Elastic recoil detection analysis of hydrogen with 7Li ions using a polyimide foil as a thick hydrogen reference
Elastic recoil detection analysis (ERDA) with an absorber foil using a 4.2 MeV 7Li2+ beam was utilized for evaluation of hydrogen depth profiles. Since recoil cross-sections when using Li ions as projectiles are not well known, the energy dependent ratio between the experimental yield and the yield calculated using the Rutherford recoil cross-section was obtained from an ERDA spectrum of a thick polyimide (Kapton) sample. It was estimated that this ratio does not significantly depend on sample composition. Therefore it was used for correction of measured spectra analyzed by existing simulation and evaluation programs in which the Rutherford recoil cross-sections were applied. The correction procedure has been verified in round-robin measurements of well-characterized Si:H thin layers. Application of the method for determination of a hydrogen depth concentration profile in hydrogen-containing graphite samples is presented.
Elastic recoil detection analysis of hydrogen with
using a polyimide foil as a thick hydrogen reference
ˇPelicon, Alenka Razpet
, Sabina Markelj, Iztok C
Department for Low and Medium Energy Physics, Joz
ˇef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
Received 27 July 2004; received in revised form 17 September 2004
Elastic recoil detection analysis (ERDA) with an absorber foil using a 4.2 MeV
beam was utilized for evalu-
ation of hydrogen depth proﬁles. Since recoil cross-sections when using Li ions as projectiles are not well known,
the energy dependent ratio between the experimental yield and the yield calculated using the Rutherford recoil
cross-section was obtained from an ERDA spectrum of a thick polyimide (Kapton) sample. It was estimated that this
ratio does not signiﬁcantly depend on sample composition. Therefore it was used for correction of measured spectra
analyzed by existing simulation and evaluation programs in which the Rutherford recoil cross-sections were applied.
The correction procedure has been veriﬁed in round-robin measurements of well-characterized Si:H thin layers. Appli-
cation of the method for determination of a hydrogen depth concentration proﬁle in hydrogen-containing graphite sam-
ples is presented.
2004 Elsevier B.V. All rights reserved.
PACS: 82.80.Yc; 68.55.Nq; 06.90.+v; 85.75.Dd
Keywords: Thin ﬁlms; Ion beam analysis; Elastic recoil detection analysis; Recoil cross-section
Elastic recoil detection analysis (ERDA) with an
absorber foil is a well established technique for
measuring hydrogen depth proﬁles in samples .
Conventionally, helium ions are used as projectiles.
The possibility of employing lithium ions has
recently been explored to achieve better depth
resolution and enhance separation of hydrogen iso-
topes . Besides, lithium ions can easily be ob-
tained using sputtering negative ion sources
available at most tandem accelerator facilities.
The major drawbacks of hydrogen-ERDA with a
Li beam, as pointed out by Mayer et al. , is the
lack of recoil cross-section data and increased
background in the spectra due to nuclear reactions.
0168-583X/$ - see front matter 2004 Elsevier B.V. All rights reserved.
Corresponding author. Tel./fax: +386 1 5612 335.
E-mail addresses: firstname.lastname@example.org (P. Pelicon), alenka.
email@example.com (A. Razpet).
Nuclear Instruments and Methods in Physics Research B 227 (2005) 591–596
In this work, we present a quantitative hydro-
gen depth analysis using polyimide (Kapton) as a
reference material for evaluation of hydrogen-
ERDA spectra measured with a 4.2 MeV
beam. The accuracy of the procedure has been
conﬁrmed in ‘‘Hydrogen in Silicon’’ round-robin
measurements, organized by Bundesanstalt fu
Materialforschung und -pru
¨fung (BAM), Berlin,
where Si:H thin ﬁlms were distributed for hydro-
gen analysis .
The applicability of the method was demon-
strated for a graphite sample from the test limiter
used in TEXTOR, a research tokamak at IPP-
¨lich . This study was initiated in order to
determine precisely the depth proﬁles of hydrogen
in typical tokamak material, graphite, that is fre-
quently used as a plasma facing material.
