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Improved FIFRELIN de-excitation model for neutrino applications
H. Almaz´
ana,1, L. Bernardb,2, A. Blanchetc,3, A. Bonhomme1,3, C. Buck1, A. Chalild,3,
A. Chebboubi4, P. del Amo Sanchez5, I. El Atmanie,3, L. Labit5, J. Lamblin2, A.
Letourneau3, D. Lhuillier3, M. Licciardi2, M. Lindner1, O. Litaize4, T. Materna3, H.
Pessard5, J.-S. R´
eal2, J.-S. Ricol2, C. Roca1, R. Rogly3, T. Salagnacf,2, V. Savu3, S.
Schoppmanng,1, T. Soldner6, A. Stutz2, L. Thulliez3, M. Vialat6
1Max-Planck-Institut f¨
ur Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
2Univ. Grenoble Alpes, CNRS, Grenoble INP, LPSC-IN2P3, 38000 Grenoble, France
3IRFU, CEA, Universit´
e Paris-Saclay, 91191 Gif-sur-Yvette, France
4CEA, DES, IRESNE, DER, Cadarache, 13108 Saint-Paul-Lez-Durance, France
5Univ. Savoie Mont Blanc, CNRS, LAPP-IN2P3, 74000 Annecy, France
6Institut Laue-Langevin, CS 20156, 38042 Grenoble Cedex 9, France
Received: date / Accepted: date
Abstract The precise modeling of the de-excitation of Gd
isotopes is of great interest for experimental studies of neu-
trinos using Gd-loaded organic liquid scintillators. The FIFRE-
LIN code was recently used within the purposes of the STEREO
experiment for the modeling of the Gd de-excitation after
neutron capture in order to achieve a good control of the de-
tection efficiency. In this work, we report on the recent addi-
tions in the FIFRELIN de-excitation model with the purpose
of enhancing further the de-excitation description. Experi-
mental transition intensities from EGAF database are now
included in the FIFRELIN cascades, in order to improve
the description of the higher energy part of the spectrum.
Furthermore, the angular correlations between γrays are
now implemented in FIFRELIN, to account for the relative
anisotropies between them. In addition, conversion electrons
are now treated more precisely in the whole spectrum range,
while the subsequent emission of X rays is also accounted
for. The impact of the aforementioned improvements in FIFRE-
LIN is tested by simulating neutron captures in various po-
sitions inside the STEREO detector. A repository of up-to-
date FIFRELIN simulations of the Gd isotopes is made avail-
aPresent address: Donostia International Physics Center, Paseo Manuel
Lardizabal, 4, 20018 Donostia-San Sebastian, Spain
bPresent address: Ecole Polytechnique, CNRS/IN2P3, Laboratoire
Leprince-Ringuet, 91128 Palaiseau, France
cPresent address: LPNHE, Sorbonne Universit´
e, Universit´
e de Paris,
CNRS/IN2P3, 75005 Paris, France
dCorresponding author: achment.chalil@cea.fr
ePresent address: Hassan II University, Faculty of Sciences, A¨
ın
Chock, BP 5366 Maarif, Casablanca 20100, Morocco
fPresent address: Institut de Physique Nucl´
eaire de Lyon,
CNRS/IN2P3, Univ. Lyon, Universit´
e Lyon 1, 69622 Villeurbanne,
France
gPresent address: University of California, Department of Physics,
Berkeley, CA 94720-7300, USA and Lawrence Berkeley National Lab-
oratory, Berkeley, CA 94720-8153, USA
able for the community, with the possibility of expanding
for other isotopes which can be suitable for different appli-
cations.
Keywords neutron-capture ·gamma-cascade
1 Introduction
FIFRELIN [1–3] is a Monte Carlo code developed to model
the fission process for reactor applications. FIFRELIN has
the capability to be run in a decay mode, allowing the mod-
eling of the de-excitation of any isotope. Recently, simulated
cascades for the isotopes 156,158Gd have been used in the
STEREO experiment, yielding an improved agreement with
the data [4]. These cascades have been made publicly avail-
able for use in other suitable applications [5]. Moreover,
within the CRAB method, which was recently proposed for
the calibration of bolometers in the 100 eV region [6], the
FIFRELIN cascades of W isotopes were used for its feasi-
bility study.
