Last version:
FLUKA 2021.2.5, February 18th 2022
(last respin )
flair-2.3-0b 30-Jul-2021


-- Fluka Release
( 18.01.2022 )

FLUKA 2021.2.5 has been released.

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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 2000–2022

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