.0 (no extra factor)


  • 1) This option applies only to low-energy neutrons. It is used to refine the basic bias setting defined by two other options: WW-FACTOr and BIASING.

  • 2) WHAT(1) refers to WW-FACTOr: it allows the user to tune the weight window by energy group (WW-FACTOr does the same by region). The profile defined will be applied to raise or lower the weight-window levels (for low-energy neutrons only) in a group of regions selected by means of WHAT(4-6) and SDUM in option WW-FACTOr.

  • 3) WHAT(2) refers to BIASING: its aim is to define a reference weight level in each region, which is used by the program to avoid excessive biasing in some critical cases. If the user has defined a weight-window (options WW-FACTOr and WW-THRESh), the reference weight level is not needed because it is derived directly from the window parameters. If the user has not defined a weight-window but has defined region importances (option BIASING), the reference weight level for a region is assumed in most cases to be the inverse of the corresponding importance. However, since importance biasing is not based on absolute values of importance but on importance ratios, in some rare cases the user may give importances which are not equal to the inverse of the average particle weight, but only proportional to it. (This is in order to better exploit the full importance range, since for technical reasons in FLUKA allowed importance values range only from 0.0001 to 10000.). In such cases it is possible to multiply all the importances by a factor WHAT(2) ONLY FOR THE PURPOSE OF CALCULATING THE REFERENCE WEIGHT LEVEL. Modification of importances by a factor WHAT(2) apply to ALL regions (but only for low-energy neutrons). If neither weight-window nor importances have been given, FLUKA still calculates a weight reference level from the ratio of physical to biased non-absorption probability. If a particle's weight exceeds the reference level in a given region by more than a factor calculated at run time, non-absorption probability biasing is switched off and transport continues according to the physical absorption probabilities (analogue transport).

Example 1:

 WW-PROFIle       0.9       0.0        1.       11.       0.0       4.0
 WW-PROFIle       0.7       0.0       12.       70.       0.0       4.0
 WW-PROFIle       0.5       0.0       71.      260.       0.0       4.0
 *  Profile n. 4 is defined a multiplication factor for weight windows, where
 *  the upper and the lower weight limits (as defined by WW-FACTOr and
 *  WW-THRESh) are multiplied by 0.9 for the first 11 neutron groups, by 0.7
 *  for groups 12 to 70, and by 0.5 for groups from 71 to 260.

Example 2:

 WW-PROFIle       0.0       1.8        1.       65.       0.0       2.0
 WW-PROFIle       0.0       2.3       66.      260.       0.0       2.0
 *  Profile n. 2 is defined a multiplication factor for importances, where
 *  the importances (as defined by BIASING) are multiplied by 1.8 for the first
 *  65 neutron groups, and by 2.3 for groups 66 to 260.

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