FLUKA: 18.15} Low-energy neutrons
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18.15} Low-energy neutrons
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The 50 MeV energy cutoff was one of the most important limitations of
the FLUKA87 version. The cutoff concerned muons and all hadrons, but
it was the absence of neutron transport below 50 MeV which constituted
the most serious drawback for many applications. The limitations stemmed
from the increasing inadequacy of the hadron interaction models in dealing
with interactions below 1 GeV and with the lack of any detailed
nuclear physics treatment, i.e. the lack of an evaporation model and low
energy particle production, at all energies.
Actually, several early attempts to overcome these weaknesses of the code
had been made by H.-J. Moehring, H. Kowalski and T. Tymieniecka
(code NEUKA [Kow87,Tym90], for Uranium/Lead scintillator only) and J. Zazula
(code FLUNEV [Zaz90,Zaz91]), with mixed results. The most promising approach
was that of Jan Zazula, of the Institute of Nuclear Physics in Cracow: he had
coupled Fluka87 with the Evap-5 evaporation module which he had extracted from
the Hetc/KFA code, and then interfaced the code with the only available
multi-group neutron cross section library extending to 50 MeV and beyond,
the HILO library.
The main limitations of these approaches, was their inability to address
the real deficiencies of the FLUKA87 hadron interaction model, their lack
of nuclear physics details and therefore the unreliability of their
excitation energy predictions, which indeed were never intended by
the original authors for any real use.
Furthermore, it became apparent that HILO had several weaknesses:
the cross section set had been put together by extending a low-energy
one of rather coarse structure based on evaluated experimental data
with the addition of much less accurate data calculated with an
intranuclear cascade code (HETC); for the same reason the library did
not contain any information on (n,gamma) generation above 20 MeV and
was based on a different Legendre angular expansion below and above
that energy. And because the library contained a very small number of
materials, the possibilities of application were rather limited.
The approach followed by Ferrari and Sala to overcome those
shortcomings was two-fold:
with reasonable accuracy low energy particle production and
medium-low energy particle interactions
up to 20 MeV (in collaboration with G.C. Panini and M. Frisoni of
ENEA - Bologna [Cuc91])
The former point is discussed in detail in the section on hadronic
models, the latter in the following.
Since Ferrari and Sala had started to work on a preequilibrium
model (later known as PEANUT, see next section) which was expected to cover
intermediate energies more accurately than the traditional
intranuclear cascade codes, it was decided to lower the FLUKA energy
cutoff to 20 MeV (thus making HILO unnecessary) and to create a
dedicated multigroup neutron cross section library to be used with FLUKA,
with the more usual upper energy limit of 20 MeV. The task was
carried out with the essential collaboration of G.C. Panini, an expert of
an ENEA laboratory in Bologna specialised in nuclear data processing
for reactor and fusion applications. Several neutron cross section
libraries have been produced for FLUKA over the years as a result of a
contract between INFN-Milan and ENEA [Cuc91].
These libraries, designed by Ferrari, had a format which was similar to
the ANISN one [Eng67] used for example by MORSE [Emm75], but which was
modified to include partial cross sections and kerma factors for dose
calculations (critically revised). Because at that time there was still
a US embargo on the most recent ENDF/B evaluated file, the cross sections
were originally derived from the European compilations JEF-1 and JEF-2.
(Later, they were regularly updated with the best ones available from JEF,
ENDF, JENDL and others). The choice of materials was particularly tailored
on detector and machine applications for high-energy colliders,
including also cryogenic liquids at various temperatures, and was much
wider than in most other libraries: it contained initially about 40
different materials (elements or isotopes), which became soon 70 (in
1991) and are now more than 130. Hydrogen cross sections were also
provided for different H molecular bonds (H gas, water, polyethylene).
Doppler reduced broadening was implemented for a few materials
at liquid argon (87 K) and liquid helium (approximately 0 K) temperatures.
The incorporation of the neutron multigroup transport module into FLUKA
by Ferrari was loosely based on the approach followed in the MORSE and
other multigroup codes, Ferrari and Fasso` had a deep expertise about.
The low energy neutron transport and interaction routines had been
rewritten from scratch progressively introducing many extra features
which are detailed in the following. Capture and inelastic gamma
generation was still implemented in the multigroup framework,
but gamma transport was taken care of by the EMF part of FLUKA.
Survival biasing was left as an option to the user with the possibility
to replace it by analogue survival.
Energy deposition computed via kerma factors was preserved, but in the
case of hydrogen the recoiling protons were explicitly generated and
transported. The same was done with protons from the 14-N(n,p) 14-C
reaction due to its importance for tissue dosimetry, and later for
all reaction on 6-Li.
The new low energy neutrons transport was ready at the end of
1990 [Fer91b]. The corresponding FLUKA version was called FlukaN
for a while to underline the neutron aspect, but the name was soon
abandoned.
At the beginning of 1997, the possibility to score residual nuclei
produced by low energy neutrons was introduced. Many improvements were
made in that same year. Federico Carminati, who was using FLUKA for
calculations related to C. Rubbia's Energy Amplifier, added to the
program a few routines and nuclear data necessary to score low-energy
fission products. Pointwise cross sections were introduced for the
neutron interactions with hydrogen. Ferrari and Sala also developed a
model to sample gammas from neutron capture in Cadmium, an important
reaction for which no data were available in evaluated cross section
files. Similar schemes for capture photon generation were established
in 2001 for other nuclei (Ar, Xe) [Fas01b]. Pointwise transport and
interactions for 6-Li were also provided.
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