The wide use of Gd-loaded organic liquid scintillators
require a precise knowledge of the nuclear de-excitation of
Gd. Such detectors are used for the measurement of the an-
tineutrino flux coming from nuclear reactors. Experiments
such as STEREO [7–9], Daya Bay [10,11] and RENO [12]
have employed Gd-loaded liquid scintillators for their mea-
surements. In these cases, the detection of an electron an-
tineutrino νeis registered through the Inverse-Beta-Decay
process (IBD) on the protons of the liquid:
νe+p→e++n(1)
Despite the improved description of FIFRELIN when
compared to data from the STEREO experiment [4], there
arXiv:2207.10918v1 [hep-ex] 22 Jul 2022
2
were still missing aspects of the de-excitation process that
could be added to further improve the overall description
of the de-exciting Gd nuclei. Examples of such aspects are
the anisotropic emission of the γrays with respect to the
previously emitted ones and the X ray emission after Inter-
nal Conversion (IC). Furthermore, the reliance on theoreti-
cal models for the primary γrays could be further improved
with the addition of evaluated transition intensities. More-
over, the version of FIFRELIN used in [4] uses an outdated
version of RIPL-3 database (RIPL-3 v.2015).
In this work, we describe the latest improvements to the
FIFRELIN de-excitation model. An updated version of RIPL-
3 database (RIPL-3 v.2020) [13] is now used as input for
the present FIFRELIN cascades. In addition, the EGAF [14]
database has been implemented in FIFRELIN, to account for
the primary γray intensities, i.e. the transitions that depop-
ulate the neutron-separation energy level, for the isotopes
156,158Gd. This is expected to improve the de-excitation de-
scription of primary γrays, as only theoretical models were
used in the previous case [4]. Furthermore, the physics of γ
directional correlations has been implemented in order to ac-
count for the anisotropic emission of the γrays with respect
to the previously emitted ones. Internal Conversion (IC) and
Internal Pair Conversion (IPC) coefficients are updated up to
6 MeV, using the BrIcc code [15] and by taking into account
the electronic binding energies. The subsequent X ray emis-
sion after IC is also accounted for in the new versions of the
cascades. Furthermore, during the IPC process, a positron is
now emitted along with the electron, in opposite directions.
All the aforementioned improvements are now available
in an updated repository of Gd cascades, to be used for any
suitable application [16], with the potential of expanding the
repository in order to include cascades for other nuclei that
are used in similar applications.
2 The FIFRELIN de-excitation model
The FIFRELIN Monte Carlo code builds the low-energy
level scheme with extensive use of known evaluated nuclear
levels from the RIPL-3 database. RIPL-3 contains the neces-
sary input parameters for nuclear reactions and nuclear data
evaluations [13]. For this work, the updated version RIPL-3
v.2020 has been used. FIFRELIN builds the de-excitation
level scheme of a nucleus for three different regions: for
(E<ERIPL ), where ERIPL corresponds to the level below of
which the level scheme is considered completely known, the
level scheme is constructed only from the RIPL-3 database
[13]. In the intermediate energy range, ERIPL <E<Elimit ,
a combination between levels from RIPL-3 and theoreti-
cal levels sampled from spin-dependent level density mod-
els are used. Here, Elimit corresponds to a level density of
5×104levels/MeV.
Table 1: Updated table of critical energy values (in MeV) for
the 156,158Gd, as used in FIFRELIN.
Isotope ERIPL Elimit SnEM
156Gd 1.366 5.117 8.536 6.439
158Gd 1.481 5.191 7.937 6.633
In the high energy interval, Elimit <E<Sn, where Snis
the neutron-separation energy, FIFRELIN samples the en-
ergy levels exclusively from level density models. The up-
dated values of ERIPL,El imit and Snfor the isotopes 156,158Gd
are tabulated in Table 1. The reason for the addition of theo-
retical models is that the complete level scheme of a nucleus
is not known. This holds especially for the continuous part
of the spectrum which corresponds to transitions from levels
near the neutron separation energy Sn.
In the higher energy region, Elimit <E<Sn, the energy
spectrum is considered continuous and the energy levels are
described in bins with a bin width of 1 keV, in order to re-
duce computation time. The initial and final energies of a
transition are sampled inside the bin. Each level is assigned
a spin and parity given by a model.
