Study of Helium Migration in Nuclear Materials at Jannus-Saclay
P. Trocellier1*, S. Miro1, Y. Serruys1, S. Vaubaillon1,2, S. Pellegrino1,2, S. Agarwal1,
S. Moll3and L. Beck1
1. CEA, DEN, Service de Recherches de Métallurgie Physique, Laboratoire JANNUS
2. CEA, INSTN, UEPTN, Laboratoire JANNUS
3. CEA, DEN, Service de Recherches de Métallurgie Physique
F-91191 Gif sur Yvette, France
* Author to whom correspondence should be sent (patrick.trocellier@cea.fr).
Abstract
The Jannus multi-ion beam facility at Saclay allows performing as well single, dual or triple
beam irradiation, ion implantation and ion beam analysis. This versatility is due on one hand
to the large range of ion beams available from the three coupled accelerators and on the other
hand to the possibility to use several dedicated or multi-purpose vacuum chambers. It is thus
possible to investigate the radiation tolerance of inorganic or organic materials and the
specific effect of an atomic species of nuclear interest like hydrogen, helium, fission or
transmutation products under thermal treatment or ion irradiation. This paper discusses the
capabilities offered by the Jannus facility to study helium mobility in advanced nuclear
materials.
Key words: diffusion, helium implantation, iron, irradiation, nuclear reaction analysis, silicon
carbide
1
1. Introduction
Whatever its origin, the consequences of the presence of helium in advanced nuclear materials
such as pure metals, alloys, refractory ceramics, composites or waste forms can be observed at
a macroscopic level as for example surface blistering, swelling, embrittlement and fracture [1,
2]. The progress in our knowledge on helium behaviour in this type of materials depends on
the possibility to access experimental facilities able to undertake helium implantation and
analysis. For this purpose, high enrgy ion implantation is preferred in order to prevent any
perturbating surface effects on the migration mechanisms of helium.
MeV accelerated ion beams are known for a long time to offer a wide range of implantation
and analysis capabilities for light element investigations [3, 4]. Considering the specific
question of helium behaviour, the multi-irradiation facility Jannus, located at Saclay,
constitutes a unique experimental tool by coupling three accelerators with different ion nature
and energy range.
Helium isotopes are easily analyzable using ion beam techniques as clearly demonstrated in
several pioneering works [5, 6, 7]. Non Rutherford proton backscattering spectrometry (NBS
or PES) was applied for depth profile determination to both isotopes for example in pyrolitic
graphite [8] or in silica [9]. Medium or heavy ion elastic recoil detection analysis (MI- or HIERDA) were used to study helium diffusion in fluorapatite [10] or spinel [11]. Nuclear
reaction analysis (NRA) was extensively applied to measure both diffusion coefficient and
thermal activation energy of helium in several inorganic media [12, 13].
After a brief description of the Jannus facility, we present what we consider to be an ideal
experimental approach to study the behaviour of helium in advanced nuclear materials
submitted to different solicitations like thermal annealing and/or irradiation.
2
2. Description of the Jannus-Saclay facility and implantation capabilities
2.1 Jannus-Saclay general layout
Figures 1a and 1b display the complete layout of the multi-irradiation facility at Jannus Saclay
[14]. Three accelerators are coupled: a 3 MV Pelletron™ named Épiméthée, a 2 MV
Pelletron™ tandem named Japet and a 2.5 MV single ended Van de Graaff named Yvette.
Épiméthée is equipped with an electron cyclotron resonance ion source able to produce multicharged ions. Japet is equipped with a charge exchange ion source operating with Cs vapour
able to produce initially single charged negative ions that are then converted into positive ions
by a stripping process through a very low pressure argon leak. Yvette includes at its terminal a
conventional radiofrequency ion source used to produce protons, deuterons, helium-3 and
helium-4 ions.
A triple beam chamber receives one beam line coming from each of the accelerators, allowing
single, dual or triple beam irradiation. This chamber is implemented with a movable array of
Faraday mini-cups allowing a periodic control of the ion flux in each beam. The sample
holder operates from liquid nitrogen temperature to 850°C. Three energy degraders
constituted by rotating wheels mounted with suitable thin metallic layers give the possibility
to broaden the damage profile accumulated into the sample under investigation. Each of the
beam line converging towards the triple beam chamber is equipped with a raster scanner unit
able to move the beam inside a 2 x 2 cm2 area onto the sample surface. A second vacuum
chamber is linked to Épiméthée. It can be used for single beam irradiation or ion beam
analysis. A Faraday mini-cups device and a heating/cooling stage sample holder are also
available in this chamber. A third vacuum chamber has been implemented on Yvette. It is a
multipurpose ion beam analysis chamber equipped with two X-ray detectors, a high purity
germanium detector for gamma-ray detection and two surface barrier detectors (100 and 1500
3
µm) usable for Rutherford backscattering, elastic recoil detection and nuclear reaction
analysis measurements.
2.2 Implantation capabilities
To obtain quantitative data on helium mobility in different types of inorganic materials, it is
of primary importance to own a reliable standard to evaluate the uncertainty as well on helium
ion implanted dose as on helium depth profiling. For this purpose, we have implanted a thin
tungsten foil (50 µm) with 1.5 MeV 3He+ at room temperature to a dose about 1 x 1017
ions/cm2 with an average dose rate around 1 x 1013 ions/cm2/s using the 3 MV Pelletron
Épiméthée. The value of the projected range extracted from SRIM calculations is (2.00 ±
0.25) µm [15].