Measurements were performed at the 2 MV
Tandem accelerator at the Joz
ˇef Stefan Institute
(JSI) in Ljubljana. Lithium ions were obtained
from the sputtering source using a mixture of
LiOH and Ag powder as a target. The energy of
the incident beam was 4224 keV for all the mea-
surements presented in this paper. The beam was
collimated by a 1 ·4 mm rectangular shaping slit
placed in front of the entrance of the experimental
chamber which was equipped with two conven-
tional silicon detectors; the RBS detector at the
scattering angle h=150and the ERDA one at
the recoil angle u=30(Fig. 1). The incident beam
angle aand the exit angle bas measured from the
normal to the sample surface were both 75.An
11 lm thick aluminum absorber foil was inserted
in front of the ERDA detector to separate hydro-
gen recoils hitting the ERDA detector from scat-
tered Li ions.
A mesh charge collector set behind the shaping
slit was used to measure the number of ions hitting
the target . It consisted of a tungsten mesh of
80% transmission placed between two cylindrical
electrodes at a negative voltage (800 V) to sup-
press the secondary electron current from the
mesh. The normalization parameter of the mea-
sured spectra, i.e. the number Nof ions hitting
the sample multiplied by the detector solid angle
, is proportional to the charge qcollected by
the mesh charge collector:
The coeﬃcient Rwas calibrated by measuring the
RBS yield from thin standard targets.
The ratio of RBS and ERDA detector solid an-
, which in our case was equal to
0.82, was determined by an
Am source posi-
tioned on the target wheel. The spectra of alpha
particles were accumulated simultaneously until
the statistical uncertainty in the solid angle ratio
was lower than 1%. The normaliza-
tion parameter NX
needed for quantitative
analysis of spectra was calculated from the accu-
mulated charge on the mesh collector using rela-
3. Results and discussion
The energy spectrum of hydrogen recoils from a
polyimide ﬁlm measured by the ERDA detector is
displayed and compared to a simulated spectrum
in Fig. 2. The calculation was done using the SIM-
Fig. 1. RBS-ERDA set-up used in this work.
592 P. Pelicon et al. / Nucl. Instr. and Meth. in Phys. Res. B 227 (2005) 591–596
NRA code  with an experimentally determined
value of NX
, Rutherford recoil cross-sections
(E) and correction factors for the stopping
powers in polyimide . The computed spectrum
diﬀers from the measured scattering yield since
the actual diﬀerential recoil cross-section r(E)is
non-Rutherford. A discrepancy between the exper-
imental and Rutherford values was also observed
when using He projectiles. For energies of He
above 2 MeV the measured cross-sections were
about twice as high as Rutherford predictions .
There was a hump present in the measured spec-
trum of polyimide between channels 10 and 40 (en-
ergy 300 keV). To investigate the origi n of this
hump, several thick homogenous samples contain-
ing hydrogen, such as SiN:H, were analyzed. The
shape of the recoil spectra matched that of the
hydrogen spectrum of polyimide, therefore it can
be concluded that the origin of the hump is a res-
onance in the recoil cross-section.
In order to use available simulation programs
based on the Rutherford recoil cross-sections, the
) at the energy E
of detected recoils
was divided by the corresponding correction factor
) to compensate for the non-Rutherford val-
ues. The factor F(E
) was determined as the ratio
between the measured yield in the spectrum of
polyimide and the calculated yield assuming Ruth-
erford cross-sections (Fig. 3).
Since the correction factor Fwas calculated at
the recoil energy E
, which depends on the stop-
ping power, rather than at the energy of Li ions
at the recoil event, the question arises if the estima-
tion of Ffor polyimide can be applied to other
materials. The energy of a hydrogen recoil hitting
the detector is
mðEÞis the stopping power in the material
mfor hydrogen and SH
AlðEÞthe stopping power for
recoils in the absorber foil of thickness d. The
of the lithium when it knocks out a
hydrogen atom at depth x
is determined by the
is the initial energy of the incident ion
mðEÞthe stopping power for Li in m.
The dependence of the recoil energy before
entering the absorber foil E
in front of the detec-
tor has been computed for the geometry used in
our set-up for polyimide, carbon and molybdenum
taking into account Ziegler–Biersack stopping
data . These data for Li ions with initial energy
of 4 MeV are shown in Fig. 4.
0 102030405060 708090
Fig. 3. Correction factor F=Y/Y
as a function of the
channel number in the ERDA spectrum.
