# Computer Modeling of Displacement Cascades in Beryllium Irradiated with Intensive Neutron Flux

Authors: T. Troev; N. Nankov; L. Petrov; E. Popov
Journal: Research Letters in Physics
 (2008)

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## Abstract

Computer simulations of the radiation defects created in beryllium irradiated by fast neutrons (E>0.1 MeV) using
the Geant4 and SRIM packages were carried out. The atom cascade displacements in Be at a neutron fluence
of 1.6×1020 n/cm2 were determined to be 0.06 dpa and the helium concentration was calculated to be 168 appm. The
concentration of L6i has been estimated to be 5% in comparison to the He concentration. Nanoscale calculations
were done in 30×30×30 nm cube of fast neutron-irradiated Be. A correlation between the Be primary knock-on atom
(PKA) energies and the damage cascades has been established. The final defect distributions of single vacancies,
divacancies, and small vacancy clusters were examined. Our results indicate that the damages caused by He atoms
are about 3 times less than damages caused by Be primary knock-on atoms (PKAs).


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## Body

## 1. Introduction

The purpose of our study is the computer modeling of
the damage of a Be sample caused by intensive fusion neutron irradiation. In
order to achieve our goal, we have adopted similar approach used by Becquart
[1]. First, we have simulated the interaction of neutrons with a macroscopic Be
target at high energy (E>0.1 MeV). For this purpose, the fusion neutron spectrum data was taken from the experiment as
an input for the simulation. We have recorded the initial energy and position
of Be primary knock-on atoms (PKAs) and of all isotopes created in the nuclear
reactions at the output of the calculation. The results obtained allowed us to
calculate the concentration of He and Li isotopes and use it as an input for
the next step in our simulation. Using the Monte-Carlo (MC) SRIM
[2], code we have calculated the atom displacements caused by the Be and He
PKAs. At the end of the simulation, the final
positions of the atoms involved in damage cascades have been recorded. Finally,
the atom displacements and the number of produced vacancies and interstitials
as a function of the PKA energy for the isotopes created were calculated.


## 2. The Methodology of the Simulations

We created a computer Monte-Carlo (MC) code based on the Geant4 CERN package [3] to simulate
ion and neutron interactions at high energies (E>0.1 MeV) in a
Be homogeneous target. We took the initial neutron energy distribution for the
simulation from [4]. Neutron elastic and inelastic scattering, neutron capture
with and without emission of charged particles were taken into consideration.
Gamma ray and secondary electron (SE) emission as well as all types of
electromagnetic interaction of charged particles and photons with matter were
included in the present model. The high-precision database for low-energy
neutron cross-section G4NDL [3] based on the ENDF/B-VI [5] database has been
used with the purpose to achieve high accuracy of the neutron interaction in Be
at energies between 0.1 MeV and 14 MeV. In our numerical calculations, we have
considered the production of 3He, 3He, and 6Li. The 7Li production has a threshold
of 12 MeV, and therefore it takes place in a fusion reactor. Our computer
simulations were carried out for a macroscopic cylindrical Be target with
diameter and length of 5 cm. We should point out that the Be PKAs for the
damage cascades due to neutron scattering and capture were considered for a
neutron fluence of about 1.6×1020 n/cm2 (E>0.1 MeV) and for small cubic volume with dimensions 30×30×30 nm inside the target. We have adopted the procedure, described in
[4] for estimation of the damage in beryllium due to intensive neutron
irradiation. The results obtained for dpa (displacement per atom) cross-section
versus various neutron energy are shown in Figure 1 where
it is seen that the elastic neutron scattering (n,n)
cross-section is more than an order of magnitude higher than the cross-section
of the neutron inelastic scattering (n,n′). The
magnitude of the neutron capture cross-section in Be with gamma ray
emission is relatively small but it is important for the tritium production.
Our calculations show that the threshold for the (n,n′)-induced dpa
is about 2 MeV and for the (n,γ) is about 1
MeV. The dpa due to elastic scattering was present at over whole energy range
(0.1–14 MeV) in the
present simulation. The dpa cross-sections for (n,n′) and (n,γ) grow rapidly up to about 4 MeV, after that they reach saturation. The next step in our model
was to carry out calculations of damages in Be using the results from the
described high-energy neutron interactions. We used the binary collision
approximation (BCA) and the SRIM-MC code [2] which relies on the universal pair
potential of Ziegler et al. [6] and Robinson [7]. The SRIM code allows [1] the
calculation of the stopping of the PKA at energies higher than 10 keV/u with an
accuracy of 10%. In this approach, the crystal structure of Be has been
neglected. Every PKA created by the high-energy neutron interaction with Be
atoms was tracked in SRIM to its stopping in the Be target. According to the
model, the target atom which is hit by PKA and starts a recoil cascade is
identified by its recoiling energy. The number of displacement collisions
records how many target atoms were set in motion in the cascade with energies
above their displacement energy which is specified at the input. The Be target
vacancies are the next items which were recorded. The replacement collisions on
the other hand may reduce the total vacancies up to 30%. In the model, the
average displacement energy Ed has been chosen to be 40 eV.
The lattice binding energy was taken to be 3.32 eV equal to the cohesive energy
of the Be crystal. At the end of every cascade, the ion type, the PKA energy,
the recoil energy, and the atom positions have been recorded for further
processing.Figure 1
dpa cross-section for a typical deuterium-tritium reactor for different types of
neutron interactions as a function of the neutron energy.