After implantation, the 3He content was determined in the multipurpose IBA chamber
implemented
3
on
the
2.5
MV
Van
de
Graaff
Yvette
by
using
the
He(d, p0)4He nuclear reaction [160] characterized by a wide resonance centered on Ed = 450
keV (Figure 2). The deuteron energy was fixed at 0.9 MeV in order to reach 0.45 MeV around
2 µm in depth. The surface barrier detector used was located at an angle of 150° and covered
for this experiment with a 23 µm thin mylar (C10H8O4) foil.
Helium-3 analysis was performed at nine different points on the surface of the 3He implanted
tungsten foil. Figure 3 gives the 3He content derived from these measurements using the
reconstitution code SIMNRA [17]. It is clear that the obtained data are in very close
agreement with the expected dose of 1 x 1017 3He/cm2: average content = 1.02 x 1017/cm2,
standard deviation = 0.05 x 1017/cm2.
4
3. Study of helium mobility in advanced nuclear materials
3.1 General considerations
As we mentioned in section 1, both helium isotopes can be analyzed using either deuteron
induced NRA for 3He or HI-ERDA for 4He. Depending of the emitted particle detected,
3
He(d, p0)4He NRA allows probing the first micrometer below the sample surface (detection
of the 4He particle) or a very large depth in the range 5 to 10 µm, due the high energy of the
emitted proton ( 13 MeV) [12, 18 - 21]. The analyzed depth using MI- and HI-ERDA is
limited to the first micrometer below the surface due to the grazing angle geometry, the
maximum energy reachable with medium mass or heavy ion, typically around 27 MeV for Ar
on Épiméthée, and also the type of detection device used, i.e. surface barrier detector with a
thin filter foil, E-E telescope or time of flight (TOF) spectrometer [22 - 24]. Thus, by
changing the nature of the helium isotope implanted in the analyzed materials, different
mechanisms able to drive the helium mobility can be investigated including surface effects.
We have performed two series of implantation/annealing experiments devoted to study the
thermally activated mobility of helium in different model nuclear materials: pure -Fe (99,95
%), polycrystalline 3C-SiC and 4H- or 6H-SiC single crystals. Fully controlled annealing tests
under vacuum or inert gas partial pressure were conducted after helium implantation. Table 1
summarizes the different implantation configurations and the different annealing conditions
that have been selected. For all the investigated materials, the applied NRA configuration
corresponds to: i) deuteron energy range from 900 keV to 1.8 MeV; ii) detection angle 150°;
iii) 1500 µm thick surface barrier detector; iv) solid angle sustended by the detector 2.44 msr;
v) thickness of the mylar foil in front of the detector 29 µm to be sure to stop backscattered
deuterons having an initial kinetic energy of 1.8 MeV.
Figure 4 displays the flow chart of the scientific approach we have developed for helium
mobility study at JANNUS Saclay. It includes two complementary options. The first one is
5
based on single deuteron energy analysis of the emitted proton spectrum from the nuclear
reaction 3He(d, p0)4He. The deuteron energy is chosen to induce the nuclear reaction with its
maximum cross section near the range of the implanted 3He atoms. The recorded energy
spectrum is then iteratively reconstructed starting from an initial Gaussian helium profile
using the SIMNRA code [17]. The second option is based on the determination of the
excitation curve, i.e. the measurement of the proton yield as a function of the kinetic energy of
the incident deuteron beam. This multi-energy analysis is both justified by the width of the
resonance shown in Figure 2 and by the low energy loss of the high energy emitted proton
that makes difficult the exact depth location of helium. Then, the excitation curve can be
interpreted in terms of helium depth profile using a pure analytical procedure based on
polynomial fitting [18]. More recently, a dedicated simulation code “AGEING” has been
developed. It combines a least square minimization algorithm and a user defined
mathematical description of the migration model [19, 25, 26, 27].
In order to keep things more clear, we have chosen to first describe the basic principles of the
different available options in the following sub-sections. Application examples and discussion
of the obtained results will be presented in section 4.
The main advantage offered by the analytical method based on the 3He(d, p0)4He nuclear
reaction coupled with the detection of the high energy proton lies in its sensitivity around 10 15
3
He/cm2 in standard geometrical conditions. The main drawback of this approach lies in its
poor depth resolution that is controlled by: 1) the low energy loss undergone by the high
energy emitted proton through the sample, 2) the energy straggling undergone by the incident
deuteron before its interaction with 3He nucleus, and 3) the presence of the 29 µm mylar foil
in front of the detector which induces an additional energy straggling for the detected protons.
Each contribution can be separately calculated and then combined to evaluate the final energy
6
dispersion synonymous of helium depth location dispersion. This particular point is discussed
more in details in subsection 4.6.
3.2 Single deuteron energy analysis
In this case, NRA measurement is carried out at only one deuteron energy. This energy value
is calculated with SRIM code [15], in order to match with the depth corresponding to the
maximum of the helium distribution that means at an energy as close as possible to the
resonance of the 3He(d, p0)4He reaction located at 440 keV. The SIMNRA simulation code is
then used to reproduce the experimental spectrum starting from an initial depth profile
obeying a pure Gaussian shape. This data processing approach first gives the helium
distribution histogram composed by a series of layers of different widths assumed to be
homogeneous in composition and finally a smoothed concentration depth profile which is
generally far from the initial Gaussian hypothesis. It must be noticed that the ultimate sublayer width cannot be less than the depth resolution calculated using the RESOLNRA routine
implemented in SIMNRA code [28] (see subsection 4.1 for a precise determination of the
depth resolution). By applying exactly the same reconstitution method for the energy spectra
recorded for as implanted and annealed samples. Considering a Gaussian behaviour for the
core part of the depth profile centered near the end of range, it is possible to evaluate the
broadening of the full half width maximum of the Gaussian peak (FWHM) with temperature.