0 20 40 60 80 100 120 140
exper imenta l
calculation ( Ruth )
Fig. 2. Experimental (thick solid line) and calculated (thin solid
line) ERDA spectra of hydrogen recoils from polyimide
(Kapton) sample (energy per channels is 9.15 keV).
P. Pelicon et al. / Nucl. Instr. and Meth. in Phys. Res. B 227 (2005) 591–596 593
For simplicity, it was assumed that the hydro-
gen content in carbon and molybdenum sample
does not inﬂuence the stopping power. From the
results shown in Fig. 4 it can be seen that the dif-
ferences between E
for diﬀerent materials at a
are smaller than 20 keV, which is a
value comparable to the detector resolution. The
three materials taken into consideration in Fig. 4
actually cover a wide range of stopping powers.
Therefore, it can be concluded that the correction
) calculated for polyimide can be used
for other samples as well. The correction factor
) is a fair approximation of the ratio of the re-
coil cross-sections r/r
In order to assure that polyimide can be used as
a reference for ERDA analysis, the stability of this
material under irradiation with the
Li beam was
investigated. ERDA spectra of polyimide were col-
lected in several consecutive steps at the same spot
on the sample. The yield in a selected energy inter-
val, corresponding to the hydrogen located at
depths from zero to 500 nm in the polyimide, was
measured as a function of cumulative ion dose.
The results representing the hydrogen loss in the
upper 500 nm layer during irradiation of the sam-
ples by 4.2 MeV
Li ions, is given in Fig. 5 together
with the loss from two other thin ﬁlm samples
[3,8]. From Fig. 5 it can be seen that the hydrogen
content in the uppermost 500 nm of the polyimide
foil decreases with cumulative dose. However, dur-
ing a typical spectrum acquisition time, when
the sample is exposed to a dose of less than
, the change in hydrogen concen-
tration is within the experimental error. To accu-
mulate hydrogen spectra of polyimide for
determination of the correction function F(E
we therefore suggest measuring a series of spectra
with doses preferably below 5 ·10
several initially unexposed spots on the polyimide
foil. As also shown in Fig. 5, hydrogen loss from
the two thin ﬁlm samples was not observed for
typical doses required to acquire hydrogen spectra
with good statistics.
The evaluation procedure applying Kapton
correction to measured recoil yields has been
tested in ‘‘Hydrogen in Silicon’’ round-robin
measurements, organized by Bundesanstalt fu
Materialforschung und -pru
¨fung (BAM), Berlin.
Amorphous Si:H thin ﬁlms on silicon were distrib-
uted for hydrogen analysis .Fig. 6(a) shows the
measured spectrum of a Si:H sample labelled 99.
The full line represents the best simulation by
SIMNRA where the target was modelled as a ﬁlm
consisting of three layers and the dotted line repre-
sents single layer simulation. It can be seen that the
three-layer model simulation matches the experi-
mental values better. This suggests that the hydro-
gen concentration in the ﬁlm is not homogeneous.
Four concentration proﬁles are plotted in Fig.
6(b): (i) the result of the three-layer simulation,
1000 2000 3000 4000
in c arbon
in molibd en
in k apton
Fig. 4. Dependence of the energy of recoils before entering the
absorber foil E
on the energy of Li ions in the recoil event E
for diﬀerent target materials.
Si:H on Si (BAM 99) 
alumoox ynitride film , ITN S5 
polyimide (kapt on)
total H conc [1017 at./cm2]
H atomic concentration in kapton [%]
Cumulative dose [1013 ions/cm2]
Fig. 5. Measured hydrogen concentration by ERDA as a
function of cumulative incident dose of
Li ions for polyimide
(right axis) and two other targets (left axis).
594 P. Pelicon et al. / Nucl. Instr. and Meth. in Phys. Res. B 227 (2005) 591–596
(ii) the result of the single-layer simulation, (iii)
BAM reference value and (iv) the round-robin
One of the motivations for this work was to
study the interaction of a hydrogen plasma with
solid surfaces. The hydrogen depth proﬁle mea-
sured for the graphite sample from the test limiter
of the TEXTOR tokamak  that was exposed to
a hydrogen plasma for many hours is shown in
Fig. 7. The energy spectrum was also converted
into a depth concentration proﬁle by an iterative
algorithm originally written for conversion of
ERDA multielemental spectra . In this calcula-
tion the sample is divided into several layers of
. An energy interval DE
ing to Dx
is calculated in the ﬁrst order approxi-
mation using stopping power values of the initial
element composition. New concentration values
are then obtained by comparison of the calculated
yield from the layer and the measured yield in the
corresponding energy interval. The results of the
iterative method were compared to SIMNRA val-
ues obtained from the four-layer simulation. It can
be concluded that the iterative algorithm gives
comparable results and can be used when fast
analysis of data is needed.