The nuclear level density model used is given by the re-
lation:
ρ(E,J,π) = ρtot (E)P(J|E)P(π),(2)
where ρtot (E)is the total nuclear level density, which corre-
sponds to the composite Gilbert-Cameron Model [17]:
ρCGCM
tot (E) = ρCT M
tot (E)if E≤EM
ρFGM
tot (E)if E>EM
(3)
Here, ρCT M
tot is the level density as predicted by the Constant
Temperature Model (CTM) [18] and ρFGM
tot is the level den-
sity as predicted by the Fermi Gas Model. The value EMis
the matching energy, and is defined as the energy where the
two models have equal values of level density and its first
derivative to ensure continuity. A complete description of
the models can be found in [13].
The spin distribution function P(J|E)is given by the re-
lation:
P(J|E) = 2J+1
2σ2(E)exp−(J+1/2)2
2σ2(E),(4)
where σ(E)is the spin-cutoff parameter [13,19]. The parity
distribution P(π)is taken as 0.5 for both parity signs, mean-
ing that a level with an unknown parity has equal probability
of having a positive or a negative parity value.
Once the complete level scheme of the nucleus is gener-
ated, the code samples one transition from all possibilities,
depending on the values of the emission probabilities. For
higher energies near the continuum a statistical treatment is
3
Sn
g.s.
+
Experimental Theoretical Merged
Fraction: (1-x)Fraction: x
Fig. 1: Procedure to simulate the level scheme using both
RIPL3 and EGAF databases. A purely “experimental” sim-
ulation using the primary transitions from EGAF (in cyan) is
combined with a simulation using RIPL-3 (red) and theoreti-
cal models (black). The weighted sum of the two simulations
give the final ”merged” simulation. Primary transitions that
are present in both the experimental and theoretical simula-
tions are removed from the theoretical part, to avoid double
counting. See text for details.
necessary for γde-excitation with the use of γ-ray strength
function models.
The statistical treatment of an excited nucleus near con-
tinuum requires the use of the average transmission coeffi-
cient Tγ, which is given by the relation:
Tγ(Eγ,X L) = E2L+1
γfXL (Eγ)/ρ(Ei,Jπ
i)(5)
where Eγis the transition energy depopulating the level with
energy Eiand spin-parity Jπ
i,Xis the type (X=Efor elec-
tric and X=Mfor magnetic transitions), Lthe multipolar-
ity of the transition and fXL is the γ-strength function. In
this work, FIFRELIN uses two models for the γ-strength
function depending on their multipolarity. For electic dipole
transitions, the γ-strength function is given by the Enhanced
Generalized Lorentzian model (EGLO) [20]. For other types
of multipoles, the Standard Lorentzian model (SLO) is used
[21].
3 Upgrade of the de-excitation process in FIFRELIN
3.1 Implementation of EGAF database
The Evaluated Gamma Activation File (EGAF) [14,22] is a
database of prompt and delayed neutron-capture γ-ray cross
sections. The database consists of data acquired from mea-
surements performed at the Budapest Research Reactor in
combination with data from literature [22]. The measure-
ments were performed on natural elemental targets. Addi-
tional γrays were placed into the Budapest dataset by com-
parison with expected transitions from the Table of Isotopes
[23].
4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9
MeV
1
10
2
10
3
10
4
10
5
10
Counts
FIFRELIN
ENSDF
(a)
4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9
MeV
1
10
2
10
3
10
4
10
5
10
Counts
FIFRELIN (+EGAF)
ENSDF
(b)
Fig. 2: (a) Higher energy γspectrum of 156Gd with FIFRE-
LIN, using only theoretical models (b) The same but with
FIFRELIN using a combination of EGAF transition intensi-
ties and theoretical models.
The implementation of the EGAF database in FIFRE-
LIN accounts for the primary γrays emitted from the ini-
tial excited state. In the case of thermal-neutron capture,
this level corresponds to the neutron-separation energy Sn.
The procedure to simulate the level scheme is illustrated in
Fig. 1. Firstly, an ”experimental” simulation is performed
with FIFRELIN, using only levels from the EGAF database.
