LOW–NEUT

Summary

Activates low-energy neutron transport (which in fact is already activated by several DEFAULTS choices), dumps on request information concerning groupwise treatment ingredients, and allows to activate generation of secondary charged particles and correlated photon cascades, as well as pointwise cross section treatment, both available only for special cases ​

See also LOW–BIAS, LOW–MAT

​

WHAT(1) =

number of neutron groups​ in the neutron cross section library​ used. The FLUKA ​ standard neutron library has 260 groups (see Low-energy neutrons in FLUKA).

Default

=  260.0

WHAT(2) =

number of gamma groups​.

Default

:  No default if WHAT(1) is given, 42.0 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

WHAT(4) =

printing flagfor a detailed description of the printed output):​

from 0.0 to 4.0 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 to the above, downscattering matrices ​ and group neutron-to-gamma transfer ​

probabilities are printed

3.0: In addition to the above, scattering probabilities and angles are printed ​ ​

4.0: In addition to the above, information about residual nuclei is printed (see Note 10 below)​

Default

: 0.0 (minimum output)

WHAT(5) =

number of neutron groups to be considered as thermal​ ones. (The standard FLUKA neutron library has 31 thermal groups).

= 0.0:

ignored

> 0.0:

resets to the default = 31.0

Default

= 31.0

WHAT(6) =

\({\texttt{i}}_0 + {\texttt{10}}\cdot {\texttt{i}}_1\):

i\(_0\)

> 0: if no full pointwise treatment is activated (see LOW–PWXS) a few pointwise cross sections​, (\(^1\)H, \(^2\)H, \(^3\)He, \(^4\)He, and \(^{12}\)C/C\(_{nat}\)) are used in a mixed approach with the groupwise treatment (see important details in Note 5 below), explicit and correlated secondary generation for \(^{10}\)B(n,\(\alpha\))\(^7\)Li ​ is activated, as well as correlated photon cascade for \(^x\)Xe(n,\(\gamma\))\(^{x+1}\)Xe ​ and \(^{113}\)Cd(n,\(\gamma\))\(^{114}\)Cd​

= 1: lower thresholds for pointwise treatment including when full pointwise treatment is invoked with LOW–PWXS, set at default values: \(10^{-5}\) eV for free gas materials 3.059023 eV (260 group library) for materials for which \(S(a,b)\) treatment is available (default for some DEFAULTS, PRECISIOn …)

= 2: same as for i\(_0\)=1

= 3: lower thresholds for pointwise treatment, including when full pointwise treatment is invoked with LOW–PWXS, set at: 3.059023 eV (260 groups) for all materials

= 4: lower thresholds for pointwise treatment, including when full pointwise treatment is invoked with LOW–PWXS, set at: \(10^{-5}\) eV for all materials

= 0: ignored

≤ -1: resets to the default (pointwise cross sections are not used)

i\(_1\)

= 1: fission neutron multiplicity ​ is forced to 1, with the proper weight

= 0: ignored

≤ -1: resets to the default (normal fission multiplicity)

Default

= -11., unless option DEFAULTS has been chosen with SDUM = CALORIMEtry, DAMAGE, HADROTHErapy, ICARUS, NEUTRONS or PRECISIOn, in which case the default is 1.0 (pointwise treatment – see Note 5 – and generation of secondary charged particles and correlated photon cascades are performed when available, and fission multiplicity is not forced)

SDUM

: not used

Default

(option LOW–NEUT not given): if option DEFAULTS has been chosen with SDUM = CALORIMEtry, DAMAGE, EET/TRANsmut, HADROTHErapy, ICARUS, NEUTRONS, NEW–DEFAults, PRECISIOn or SHIELDINg, low-energy neutrons are transported and a suitable cross section library must be available.

In all other cases, low-energy neutrons are not transported, and their energy is deposited as explained in Note 2 below.

Notes

​

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 Low-energy neutrons in FLUKA.

2. If low-energy neutrons are not transported (because of the chosen DEFAULTS, or because so requested by the user, see Note 3) the energy of neutrons below threshold (default or set by PART–THR) is deposited on the spot. This is true also for evaporation neutrons​.