Experimental hydrogen elastic recoil spectra
obtained using a 4.2 MeV Li beam cannot be eval-
uated directly by existing simulation codes since
hydrogen recoil cross-sections for ERDA with
Li ions at the relevant energies and angles are
not available in the existing data bases [6,10].As
shown in the case of polyimide they deviate from
the Rutherford values. We estimated that the cor-
rection factor which follows from the measured
ERDA spectrum on polyimide (Kapton) is not
20 40 60 80 100 120
Three-layer simulatio n
Single layer simulat ion
0 50 100 150 200 250
H concentration [at.%]
Li-ERDA. three-layer model
Li-ERDA. one-layer model
BAM reference value
Round-robin average value
Fig. 6. (a) Measured hydrogen-ERDA spectrum of Si:H thin ﬁlm, labelled 99 and selected as a potential thin ﬁlm hydrogen standard
within the round-robin ‘‘Hydrogen in Silicon’’. Measured energy spectrum of a standard sample compared to a model spectrum
calculated by SIMNRA  and ‘‘Kapton correction’’. (b) Hydrogen concentration proﬁles obtained from the single and three layer
models compared to the BAM reference value and the average value of the round-robin.
0 1000 2000 3000 4000 5000 6000 7000
H concentration [at. %]
iter. algorithm 
Fig. 7. Hydrogen depth proﬁles in a graphite sample exposed to
a hydrogen plasma as calculated by SIMNRA and a depth
conversion algorithm from .
P. Pelicon et al. / Nucl. Instr. and Meth. in Phys. Res. B 227 (2005) 591–596 595
strongly dependent on the sample composition.
Therefore, the values obtained for a thick bulk ref-
erence sample such as polyimide can serve for con-
version of energy spectra into hydrogen depth
proﬁles. The accuracy of the depth scale is limited
to the reliability of the stopping power data, simi-
larly as for other heavy ion ERDA techniques.
Our results conﬁrm the predictions of Mayer
et al.  that a Li beam is a suitable alternative
to a He beam for hydrogen-ERDA. The method,
with a carefully performed normalization proce-
dure, yielded very accurate results in the recent
round-robin measurements on thin hydrogen
layers in silicon, initiated by BAM Berlin .
Further experiments with samples also containing
deuterium are planed in order to be able to inves-
tigate the adsorption of hydrogen isotopes in
fusion relevant materials.
The authors would like to thank U. Reinholtz
and P. Weise from Bundesanstalt fu
chung und -pru
¨fung (BAM), Berlin. We are in-
debted to I. Bogdanovic
about recoil cross-sections. Part of this research
was supported by EURATOM, contract no.
 J. Tirira, Y. Serruys, P. Trocellier, Forward Recoil
Spectrometry, Plenum Press, New York, 1996.
 M. Mayer, J. Roth, K. Ertl, Nucl. Instr. and Meth. B 190
 U. Reinholtz, H.P. Weise, Bundesanstalt fu
chung und -pru
¨fung (BAM), Berlin, round robin test
‘‘Hydrogen in Silicon’’, results sent to the participants of
the round-robin, unpublished.
 M. El Bouanani, University of North Texas, in press.
 M. Mayer, Tech. Rep. IPP 9/113, Max-Planck-Institut fu
Plasmaphysik, Garching, 1997.
 J.F. Ziegler, J.R. Biersack, U. Littmark, The Stopping and
Ranges of Ions in Matter, Vol. 1, Plenum Press, New York,
1985, see also http://www.srim.org/.
 N.P. Barradas et al., Nucl. Instr. and Meth. B, submitted
 C. Spaeth, F. Richter, S. Grigull, U. Kreissig, Nucl. Instr.
and Meth. B 140 (1998) 243.
 Sigmabase, Available at http://www.physics.isu.edu/sigma-
596 P. Pelicon et al. / Nucl. Instr. and Meth. in Phys. Res. B 227 (2005) 591–596