Taking into account the relationship between the FWHM and the standard deviation () of the
distribution (FWHM = 2.355 ) and using the following classical formalism already applied
on a wide range of materials [18 – 20]:
D = (T2 – 02)/(2 t)
(1)
with D the apparent diffusion coefficient, T and 0 the standard deviations corresponding to
the samples annealed at temperature T or as implanted and t the annealing time. The evolution
7
of the apparent diffusion coefficient with T can be further interpreted in the frame of the
classical Arrhenius assumption to derive an average activation energy value (Ea) according
to:
D = D0 exp (- Ea /kB T)
(2)
with D0 a pre-exponential factor and kB the Boltzmann constant. This approach was used to
study helium migration in various oxides considered as potential transmutation targets or
waste matrices [12, 18 – 20].
Due to the width of the nuclear resonance 3He(d, p0)4He, the assumption of a Gaussian
behavior for the core part of the helium depth profile tends to neglect the presence of eventual
tails located towards the surface and even towards depth. The data processing of the
histogram distribution extracted from SIMNRA energy spectrum reconstitution can be deeply
pursued using the fitting code FITYK specifically dedicated to the decomposition of
analytical binary spectra into its different components selected from a library of mathematical
functions and coupled with a least square calculation procedure based on a Levenberg–
Marquardt algorithm [29].
3.3 Excitation curve analysis
3.3.1) General considerations
The excitation curve method has been described elsewhere [25 - 27, 30]. It consists in varying
progressively the incident deuteron energy and simultaneously detecting the corresponding
emitted proton spectrum. The direct comparison of the total area under the excitation curve
obtained for the as implanted and the annealed samples allows us to evaluate the helium loss
fraction and to describe its evolution with temperature. More precisely, considering that
helium release obeys to a 1st order kinetics [19, 26], one can fit the decrease of the helium
content in the distribution by the following equation:
dC(He)/dt = - C(He) 0 exp(- H / kB . T)
(3)
8
with 0 a classical frequency factor and H the activation enthalpy for helium release. For a
given annealing time ta at a temperature T, the integration gives
f = C0(He) – CT(He) = 1 – exp[-  . exp(-H / kB . T)]
(4)
with a pre-exponential factor  = 0 . ta.
As the detected proton yield I0 (E0) at a given incident deuteron energy E0 is the convolution
of the 3He depth profile (x) with the cross section (E(x)) of the nuclear reaction
3
He(d, p0)4He, it is theoretically possible to extract the depth profile distribution.
Nevertheless, this approach requires to have a good knowledge of the stopping power of the
incident deuterons in the material and of the (d, p) reaction cross-section. These key
parameters need to be expressed as polynomial fitting curves and then combined as
demonstrated by Gosset et al. [18] in zirconia and britholite. The comparison between this
purely analytical data treatment of the excitation curve and the single deuteron energy
approach leads to a rather good convergence when the depth profiles exhibit Gaussian-like
shapes [12, 18].
3.3.2) Capabilities offered by the AGEING code
An alternative to the purely analytical data processing method described above consists in
reconstructing the excitation curves using the AGEING code dedicated to both depth profile
extraction and migration parameters determination [19, 25 - 27]. Data treatment starts from
the same theoretical basis. The initial depth profile is assumed to be Gaussian, and is
characterized by three parameters; the amplitude A, the centroid position x c and the standard
deviation s. The first step of the analysis consists in fitting the three parameters (A, x c and s)
defining the Gaussian profile (x) by using in the first modulus of AGEING computer code, a
modified Levenberg–Marquardt algorithm called NLINLSQ which minimizes an error
function between the experimental and the calculated curves. The second modulus of the
9
AGEING code called PDE_MOL permits to define a mathematical model of helium
migration. A series of differential equations can be written to describe the different physical
mechanisms involved as for example pure diffusion, atomic transport and exchange coupled
with boundary conditions. Among the latter, we assume that the depth distribution must have
a zero value at x = 0, because helium cannot accumulate at the surface. However, no
additional assumption is taken on the diffusion profiles: only the Gaussian assumption on the
implantation profile was kept. The iterative reconstruction of the depth distribution in the
frame of the selected migration model finally allows us to extract the relevant migration
parameters such as apparent diffusion coefficient, transport rate, exchange factor, loss
fraction, etc. This approach was both applied on pure polycrystalline -Fe samples and 4Hand 6H-SiC single crystals as implanted and post annealed.
4. Results and discussion
4.1 SIMNRA depth profiling
Figure 5a shows the result of the iterative reconstitution of the proton spectrum obtained for
the pure -Fe sample implanted by 3 MeV 3He ions at room temperature with a fluence of
about 2.5 x 1016/cm2. The deuteron integrated charge is 20 µC. The corresponding 3He
concentration histogram is displayed on Figure 5b together with the histogram derived from
the analysis of the sample annealed for 1 hour at 1250°C. It is important to keep in mind that
at this temperature, the crystalline structure of iron has changed from -Fe (ferritic) to -Fe
(austenitic). After annealing, the intensity of the helium depth profile is decreased by a factor
about 5 and its width increased by a factor of 1.4. This evolution versus T puts clearly in
evidence the thermal activation of the 3He migration process.