No theoretical models are used in this simulation. Then,
a ”theoretical” simulation using the standard procedure of
FIFRELIN is simulated, using the RIPL-3 database along
with the theoretical models described in Section 2. In order
to merge the simulation, a weighting factor xhas to be esti-
mated, which corresponds to the percentage of the primary
transitions of EGAF. Then, the merged simulation is con-
structed by using a percentage of xcascades from the EGAF
simulation and 1 −xcascades from the theoretical one. For
156Gd, x=12.64% while for 158 Gd, x=18.16%.
The combination of these two databases has yielded a
much better description in the higher energy part of the gamma
spectrum. Fig 2shows that the inclusion of the EGAF data in
the FIFRELIN simulation brings the description of all dis-
crete levels a lot closer to the ENSDF [24] values, which
4
are taken as a reference. Work is in progress for an auto-
mated interface with ENSDF that would allow an easier use
of FIFRELIN for any target isotope.
3.2 γ-directional correlations
In parallel to the improved set of discrete levels taken from
the nuclear databases, the treatment of the direction of emis-
sion of the γrays has been refined. The formal theory of
angular correlations has been used, based on the statistical
tensor formalism [25,26]. Their calculation is essential for
the determination of the probability distribution functions
which describe the directions of γrays in the cascade. For a
cascade of γrays starting from an initial state J0and ending
to a state Jn:
J0
γ0
−→ J1
γ1
−→ ... γn−1
−−→ Jn(6)
a set of statistical tensors can be calculated, containing the
information on the orientation of the initial state J0. Then,
the probability distribution functions for each γcan be eval-
uated and used for the generation of directions (θi,φi)of the
i-th γray. The implementation of the angular correlations
on FIFRELIN, along with its theoretical description, is de-
scribed in detail in [27].
3.3 Treatment of conversion electrons and X rays
IC and IPC coefficients are accounted for using BrIcc code
based on the Dirac-Fock calculations under the ”Frozen Or-
bital” approximation [15] within the energy interval from
(Eshell +1) keV up to 6 MeV, where Eshell is the electron
shell energy. In the previous version of FIFRELIN cascades [4],
the kinetic energy of the conversion electron was assigned
the energy of the corresponding transition. In the present
version, the process is treated more accurately by account-
ing the electronic binding energies. For the case of IPC, a
positron is now emitted together with the electron, in oppo-
site directions.
In addition, a treatment of X ray emission has been also
implemented in the newer FIFRELIN cascades. When a con-
version electron is emitted, the remaining residual energy is
then used to emit an X ray. In this way, the sum of the total
energy of the cascade is always equal to the initial energy of
the nucleus Sn.
4 Application to the STEREO detector
The final test of all these new ingredients of FIFRELIN is
the comparison between the simulated and measured spec-
tra after neutron-capture inside a Gd-loaded scintillator. For
Gamma-Catcher
------
scintillator liquid
(no Gd)
Target
------
6 cells
scintillator liquid
(Gd loaded)
Mineral oil
Acrylic buffers
PMT
Fig. 3: Layout of the STEREO detector. The six central Gd-
loaded target cells constitute the neutrino target, and are sur-
rounded by a crown of unloaded liquid to mitigate gamma
leakages out of the target volume and impact of external
background.
this purpose, data taken from the STEREO detector are com-
pared with simulations using the updated FIFRELIN cas-
cades. This is an important aspect for all neutrino detection
experiments by the IBD process since the accuracy of the
detection efficiency directly depends on the control of these
spectra. The layout of STEREO detector is shown on Fig 3.
The detector is composed of six cells of Gd-loaded liquid
scintillator (target), surrounded by four cells of Gd-free scin-
tillator (gamma catcher). More details about the STEREO
detector can be found in [7].
In the STEREO experiment the neutron response is mon-
itored with regular calibration runs where an americium-
beryllium (AmBe) source is deployed in 5 of the 6 cells,
successively at 5 different heights (10, 30, 45, 60 and 80 cm
from the bottom). The neutrons are produced through a two-
step process: an Am decay first emits an αparticle, which
then interacts with the a Be nucleus α+9Be →12C + n. In
about 60% of the cases the 12C isotope is produced in an
excited state decaying with a 4.4 MeV γray. The prompt
signal of this source is thus the sum of the energy deposits
from proton recoils induced by the few MeV neutron and
the high energy γray. The neutron capture signal is selected
by requesting a second energy deposit in a 100 µstime win-
dow following a first large energy deposit (between 4 and 7
MeV). The size of this time window is set according to the
16 µscapture time of a neutron in the target volume of the
STEREO detector. Because of the high activity of the source
(∼15 ×103n/s) special care is taken to the statistical sub-
traction of accidental pairs of events. The complete descrip-
tion of the statistical analysis of the STEREO data is beyond
the scope of present work, and will be thoroughly presented
in an upcoming publication of the STEREO collaboration.
