Last version:
FLUKA 2021.2.2, September 25th 2021
(last respin )
flair-2.3-0b 30-Jul-2021


-- Fluka Release
( 25.09.2021 )

FLUKA 2021.2.2 has been released.
Fluka Release 30.07.2021 FLUKA 2021.2.1 has been released.
Fluka Major Release 18.05.2021 FLUKA 2021.2.0 has been released.
Congratulations from INFN: ,
Dear Paola,
I wish to congratulate you and all the authors and collaborators for this new Fluka release, which looks at the future and confirms the support of INFN in the development and continuous improvement of this code.
best regards
Diego Bettoni
INFN Executive Committee

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defines a detector for a track-length fluence estimator


The full definition of the detector may require two successive cards (the second card, identified by the character '&' in any column > 70, must be given unless the corresponding defaults are accepted)

First card:

     WHAT(1) =  1.0 : linear binning in energy
             = -1.0 : logarithmic binning in energy
             Default = 1.0 (linear binning)

     WHAT(2) = (generalised) particle type to be scored
             Default = 201.0 (all particles)

     WHAT(3) = logical output unit:
             > 0.0 : formatted data are written on WHAT(3) unit
             < 0.0 : unformatted data are written on |WHAT(3)| unit
             Values of |WHAT(1)| < 21 should be avoided (with the
             exception of +11).
             Default = standard output unit

     WHAT(4)  > 0: region defining the detector
             = -1: all regions (see Note 7)
             Default = 1.0

     WHAT(5) = volume of the detector in cm**3
             Default = 1.0

     WHAT(6) = number of energy bins
             Default = 10.0

     SDUM    = any character string (not containing '&') identifying the
               track-length detector. Max. 10 characters.

Continuation card:

     WHAT(1) = maximum kinetic energy for scoring
               Default: Beam particle total energy as set by the BEAM
               option (if no BEAM card is given, the energy corresponding
               to 200 GeV/c momentum will be used)

     WHAT(2) = minimum kinetic energy for scoring
               (default: 0 if linear binning, 0.001 GeV otherwise)
               Note that the lowest energy limit of the last neutron group
               is 1.E-14 GeV (1.E-5 eV) for the 260 data set.

     WHAT(3)..WHAT(6) : not used

     SDUM    = & in any position in column 71 to 78



  • 1) IMPORTANT! The results of USRTRACK are always given as DIFFERENTIAL distributions of fluence in energy, in units of cm-2 GeV-1 per incident primary unit weight), Thus, for example, when requesting a fluence energy spectrum, to obtain INTEGRAL BINNED results (fluence in cm-2 PER ENERGY BIN per primary) one must multiply the value of each energy bin by the width of the bin (even for logarithmic binning).

  • 2) If the generalised particle is 208 (ENERGY) or 211 (EM-ENRGY), the quantity scored is differential energy fluence, expressed in GeV per cm2 per energy unit per primary. That can sometimes lead to confusion since GeV cm-2 Gev-1 = cm-2, where energy does not appear. Note that integrating over energy one gets GeV/cm2.

  • 3) The maximum number of tracklength + collision detectors (see option USRCOLL) that the user can define is 400. This value can be changed by modifying the parameter MXUSTC in member USRTRC of the flukadd library or directory and then re-compiling and linking FLUKA.

  • 4) The logical output unit for the estimator results (WHAT(3) of the first USRTRACK card) can be any one of the following:
  • the standard output unit 11: estimator results will be written on the same file as the standard FLUKA output
  • a pre-connected unit (via a symbolic link on most UNIX systems,
    ASSIGN under VMS, or equivalent commands on other systems)
  • a file opened with the FLUKA command OPEN
  • a file opened with a Fortran OPEN statement in a user-written initialisation routine such as USRINI, USRGLO or SOURCE (see (13))
  • a dynamically opened file, with a default name assigned by the Fortran compiler (typically fort.xx or ftn.xx, with xx equal to the chosen logical output unit number).

The results of several USRTRACK and USRCOLL detectors in a same FLUKA run can be written on the same file, but of course only if they are all in the same mode (all formatted, or all unformatted). It is also possible in principle to write on the same file the results of different kinds of estimators (USRBDX, USRBIN, etc.) but this is not recommended, especially in the case of an unformatted file, because it would make very difficult any reading and analysis.

  • 5) When scoring neutron fluence, and the requested energy bin structure overlaps with that of the low energy neutron groups, bin boundaries are forced to coincide with group boundaries and no bin can be smaller than the corresponding group. Actually, the program uses the requested energy limits and number of bins to estimate the desired bin width. The number of bins above the upper limit of the first low-energy neutron group is recalculated according to such width. To preserve the requested upper energy limit, the width of the first bin above the low energy group may be smaller than that of the others. Note that the lowest energy limit of the last neutron group is 1.E-14 GeV (1.E-5 eV) for the 260 data set. All group energy boundaries are listed in two Tables in (10).

  • 6) A program USTSUW is available with the normal FLUKA code distribution in directory $FLUPRO/flutil. USTSUW reads USRTRACK results in binary form from several runs and allows to compute standard deviations. It returns differential and cumulative fluence, with the corresponding percent errors, in a file, and differential fluence in another file formatted for easy plotting. It also returns a binary file that can be read out in turn by USTSUW. The content of this file is statistically equivalent to that of the sum of the files used to obtain it, and it can replace them to be combined with further output files if desired (the USTSUW program takes care of giving it the appropriate weight).

  • 7) Setting WHAT(4) = -1 will provide the sum of the track-lengths in all regions, divided by the value set by the user for WHAT(5).


 USRTRACK         1.0    PHOTON     -24.0      16.0    4500.0      150.PhotFlu
 USRTRACK         1.5       0.0       0.0       0.0       0.0       0.   &
 *   Calculate photon fluence spectrum in region 16 from 0 to 1.5 GeV, in
 *   150 linear energy intervals. Write unformatted results on
 *   unit 24. The volume of region 16 is 4500 cm3.

© FLUKA Team 2000–2021

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