As we mentioned in subsection 3.1, several factors are responsible of the poor depth
resolution of 3He NRA profiling for a high implantation depth, 6 to 8 µm in our case. The
10
energy straggling undergone by the incident deuterons within the target to reach the 3He Rp
region has been evaluated at about 50 keV and the subsequent proton energy dispersion may
reach 75 keV. The maximum values of the proton energy dispersion during its outward path
and its path through the absorber foil were respectively estimated to 12 and 13 keV using the
classical Bohr’s formalism. Then, the final energy dispersion is about 77 keV. Consequently,
we can assumed that the depth resolution is entirely controlled by the proton energy
dispersion induced by the deuteron straggling during its inward path. Using the RESOLNRA
routine [28], we found a depth resolution in the 250 – 300 nm depending on thetarget medium
considered.
4.2 FITYK decomposition
It is possible to improve the relevance of the classical data processing discussed above for
pure -Fe by taking into account the whole depth profile extracted from SIMNRA [17]. The
method is based on the decomposition of the experimental depth profile in several
components by using the FITYK software [29]. We have successively applied this procedure
to helium depth profiles determined in pure Fe (99.5 %) and 3C-SiC.
Figure 6 illustrates this approach in the case of pure Fe (99.95 %) after implantation or
annealing at 1070°C for 1 hour. Two Gaussian helium populations are necessary to reproduce
the helium profiles extracted from SIMNRA reconstitution. The first Gaussian curve is
centered on the depth corresponding to the helium range. It corresponds to the helium atom
associated with the high density of defects induced by helium implantation in the end of range
region. The second Gaussian curve is shifted toward iron surface, it corresponds to the helium
atoms trapped in single vacancies or small vacancy clusters present along the ion path. The
mobility of these helium atoms is probably very high of the order of 10 -16 - 10-15 m2/s.
Considering only the characteristic parameters of the central contribution, we were able to
derive the respective apparent diffusion coefficient (Dapp) and transport velocity of helium (v):
11
i) D1app = 3.8 x 10-18 m2 s-1 at 1070°C; ii) D1app = 7.3 x 10-17 m2 s-1 at 1250°C; iii) 0.10 < v1 <
0.25 µm/h.
The second example of “Gaussian decomposition” performed with FITYK concerns as
implanted and 1100°C annealed 3C-SiC samples. As in the case of pure iron, this
decomposition leads us to discriminate two helium populations (see Figure 7). Their
characteristic parameters are summarized in Table 2. For the as implanted sample, the
population represented by the widest Gaussian (Gaussian 1) is also identified as the helium
atoms associated to point defects and/or small vacancy clusters, able to migrate through the
grain boundaries while the narrowest Gaussian (Gaussian 2) is associated with the helium
atoms incorporated near the ion end of range probably as bigger helium-vacancy clusters and
bubbles. In the case of the 1100°C post-annealed sample a certain amount of helium atoms
located near the ion range (Gaussian 1) moves from the central part of the profile towards the
surface and joins the second population (Gaussian 2). This motion is probably driven by
detrapping mechanisms from helium-vacancy clusters and bubbles activated by thermal
annealing.
4.3 Determination of migration parameters using the AGEING code
In the case of pure Fe (99.95 %) implanted by 3 MeV 3He ions at room temperature
with 2.5 x 1016 ions/cm2, the experimental excitation curves and the corresponding AGEING
depth profiles are displayed in Figures 8a and 8b. We have here assumed that the He
migration model obeys to the following equation derived from second Fick’s law and
including transport and release processes:
C(x)/dt = D 2C(x)/x2 – v C(x)/x – FC(x)
(5)
with C(x) the helium concentration at depth x, D the apparent diffusion coefficient, v the
transport velocity and F the release factor. Table 3 summarizes the main migration data
derived from this reconstitution.
12
Concerning SiC single crystals, the main experimental data have been recently detailed [26].
Here, the assumptions were a little bit different from those admitted for -Fe because two
distinct populations have been discriminated. Population 1 corresponds to the He fraction
trapped near the Rp region and population 2 to helium atoms associated with point defects
and/or small vacancy clusters located close to the surface. Di and vi respectively denotes the
apparent diffusion coefficient and transport velocity of population i. An exchange coefficient
g12 was thus introduced in the migration model:
C1(x)/dt = D1 2C1(x)/x2 – v1 C1(x)/x – g12C1(x)
(6)
C2(x)/dt = D2 2C2(x)/x2 – v2 C2(x)/x + g12C1(x) – F2C2(x)
(7)
This model thus consists in fitting seven free parameters: D1, v1, g12, D2, v2, F2 and TP1(the
trapping fraction of He population 1) by using a trial-and-error method, based on the
minimization of an error function between the experimental and calculated curves by the
dedicated routine NLINlSQ:
Err = (1N(I0exp(n) – I0sim(n))/MAX (I0exp) * N
(8)
where N is the number of measurements, I0exp the experimental and I0sim the simulated data
points [25]. The quality of the fitted parameters is always estimated by the error term (Err).
The main results of this study are presented in Table 4 [26]. These data clearly show a
discrepancy about two or three orders of magnitude between the mobility of the helium issued
from the two populations that have been discriminated.