An identical analysis is applied on simulations and data,
and a more detailed description can be found in [7]. The re-
5
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
Normalized Entries
DATA
FIFRELIN
FIFRELIN (Updated)
AmBe Source
Cell 4 - 45 cm
1 2 3 4 5 6 7 8 9 10
MeV
0.4
0.6
0.8
1
1.2
Data/MC
(a)
0.005
0.01
0.015
0.02
0.025
0.03
0.035
Normalized Entries
DATA
FIFRELIN
FIFRELIN (Updated)
AmBe Source
Cell 2 - 80 cm
1 2 3 4 5 6 7 8 9 10
MeV
0.4
0.6
0.8
1
1.2
1.4
Data/MC
(b)
Fig. 4: (a) Reconstructed energy spectrum of the STEREO detector in coincidence with 4.4 MeV prompt γray from an AmBe
source placed at the calibration position of cell 4 at 45 cm (center of the detector cell). (b) The same for cell 2 at 80 cm (10 cm
from the top of the detector cell). The simulations with the updated FIFRELIN cascades (using RIPL-3 v.2020 and including
EGAF transitions, angular correlations and X rays) is shown in red. The previous version of FIFRELIN cascades [4], which
use RIPL-3 v.2015, are shown in blue. Experimental data are shown in black. See text for details.
sulting delayed energy spectra are presented in Fig. 4and are
compared with simulations using both the updated FIFRE-
LIN version and the previous one from [4]. The energy re-
construction corresponds to the whole STEREO detector.
The 2.2 MeV peak from H(n,γ) and the ∼8 MeV from Gd(n,γ)
are clearly visible. There is a very good agreement between
both versions of FIFRELIN, as in particular positions inside
the detector there seems to be a minor influence of the afore-
mentioned updates in FIFRELIN.
For a central position of the source (Fig. 4a), most of the
γrays of the Gd-cascade are contained in the detector and
the Gd-peak is dominant. When approaching the border of
the target volume (Fig. 4b) a large fraction of the emitted
γrays can escape the active volume, transferring the recon-
structed events from the full energy peak to the lower energy
tail. This change of shape of the neutron-capture spectrum
is well described by both FIFRELIN simulations.
However, the situation is not the same when neutron-
captures are happening near the border of the detector cells,
where there are no experimental data. In order to further
check the impact of the new improvements in FIFRELIN
beyond the calibration positions of the STEREO detector,
simulations were also run near the border of the detector. A
simulated neutron source is placed at various positions near
the border of Cell 1. A simulation on the border can provide
insight on various changes on the shape of the spectrum of
the energy deposited in the detector, as more γrays are prone
to escape. The effect of the updated FIFRELIN cascades in
the reconstructed spectrum near the border of the STEREO
detector is demonstrated in Fig. 5.
Two simulations were run in order to estimate the ef-
fect of EGAF primary transitions. In Fig. 5a, the neutron
source was placed at (5,5,5) mm with respect to the outer
corner of Cell 1. There is an observable change in the shape
of the spectrum when primary transitions from EGAF are
included in FIFRELIN. This can be expected, as now the
primary transitions are more intense, as shown in Fig. 2b,
leading to a larger amount of γrays escaping the detector.
It is important to note that in positions near the border of
the detector there are no experimental data from STEREO.
However, the very good agreement of the FIFRELIN spec-
trum with the evaluated data from ENSDF provides a strong
argument for the use of the improved cascades. The same
effect is demonstrated for a different position of the source
in Fig. 5b. The neutron source is now placed at (5,5) cm
with respect to the corner but at the center of the cell in the
z coordinate.
