FLUKA: 10.4} The FLUKA neutron cross section library
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10.4} The FLUKA neutron cross section library
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As explained in 3}, an unformatted cross section data set must be available
for low-energy neutron transport. For a description of the algorithms used
for tracking low-energy neutrons, see the beginning of this Chapter. Other
useful information can be found in the Notes to options LOW--NEUT, LOW--MAT
and LOW--BIAS}.
The Legendre expansion used in Fluka is P5, i.e. at each collision
the polar scattering angle is sampled from three discrete values, such that
the first 6 moments of the angular distribution are preserved (the
azimuthal angle is sampled instead from a uniform distribution between
0 and 2*pi). The energy group structure depends on the cross section
set used. Here below the group structure of the currently available sets
is reported, and a list of the materials they contain.
The default FLUKA neutron cross section library (originally prepared by
G. Panini of ENEA [Cuc91]) contains more than 250 different materials (natural
elements or single nuclides), selected for their interest in physics, dosimetry
and accelerator engineering. This library has a larger number of groups and a
better resolution in the thermal energy range in respect to the original one.
The cross section set is being continuously enriched and updated on the basis
of the most recent evaluations (ENDF/B, JEF, JENDL, etc.).
The preparation of the library involves the use of a specialised code [NJOY] and
several ad-hoc programs written to adjust the output to the particular structure
of these libraries. The library is continuously enriched and updated on the
basis of the most recent evaluations (ENDF/B, JEF, JENDL etc.).
The library format is similar to that known as ANISN (or FIDO) format, but it
has been modified to include kerma factor data, residual nuclei and partial
exclusive cross sections when available. The latter are not used directly by
FLUKA, but can be folded over calculated spectra to get reaction rates and
induced activities.
More materials can be made available on request, if good evaluations are
available. Some cross sections are available in the library at two or three
different temperatures, mainly in view of simulations of calorimeters containing
cryogenic scintillators. Doppler broadening is taken into account.
Note that the energy groups are numbered in order of DECREASING energy
(group 1 corresponds to the highest energy).
The FLUKA neutron cross section library has 260 neutron groups and 42
gamma groups. Gamma energy groups are used only for (n,gamma) production, since
transport of photons in FLUKA is continuous in energy and angle and is performed
through the EMF module).
Hydrogen cross sections, which have a particular importance in neutron
slowing-down, are available also for different types of molecular binding
(free, H2O, CH2).
At present, the FLUKA libraries contain only single isotopes or elements
of natural isotopic composition, although the possibility exists to
include in future also pre-mixed materials.
Neutron energy deposition in most materials is calculated by means of
kerma factors (including contributions from low-energy fission). However,
recoil protons and protons from N(n,p) reaction are produced and transported
explicitly.
Each material is identified by an alphanumeric name (a string
not longer than 8 characters, all in upper case), and by three integer
identifiers. Correspondence with FLUKA materials (pre-defined or
user-defined) is based on any combination of name and zero or more
identifiers. In case of ambiguity, the first material in the list
fulfilling the combination is selected. (See command LOW-MAT for more details).
The convention generally used (but there may be exceptions) for the 3
identifiers is:
(exceptions are possible in order to distinguish between
data from different sources referring to the same nuclide)
10.4.1} 260 neutron, 42 gamma group library
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