4.4 Helium release versus T
As detailed in section 3.3.1., the comparison of the total area under the excitation curve
obtained for the as implanted and the annealed samples allows us to evaluate the helium loss
fraction and to describe its evolution with temperature. The helium release in pure -iron can
be correctly fitted by  = 253.78 and H = (0.78  0.08) eV. In the case of 3C-SiC we
13
obtained the following adjustment:  = 61.65 and H = (0.63  0.07) eV. Figure 9 shows the
evolution of helium release versus T for the two investigated materials.
In pure iron (99.95 %), helium mobility starts only at a temperature higher than 837°C. The
helium release reaches 40 % at 1000°C and tends to 70 % at 1250°C (Figure 9a). This strong
temperature effect is probably enhanced by the  –  transition (bcc to fcc) occurring around
910°C [31, 32].
In 3C-SiC polycrystals, helium migration seems to start just below 1000°C (Figure 9b).
Helium release reaches nearly 40 % after annealing at 1200°C for 2 hours.
4.5 Arrhenius behavior
From the apparent diffusion coefficient values determined using the AGEING code for pure
Fe (99.95 %) and assuming an Arrhenius behaviour, the derived activation energy is about
(1.13 ± 0.12) eV [33] (Figure 10b) . In their review paper Lewis and Farrell reported
activation energy values in the range 0.6 – 4 eV for -Fe for annealing temperature less than
898 K [34]. Morishita reported values in the range 1.6 – 3.78 eV for -Fe annealed at
temperature less than 1250 K, depending of the nature of the trapping site [35]. Peterson gave
a range value 2.49 – 2.95 eV for ferritic and austenitic structure respectively [36]. Lefaix and
co-workers published activation energy values between 1.82 to 3.91 eV derived from TDS
measurements on pure iron (99.95 %) [37].
Concerning silicon carbide single crystals, values contained in Table 4 clearly show a small
shift of Gaussian 1 towards the sample surface associated with an increase of its FWHM.
From these data, it is possible to derive an apparent diffusion coefficient at 1100°C equal to
9.7 x 10-19 m2 s-1. Then, assuming an Arrhenius behaviour an average activation energy of
(1.06  0.11) eV was determined. This result is in rather good agreement with the result
extracted from SIMNRA processing (1.28 eV). Our data are also in good agreement with data
available in the literature, i.e. in the range 0.46 – 4.10 eV, depending on the silicon carbide
14
polytype, the chemical composition of impurities and the nature of the trapping sites generally
identified through thermodesorption spectrometry measurements [38 – 42].
Figure 10 illustrates the “Arrhenius behaviour” processing of the diffusion data available for
pure -Fe and 3C-SiC.
4.6 Comparison of the different approaches
As mentioned in section 3.2, the single deuteron energy method may present some limitations
if the investigated depth profile exhibits some tails towards the surface or towards greater
depth. In order to control the relevance of the depth profile extracted from the SIMNRA
fitting procedure, the single energy measurement can be repeated for two others deuteron
energy values located on each side of the helium peak near the half-width of the helium
distribution. In addition, it is also possible to theoretically reconstruct the excitation curve
from the histogram extracted through the single deuteron energy method. For this purpose, we
need to assume that the former depth distribution corresponds to the true helium profile as it is
shown Figure 11 for 3C-SiC as implanted. The agreement between the two curves is rather
good. It must be noticed that the calculated curve is slightly above the experimental one in the
low deuteron energy range and slightly below in the high deuteron energy range. This effect
probably results from the variation of the uncertainty affected to the proton peak area
counting rate versus energy.
To summarize, the single deuteron energy method allows the characterization of the core part
of the helium depth profile located near the ion end of range in the frame of the Gaussian
approximation. The evaluation of the increase of the profile width versus T permits to extract
apparent diffusion coefficient and transport velocity values. The description of the helium
depth profile may be strongly improved by using the decomposition code FITYK associated
with SIMNRA energy spectrum reconstitution. The existence of several helium populations is
thus possible together with the determination of both their characteristic parameters (center,
15
area, FWHM) and the correlated migration data (apparent diffusion coefficient, transport
velocity) versus T. The excitation curve method coupled with AGEING simulations
constitutes the most powerful approach. As the helium depth profile can be completely
covered by varying the deuteron energy, the choice of both the initial depth profile and the set
of differential equations describing the migration model is much more flexible. Another
advantage of the AGEING code lies in the possibility to use a trial-and-error method, based
on the minimization of an error function between the experimental and calculated curves.
All the results discussed in the present work are summarized in table 5 in order to compare the
different data processing methods: “SIMNRA + Gaussian approximation, SIMNRA + FITYK
and AGEING. Concerning pure Fe (99.95 %), only one helium population has been identified
by the different data processing methods. Activation energy, apparent diffusion coefficient
and transport velocity values are in quite good agreement. Helium lost fractions are also in
good agreement. Contrary to -Fe, two distinct helium populations have been identified for
polycrystalline 3C-SiC and 4H-/6H-Sic single crystals. This type of discrimination was
reported for the first time by Gosset in his study concerning helium diffusion in britholite
[30]. Considering the actual progress of our study on helium migration in silicon carbide, it is
difficult to conclude about a different behaviour between single crystals and polycrystals. In
terms of apparent diffusion coefficient, it seems that 3He migration is slightly more
pronounced in SiC polycrystals than in single crystals (table 5). Nevertheless, the activation
values derived from NRA analysis vary in a large range so that they do not allow us to
discriminate the helium behaviour in both media and to show a clear evidence for the eventual
role played by grain boundaries.