Two simulations were also run in order to compare the
effect of angular correlations. The results in this case show
that the changes are not statistically significant. One expla-
nation for this could be that the directions are averaged out
in the detector leading to the same energy deposition inside
6
4−
10
3−
10
2−
10
1−
10
Normalized Entries
FIFRELIN (+EGAF) Corr ON
FIFRELIN (+EGAF) Corr OFF
FIFRELIN Corr OFF
0 1 2 3 4 5 6 7 8 9 10
MeV
0.9
1
1.1
Ratio
Corr ON/Corr OFF
(FIF+EGAF)/FIF
(a)
5−
10
4−
10
3−
10
2−
10
1−
10
Normalized Entries
FIFRELIN (+EGAF) Corr ON
FIFRELIN (+EGAF) Corr OFF
FIFRELIN Corr OFF
0 1 2 3 4 5 6 7 8 9 10
MeV
0.9
1
1.1
Ratio
Corr ON/Corr OFF
(FIF+EGAF)/FIF
(b)
Fig. 5: (a) Reconstructed energy spectrum of the STEREO detector obtained from a simulated neutron source placed close
to the edge of Cell 1. ”Corr ON” refers to enabled angular correlations of the γrays while Corr OFF refers to correlations
being disabled. FIFRELIN cascades with and without the inclusion of EGAF transitions are also compared. The position of a
neutron source is located at (5,5,5) mm from the top corner of Cell 1, which constitutes also the corner of the neutrino target
of the detector. In (b) the same results are shown, but now with the neutron source located at (5,5) mm horizontally and at
the center of the cell vertically. See text for details.
the detector, with a negligible effect on the present spectra.
However, since this effect is sensitive to the geometry of the
experimental setup, other applications may benefit.
5 Discussion and future directions
New simulated cascades for the de-excitation of 156,158Gd
have been generated using the FIFRELIN code, to be used
by the community for various applications. This code pro-
vides a refined description of the γcascades following neu-
tron captures by gadolinium nuclei. Three main new features
have been implemented for the simulations discussed here:
1) A complete set of measured primary γrays is built by
merging the RIPL-3 and EGAF databases. 2) A full treat-
ment of angular correlations is implemented. 3) The physics
of IC and IPC processes is treated more accurately, includ-
ing the secondary emission of X rays.
Recent improvements towards a more realistic de-excitation
model of 156,158Gd can benefit the community in a wide
range of applications in both low- and high- energy physics.
The comparison with the STEREO data in Fig. 4show that
both versions of FIFRELIN are able to describe well the re-
constructed experimental spectrum. However, there are sig-
nificant changes in the spectrum shape when the neutron
capture is happening in positions close to the border of the
cells. When compared with evaluated data, the de-excitation
description using the new FIFRELIN cascades has been sig-
nificantly improved especially for the higher energy part of
the cascade, as seen in Fig. 2. This constitutes a strong ar-
gument for the necessity of delivering an updated version of
the previous FIFRELIN cascades to the community. A more
precise description of the primary γrays is an improvement
which can benefit applications and experimental setups that
are sensitive to the higher energy part of the Gd spectra.
The inclusion of γ-directional correlations is also an im-
portant aspect of the present work. Despite the negligible
impact when applied for the STEREO detector, such an ef-
fect may be more pronounced in different experimental se-
tups, making the angular correlations an essential aspect of
the de-excitation process. Angular correlations are sensitive
to the geometry of the detectors/setups, thus the necessity of
their inclusion depending on each application can be very
important.
The addition of X rays and the improvements on the
electron conversion processes are also new features which
improve the de-excitation description. The IC process is now
treated more accurately, allowing for the emission of an X
ray after an emission of a conversion electron. The improved
modeling of the IC and IPC processes can be suitable in
7
various experimental applications that rely on electron spec-
troscopy [28–30].
In conclusion, we make available ten millions of updated
de-excitation cascades for the isotopes 156Gd and 158 Gd [5],
free of use for any other running and upcoming projects us-
ing neutron capture on gadolinium. The generalization of
these γ-ray cascade predictions for other isotopes of interest
is underway, notably in the context of cryo-detector calibra-
tion using neutron capture [6].
Acknowledgments
We acknowledge the financial support of the Cross-Disciplinary
Program on Numerical Simulation of CEA, the French Al-
ternative Energies and Atomic Energy Commission.
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