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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Activates low-energy neutron transport


     WHAT(1) = number of neutron groups in the cross section set used.
               The FLUKA standard neutron library has 260 groups (see (10)).
               Default = 260

     WHAT(2) = number of gamma groups
               No default if WHAT(1) is given, 42 otherwise. (The standard
               FLUKA neutron library has 42 gamma groups).

     WHAT(3) = maximum energy of the low-energy cross section neutron library.
               For the standard FLUKA neutron library, the maximum energy is
               0.020 GeV.
               Default = 0.020 GeV.

     WHAT(4) = printing flag: from 0.0 to 3.0 increases the amount of
               output about cross sections, kerma factors, etc.
               1.0 : Standard output includes integral cross sections,
                     kerma factors and probabilities
               2.0 : In addition, downscattering matrices and group
                     neutron-to-gamma transfer probabilities are printed
               3.0 : In addition, scattering probabilities and angles are
               4.0 : In addition, information on residual nuclei is printed
               Default: 0.0 (minimum output)

     WHAT(5) = number of neutron groups to be considered thermal ones. (The
               standard FLUKA neutron library has 31 thermal groups).
             = 0, ignored
             < 0: resets to the default = 31.0
             Default = 31.0

     WHAT(6) = i0 + 10 * i1:
             i0 =  1: available pointwise cross sections used (see Note 4
                      below) and explicit and correlated 6-Li(n,t)4He,
                      10-B(n,alpha)7-Li, 40-Ar(n,gamma)41-Ar,
                      x-Xe(n,gamma)x+1-Xe and 113-Cd(n,gamma)114-Cd
                      photon cascade requested
                =  0: ignored
                =<-1: resets to the default (pointwise cross sections
                      are not used)
             i1 = 1, fission neutron multiplicity forced to 1, with
                     proper weight
                = 0, ignored
                =<-1: resets to the default (normal fission multiplicity)
             Default = -11., unless option DEFAULTS is present with
                     SDUM = CALORIMEtry, ICARUS, NEUTRONS or PRECISIOn,
                     in which case the default is 1.0 (pointwise cross sections
                     are used when available and fission multiplicity is not

     SDUM:   Not used

     Default (option LOW-NEUT not given): if option DEFAULTS is used with
             NEW-DEFAults, PRECISIOn or SHIELDINg, low-energy neutrons are
             transported and a suitable cross section library must be
             In all other cases, low-energy neutrons are not transported, and
             their energy is deposited as explained in Note 2).


  • 1) In FLUKA, transport of neutrons with energies lower than a certain threshold is performed by a multigroup algorithm. For the neutron cross section library currently used by FLUKA, this threshold is 0.020 GeV. The multigroup transport algorithm is described in Chap. (10).

  • 2) Evaporation option is mandatory by default or explicitly (see EVENTYPE) if LOW-NEUT is requested (by default or explicitly). If low-energy neutrons are not transported (because of the chosen DEFAULTS, or because a DEFAULTS card is absent), the energy of neutrons below threshold (default or set by PART-THR) is deposited on the spot. This is true also for evaporation neutrons. If there is no interest in low-energy neutron transport, but that feature is implicit in the DEFAULTS option chosen, it is suggested to request LOW-NEUT, and to use PART-THRes with an energy cutoff WHAT(1) = 0.020.

  • 3) Gamma data are used only for gamma generation and not for transport (transport is done via the FLUKA ElectroMagnetic module EMF using continuous cross sections). The actual precise energy of a photon generated by (n,gamma) or by inelastic reactions such as (n,n') is sampled randomly within the gamma energy group concerned, except for a few important reactions where a single monoenergetic photon is emitted. By default, for the 1-H(n,gamma)2-H reaction the actual photon energy of 2.226 MeV is used. It is possible to do the same with the capture gammas in 6-Li, 10-B, 40-Ar, x-Xe and 113-Cd, by setting WHAT(6) = 1.0 or 11.0.

  • 4) Pointwise neutron transport is available, by setting WHAT(6) = 1.0 or 11.0, for the following nuclides: 1-H (above 10 eV), 6-Li (all reactions), 10-B (only for the reaction 10-B(n,alpha)7-Li). Recoil protons are always transported explicitly, and so is the proton from the 14-N(n,p) reaction, for which a pointwise treatment is always applied

  • 5) The groups are numbered in DECREASING energy order (see (10) for a detailed description). The energy limits of the thermal neutron groups in the standard FLUKA neutron library neutron library are reported in}

  • 6) Here are the settings for transport of low-energy neutrons corresponding to available DEFAULTS SDUM options:
              CALORIMEtry, ICARUS, NEUTRONS, PRECISIOn: low-energy neutrons
                 are transported, using pointwise cross section when available
              EET/TRANsmut, HADROTHErapy, NEW-DEFAults (or DEFAULTS missing),
                 SHIELDINg: low-energy neutrons are transported using always
                 multigroup cross sections
              Any other SDUM value of DEFAULTS: no low-energy neutron transport


 LOW-NEUT       260.0      42.0     0.020       2.0      31.0      11.0
 *  The low-energy neutron library used is the (260n, 42gamma) standard
 *  multigroup library. The user requests a printout of cross sections, kerma
 *  factors, probabilities, downscattering matrices and n-->gamma transfer
 *  probabilities. Pointwise cross sections will be used where available, and
 *  only one neutron per low-energy fission will be emitted, with an adjusted
 *  weight.

© FLUKA Team 2000–2021

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