3. If there is no interest in transporting low-energy neutrons, but this feature is is implicit in the DEFAULTS option chosen, it is suggested to use PART–THRes with an energy cutoff​ WHAT(1) = 0.020. However, even in this case the availability of the low-energy neutron cross sections for the materials defined in input is checked. To avoid the run being stopped with an error message, the user should issue a LOW–MAT command for each material for which cross sections are missing, pointing them to any available material.

4. Gamma data are used only for capture gamma generation​ and not for transport (transport​ is done via the ElectroMagnetic Fluka 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, as the \(^1\)H(n,\(\gamma\))\(^2\)H​ reaction where the actual photon energy of 2.226 MeV is used. It is possible to get (single or correlated) physical gammas also for the capture in \(^{10}\)B, \(^{12}\)C, \(^x\)Xe and \(^{113}\)Cd, by setting WHAT(6)= 1.0-4.0 or 11.0-14.0 (see Note 5 for the additional requirement applying to \(^{12}\)C).

5. Full, correlated, pointwise treatment is available for all isotopes when invoking the LOW-PWXS option. Otherwise, in the framework of groupwise cross sections, special pointwise neutron transport​ is available only for a few isotopes, \(^1\)H, \(^2\)H, \(^3\)He, \(^4\)He, and \(^{12}\)C/C\(_{nat}\) by setting WHAT(6)= 1.0-4.0 or 11.0-14.0. In the case of \(^2\)H, in order to get the pointwise treatment it is mandatory to define the respective monoisotopic material through a MATERIAL card and name it DEUT... (it must start with DEUT).

   In the case of \(^3\)He, and \(^4\)He, in order to get the pointwise treatment it suffices to define the respective monoisotopic material through a MATERIAL card and associate it to the low energy materials HELIUM-3, HELIUM-4, respectively.

   For pointwise treatment for \(^1\)H, if activated, it is sufficient to name the material through a MATERIAL card ...HYDR.... If the material has natural composition, the (small) amount of \(^2\)H will be neglected. Of course one can define two monoisotopic materials for \(^1\)H and \(^2\)H and mix them in the proper proportions with a COMPOUND card.

   Pointwise cross sections for \(^{12}\)C are activated, if requested, when the low energy neutron data set associated with the material has name CARBO-12, CARBON..., for both \(^{12}\)C and natural Carbon. In the latter case the \(^{13}\)C small abundance is neglected.

6. Recoil protons​ are always transported explicitly, and so is the proton from the \(^{14}\)N(n,p)\(^{14}\)C reaction​.

7. The groups are numbered ​ in decreasing energy order (see Low-energy neutrons in FLUKA for a detailed description). The energy limits of the thermal neutron groups in the standard FLUKA neutron library are reported in Neutron and gamma energy groups.

8. 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, with generation of charged secondaries, correlated photon cascades and use of pointwise cross sections 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

9. If treatment of low energy neutrons is requested, one must make sure that the transport threshold for neutrons (set with PART–THR) be equal to the minimum energy needed for neutron transport (typically \(10^{-5}\) eV). Please note that the behaviour of the PART–THR option for neutrons has changed with respect to past releases.​

10. For a given neutron group and material (see example below), the residual nuclei production probabilities printed in the output are normalized to the non-elastic cross section (e.g. a 0.5 probability means that that nucleus is produced on average every other non-elastic interaction).

    %            Group:    1   A     Z    m    Prob
    %                         12     6     0    7.54E-01
    %                         11     5     0    9.09E-01
    %                         13     6     0    9.09E-01
    %                         12     5     0    9.36E-01
    %                          9     4     0    1.00E+00
    %                         14     6     0    1.00E+00
    %                          1     1     0    7.44E-02
    %                          2     1     0    1.07E-01
    %                          4     2     0    6.44E-02
    %
    %
    

Example:

*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+...
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 treatment and generation of secondary charged
*  particles and correlated photon cascades will be performed where available
*  (see WHAT(6) and Note 5 above), and only one neutron per low-energy fission
*  will be emitted, with an adjusted weight.