16
5. Further analytical developments
5.1 High-energy heavy ion induced elastic recoil detection analysis
Deuteron induced NRA allows the profiling of 3He through very large depth up to 10 µm. If
the implantation range decreases to about 1 µm, the same nuclear reaction can be used but in
this case one prefers detecting the recoil nuclei 4He rather than the emitted protons. As
demonstrated elsewhere [43], the depth resolution is better due to the larger energy loss of the
 particle to emerge from the target.
When the implantation depth reaches less than a few tenths of micrometers, elastic recoil
processes can be usefully investigated. Thus, we have tested the analytical capabilities of
high-energy heavy ion induced elastic recoil analysis to determine the depth profile of 4He
after ion implantation in the very near surface region of pure -Fe. 4He implantation
conditions are summarized in Table 6.
The experimental configuration adopted for high energy HI-ERDA is schematically described
in Figure 12. As mentioned in Table 6, and according to the paper from Markina [22], several
types of incident ions have been tested: 12 MeV 12C4+, 15 MeV 16O5+ and 27 MeV 40Ar9+. The
filter placed in front of the surface barrier detector was 12 µm mylar or 10 µm Al. The
characterization of a pure -Fe sample as implanted at 60 keV with 1.0 x 1016 4He/cm2 is
illustrated in Figure 12 by comparing the two energy spectra recorded. The agreement
between the two measurements appears to be excellent. Processing of this type of data using
the SIMNRA code is in progress.
The implementation of the single beam irradiation/IBA ion beam chamber connected to the
accelerator ÉPIMÉTHÉE with a fully remote controlled four stages micrometric sample
holder coupled with two surface barrier detectors for simultaneous IBA measurements is
nearly achieved. This technical improvement will allow us to determine on line the depth
17
profile of the atomic species immediately after their implantation or after post-irradiation
treatment.
5.2 Nuclear microprobe investigations
Moreover, it would be also possible to carry on complementary investigations through nuclear
microprobe measurements of the 3He volumic distribution in order to discriminate the
influence of the sample microstructure and the possible role played by grain boundaries on the
mobility of helium in polycrystalline materials as it has been already demonstrated by Miro in
apatite [44], Trocellier et al. in various ceramics [12] and Martin et al in uranium dioxide [45].
In order to complete these investigations, helium can be implanted at a lower energy so that
the depth distribution would be located within the first micrometer beneath the sample
surface. In this case, it would be possible to detect the heavy recoil nucleus 4He from the
3
He(d, p0)4He nuclear reaction and to obtain a better depth resolution for the profile as it is
done in CEMHTI Orléans on uranium dioxide [46]. Moreover, implanting helium closer to
the surface, the preparation of TEM thin foils becomes easier and complementary
transmission microscopy observations could be coupled to NRA measurements as in the work
by Lefaix-Jeuland and co-workers [47].
6. Conclusion and perspectives
The multi-irradiation platform JANNUS proves to be a very efficient experimental tool to
investigate the mobility of helium in nuclear materials activated either by thermal treatment or
under irradiation conditions. In the case of 3He, NRA measurements performed either by the
“single deuteron energy” or the “excitation curve” methods are very relevant to determine
depth profile and migration parameters. A significant difference was found in the thermal
behaviour of helium in Fe compared to SiC. In iron, only one helium atom population has
been identified centred at the implanted ion range. Its mobility is characterized by transport
18
and diffusion. For SiC, two populations have been distinguished, one centred at Rp associated
to vacancy-He clusters and bubbles and the second located near the surface and associated to
point defects and small vacancy-helium clusters.
This type of study, based on ion beam analysis can be first easily extended to other substrates.
In the frame of a PhD thesis, work is now in progress at JANNUS Saclay laboratory in order
to study the effects of thermal annealing and ion irradiation on the mobility of 3He in highly
refractory ceramics such as SiC, ZrC, TiC and TiN [48]. It can be also extended to other
atomic species as hydrogen, deuterium and further to fission products as I, Xe, or Cs. Some
studies have been already carried out on Cs behaviour in yttria-stabilized zirconia [49].
Further instrumental development based on the use of 2 or 3 particles detectors or segmented
detectors would strongly improve the sensitivity of the NRA method. Moreover, the coupling
of a multi-detection device with data processing using DataFurnace code [50] would be more
performant in terms of helium localization and migration.
Further analytical developments will be focused on the use of heavy ion-induced elastic recoil
detection analysis (HI-ERDA) to determine the depth profile of 4He after implantation and
subsequent thermal annealing. Another interesting topic to be explored would be the study of
helium migration assisted by irradiation defects by combining the possibility offered by the
different accelerators of the JANNUS Saclay platform (see figure 1).
The possibility to perform simultaneous ion implantation and irradiation also allows us to
study the combined effects able to occur under radiative environment. For example, it
includes the cross-effects between nuclear and electronic energy losses, the interactions
between defects and implanted gas atoms and the possible competition between different light
or heavy implanted chemical species.
19
Acknowledgements
Special thanks are due to the whole JANNUS team and particularly to É. Bordas, H. Martin
and F. Leprêtre for their constant support before, during and after irradiation and ion beam
analysis experiments. We are particularly indebted to Gihan Velisa for his investment in the
presentation of the figures and to Jacques Haussy for his patient support during AGEING
training. Part of this work has been performed in the frame of two internships by J. Runtz on
leave from Polytech Clermont-Ferrand and by A. Laurence on leave from ENSCBP Bordeaux
to whom the authors want to express their sincere thanks. Helium implantations and ion beam
analyses have been performed in the frame of the EMIR network program. Special thanks are
due to E. Jouanny from École des Mines Gardanne for a patient reading of the last version of
this manuscript. Last but not least, stimulating comments from the reviewer have been greatly
appreciated.