## 3. Results and Discussion

In the present simulations, the He content was estimated to be 168 appm. The6Li atom concentration was
estimated to be 5% of the He content for the neutron fluence of 1.6×1020 n/cm2 (E>0.1 MeV) used
in the simulations. If we scale the result for the neutron flux of 1.3×1015 n/cm2/s expected for the fusion
reactor first wall (FW), our data is in agreement
with the available one in the literature [8] although our results are obtained
by a different procedure. It was established in our calculations that the total
damage of 0.06 dpa for a neutron fluence of 1.6×1020 n/cm2 scales well with the already
reported results for 2.5×1022 n/cm2 [4]. We have established that
the calculated number of the replacement collisions which lead to the
recombination of vacancies and self-interstitials was only 1.8% compared to the
total number of displacements. This result for the first time indicates that
high-energy recoils generate atomic collision cascades in Be in which a
fraction of the defects recombines. The calculated results of the Be subcascades as a function of the PKA energy are
shown in Figure 4. They indicate a correlation between PKA energies and the
number of subcascades. It is seen in Figure 4 that PKAs with higher energies
produce a large number of subcascades. It is seen that Be PKA energies in the range
20–120 keV have a higher probability to induce
subcascades. Obviously, the most probable number of subcascades is around 50.
Similar results but for α-Fe are discussed in [9, 10]. The authors pointed out that for PKA with an energy
higher than some critical value in the range of
10–40 keV, the formation of subcascades is more
probable. On the other hand, PKA with higher energies do not initiate as large
a number of subcascades as one could expect. This is probably due to the fact
that the most efficient energy transfer in Be occurs at lower energies and thus
produces the greatest amount of damages. From our simulation results shown in
Figure 2, it is evident that low-energy cascades (E<100 keV) are
responsible for about 70% of the defects production. The accumulation of He
atoms in the Be sample is due to the stopped He atoms losing their energy in
the cascade collisions. The distribution of vacancies created by beryllium atom
displacements and helium atoms created by neutron capture in Be is shown in
Figure 3. We considered the He atoms as
interstitials which contribute to the interstitial clusters concentration.Vacancy distribution for PKA with energiesE<100 keV and E>100 keV. The
block size is 30×30×30 nm.
(a)(b)Figure 3
Distribution of
vacancies created by beryllium atom displacements and helium atoms created by
neutron capture in Be. The block size is60×30×30 nm.Figure 4
Correlation between the number of Be PKA subcascades and the energy.


## 4. Conclusions

Computer simulations of the creation of point defects in beryllium, irradiated by
intensive fast (E>0.1 MeV) neutron fluence up to 1.6×1020 n/cm2, have been performed. Our
numerical model includes two steps. In the first step, precise calculations of
the PKA energies and positions for Be, He, and 6Li recoils have been made. The
damage in Be due to the intensive neutron irradiation was calculated to be 0.06
dpa. The He content was determined to be 168 appm which is in agreement with
the available results. The 6Li content which is important
for the tritium production was calculated to be 5% of the He content. The
simulation results show that 70% of the damages occur at Be PKA energies below
100 keV. The number of subcascades increases with the PKA energy and reach
saturation at about 110 keV. The most probable subcascade number is 50 for PKA
energies close to 50 keV. The final Be and He atom distributions confirm that
the most likely defects are single vacancies, divacancies, and small-vacancy
nanovoids containing more than 5 atoms.



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*Source: 746892-2008-02-07.xml*