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23
Table captions
Table 1: Helium implantation configurations in -Fe, polycrystalline 3C-SiC and 4H- or
6H-SiC single crystals.
Table 2: Parameters of the best Gaussian decomposition obtained with the FITYK code
for as implanted and 1100°C post-annealed polycrystalline 3C-SiC samples.
Table 3: Summary of the data obtained in pure iron (99.95 %) by the excitation curve
method using the simulation code AGEING (after [33]). Owing to the temperature range
explored, these migration parameters are characteristic of the  phase.
Table 4: 3He diffusion parameters in 4H- and 6H-SiC single crystals obtained by the
excitation curve method using the AGEING code [26].
Table 5: Comparison of helium migration data obtained in pure Fe (99.95 %), 3C-SiC
polycrystals and 4H- or 6H-SiC single crystals by the different data processing
approaches: a) apparent diffusion coefficient and transport velocity; b) helium release
fraction and activation energy.
Table 6: Experimental implantation and analysis configurations adopted to test the high
energy HI-ERDA method.
24
Figure captions
Figure 1: General layout of the Jannus-Saclay multi-ion irradiation facility.
Figure 2 : Total cross section for the 3He(d, p0)4He nuclear reaction after Bosch & Hale
[16].
Figure 3: 3He content extracted from the nine different analysis spots performed on the
implanted tungsten foil by using SIMNRA.
Figure 4: Scientific approach developed for 3He mobility studies at JANNUS Saclay.
Figure 5: SIMNRA iterative reconstitution of the proton spectrum obtained for the as
implanted -Fe sample measured by NRA: a) comparison between experimental and
simulated spectra, b) comparison between 3He depth profiles of Fe as implanted and Fe
annealed for 2 hours at 1250°C.
Figure 6: Gaussian decomposition (G1, G2 and Sum = G1 + G2) of the 3He
concentration depth profiles deduced through SIMNRA fitting for as implanted and
1070°C annealed pure Fe (99.95 %) sample (As impl Exp and 1070 Exp) obtained by
using the code FITYK [29].
Figure 7: Gaussian decomposition (G1, G2 and Sum = G1 + G2) of the 3He
concentration depth profiles deduced through SIMNRA fitting for as implanted and
1100°C annealed 3C-SiC samples (As impl Exp and 1100 Exp) obtained by using the
code FITYK [29].
Figure 8: a) Experimental excitation curve obtained for -Fe sample implanted by 3
MeV 3He at room temperature to 2.5 x 1016 ions/cm2 and annealed at 1013 or 1120°C; b)
3He depth profile extracted for pure Fe (99.95 %) sample implanted by 3 MeV 3He at
room temperature to 2.5 x 1016 ions/cm2 and annealed at 1013, 1070 or 1120°C by using
the reconstitution code AGEING after [33].
Figure 9: Annealing temperature effect on the release of 3He in pure Fe (a) and
polycrystalline 3C-SiC (b).
Figure 10: Comparison of the Arrhenius interpretation of 3He release in pure Fe and
polycrystalline 3C-SiC.
Figure 11: Comparison of experimental (Exp.) and calculated (Calc.) excitation curve
for polycrystalline 3C-SiC as implanted by 3 MeV 3He ions at room temperature to a
fluence of 1.0 x 1016 3He/cm2 through a single energy measurement at 1300 keV.
Figure 12: HIERDA experimental configuration to determine 4He depth profile in pure
iron implanted at 60 keV and comparison of the 4He depth profiles obtained with 15
MeV 16O5+ or 27 MeV 40Ar9+ induced HIERDA on as implanted pure iron.
25
Table 1: Helium implantation configurations in -Fe, polycrystalline 3C-SiC and 4H- or
6H-SiC single crystals.
-Fe
Isotope
Energy
Dose (ions/cm2)
Rp
Rp
dpa
He content
(at. %)
Implantation T
Annealing
temperature (°C)
Annealing time
(h)
3C-SiC polycrystals
He
3 MeV
2.5 x 1016
5.3 µm
174 nm
0.65
0.88
He
3 MeV
1.0 x 1016
8.7 µm
211 nm
0.25
0.37
4H-/6H-SiC single
crystals
3
He
3 MeV
1.0 x 1016
8.7 µm
211 nm
0.25
0.37
Room T
837, 1013
1070, 1120, 1150
1
2 (for T  1070°C)
Room T
1000, 1100, 1200
Room T
1100, 1150
3
3
1 to 4
Table 2: Parameters of the best Gaussian decomposition obtained with the FITYK code
for as implanted and 1100°C post-annealed polycrystalline 3C-SiC samples.
Gaussian 1
Gaussian 2
Centre
15
(10 at./cm2)
Width
15
(10 at./cm2)
Centre
15
(10 at./cm2)
Width
15
(10 at./cm2)
As implanted
77719
Post-annealed 1100°C
77670
7672
8123
74746
72456
29166
23383
Table 3: Summary of the data obtained in pure iron (99.95 %) by the excitation curve
method using the simulation code AGEING (after [33]). Owing to the temperature range
explored, these migration parameters are characteristic of the  phase.
Annealing T
(°C)
Annealing
time (h)
Helium loss
(%)
1013
1070
1120
2
1
1
35.7
43.5
43.7
Average apparent
diffusion coefficient
(m2/s)
1.58 x 10-19
2.84 x 10-19
3.40 x 10-19
Average transport
velocity
(µm/h)
0.055
0.021
0.017
26
Table 4: 3He diffusion parameters in 4H- and 6H-SiC single crystals obtained by the
excitation curve method using the AGEING code [26].
D1
(10-19 m2/s)
1100
t
(h)
1
TP1
(%)
0.67
g12
(h-1)
9.0
D2
(10-16 m2/s)
3.0
v1
(µm/h)
0.17
6H
1100
2
3.1
0.07
0.55
6H
1100
3
3.2
0.07
6H
1100
4
2.9
6H
1150
1
6H
1150
6H
Sample
T(°C)
2.3
v2
(µm/h)
1.3
F2
(%)
8.9
Error
(%)
1.15
6H
9.1
6.8
0.4
14.0
1.49
0.50
9.1
2.4
0.1
15.9
2.39
0.07
0.36
9.1
7.8
0.1
26.8
3.28
2.9
0.18
0.48
9.0
7.6
0.9
17.8
1.73
2
2.8
0.15
0.38
9.0
2.3
0.4
29.9
1.96
1150
3
2.4
0.14
0.30
9.1
18.0
0.2
37.2
2.57
4H
1150
1
2.7
0.12
0.48
9.0
8.2
0.6
16.2
0.82
4H
1150
2
3.1
0.05
0.40
9.0
20.0
0.4
22.0
1.97
27
Table 5: Comparison of helium migration data obtained in pure Fe (99.95 %), 3C-SiC polycrystals and 4H- or 6H-SiC single crystals by
the different data processing approaches: a) apparent diffusion coefficient and transport velocity; b) helium release fraction and
activation energy.
a
Fe*
3C-SiC
6H-SiC
Method
Data processing
Apparent diffusion coefficient
(m2 s-1)
Transport velocity
(µm h-1)
Single energy
SIMNRA + Gaussian approximation
0.24 – 0.38
Single energy
SIMNRA + FITYK
Excitation curve
AGEING
3.0 10-18 (1070°C)
2.90 10-17 (1250°C)
3.84 10-18 (1070°C)
7.20 10-18 (1120°C)
4.43 10-17 (1250°C)
2.01 10-19 (1013°C)
1.87 10-18 (1070°C)
4.28 10-18 (1120°C)
Single energy
SIMNRA + Gaussian approximation
0.035 (1200°C)
Single energy
SIMNRA + FITYK
Excitation curve
AGEING
9.53 10-19 (1000°C)
1.45 10-18 (1100°C)
4.7 10-18 (1200°C)
9.72 10-19 (1000°C)
1.79 10-18 (1100°C)
8.61 10-18 (1200°C)
Single energy
Single energy
Excitation curve
SIMNRA + Gaussian approximation
SIMNRA + FITYK
AGEING
1.2 – 4.3 10-18
D2 = 9.04 10-17 (1150°C)
D1 = 3.0 10-19 to 1.5 10-18(1100°C)
D2 = 2.3 to 3.0 10-16 (1100°C)
0.5 (1100°C)
0.15 (v1) (1100°C)
0.1–0.2 (v1)
0.5 (v2)
0.11 – 0.24
0.02 – 0.06
0.03 - 0.12
b
Fe*
3C-SiC
6H-SiC
*
Owing
Method
Data processing
Helium release
(%)
Single energy
SIMNRA + Gaussian approximation
36.4 (1013°C)
42 (1120°C)
Single energy
Excitation curve
SIMNRA + FITYK
AGEING
Single energy
SIMNRA + Gaussian approximation
Single energy
SIMNRA + FITYK
Excitation curve
Single energy
Single energy
Excitation curve
to
the
Activation energy (eV)
Helium release
Core diffusion
1.10 to 1.76
0.78  0.08
35.7 (1013°C)
43.7 (1120°C)
0.58  0.06
0.44  0.04
AGEING
10 (1000°C)
39.5 (1100°C)
5.5 (1000°C)
38.3 (1100°C)
29% (1200°C)
SIMNRA + Gaussian approximation
SIMNRA + FITYK
AGEING
31.5 (1100°C-1h)
39 (1100°C-1h)
33 (1100°C-1h)
temperature
range
explored,
1.98 to 3.67
1.13  0.12
0.50 ± 0.05
these
migration
parameters
0.95  0.10
2.33  0.23
1.52  0.15
1.06  0.11
0.63 ± 0.06
0.75  0.08
4.3  0.40
1.0  0.01 (Ea1)
3.3 to 4.1 (Ea2)
are
characteristic
of
the

phase.
Table 6: Experimental implantation and analysis configurations adopted to test the high
energy HI-ERDA method.
Ion
Energy
Temperature
Dose
Rp
Rp
Implantation
4
He
60 keV
RT
1.0 x 1016/cm2
1840 nm
58 nm
12 4+
C
12 MeV
4.73 µm
226 nm
HI-ERDA analysis
16 5+
O
15 MeV
RT
100 µC
4.32 µm
223 nm
40
Ar9+
27 MeV
4.01 µm
248 nm
30
33
Figure 4
35
36
37
Figure 9a
0,6
Fe
He lost fraction (a. u.)
0,5
0,4
0,3
0,2
0,1
0,0
200
400
600
800
1000
1200
1400
1600
Temperature (K)
38
Figure 9b
He lost fraction (a.u.)
0,5
3C-SiC
0,4
0,3
0,2
0,1
0,0
200
400
600
800
1000
1200
1400
1600
Temperature (K)
39
40
41