Materials Analysis Using Fast Ions
W. BOHNE, A. DENKER, S. LINDNER, J. OPITZ-COUTUREAU,
J. RÖHRICH, E. STRUB
Ionenstrahllabor, Hahn-Meitner-Institut, Glienicker Str. 100, D 14109 Berlin, Germany
Abstract. At the Ionenstrahllabor a great variety of ions with variable energies up to several MeV/u can be produced.
Performing proton induced X-ray emission (PIXE) with protons of 68 MeV, heavy elements can be detected via the K
X-rays in addition to their L X-rays. The large proton range and the small absorption coefficients for the K X-rays result
in an analyzable depth of several millimeters. The L to K line intensity ratio yields further information on the composition of the objects. Examples of measurements on archaeological and art historical objects will be shown. The use of
heavy ion elastic recoil detection analysis (HI-ERDA) allows the determination of the distribution of all elements in materials up to a depth of several micrometers. Especially the measurement of the hydrogen concentration is possible with
high sensitivity. The experimental setup and measurements on solar cell materials will be presented.
Protons with energies of 68 MeV have much larger
ranges in the investigated materials. In addition, the
X-ray production cross-sections for the K lines of
heavy elements (Z > 50) is at least a factor of 100 larger than at 3 MeV [3,4]. Heavy elements may be detected via their K X-rays which also have a much
smaller absorption coefficient than the L X-rays in the
analyzed material [5]. E.g. it is possible to detect thin
gold foils even behind 3 mm of lead glass [6]. The
proton intensities necessary for the high-energy PIXE
analysis are small due to the large cross-sections. The
energy loss of high energy protons, i.e. the risk of radiation damage, is smaller than that of 3 MeV protons.
Hence, high-energy PIXE provides a non-destructive
analytical technique for the analysis of thick layers.
INTRODUCTION
In materials research it is of crucial importance to
determine the elemental composition and structure of
solids. At the ion beam laboratory ISL various analytical techniques using high-energy ions are provided.
Two of the techniques will be presented: The investigation of art and archaeological objects has to be nondestructive. PIXE allows elemental analysis without
sampling on the object. Complex layered structures
like solar cell materials are analyzed by heavy ion
ERDA.
HIGH-ENERGY PROTON INDUCED
X-RAY EMISSION
Experimental Setup
Proton Induced X-ray Emission (PIXE) using proton energies between 1 and 4 MeV is an established
method for analytical problems in biology, geology,
environmental issues as well as for non-destructive
analysis in art and archaeometry [1,2]. Elements with
Z > 50 are detected via their L X-rays due to the decrease of the K X-ray production cross-sections with
increasing Z. As the range of protons at these energies
is relatively small, the method is well suited for the
analysis of thin layers, e.g. inks on parchments or surfaces of metals.
The experimental setup has been described in detail
in [6]. ISL can provide proton beams up to 72 MeV.
The variations in cross sections above 40 MeV are
small for medium Z, and the increase for Z > 60 is
low. Since there are no special advantages for one
specific energy, the eye tumor therapy beam of
68 MeV is used as regular proton energy. This beam
runs one week each month. The switching time from
therapy to PIXE is about half an hour after the therapy
session has been finished.
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
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gold layer, i.e. gilding, on top of a different material
can be excluded. Similar ratios were observed for all
investigated spots. Therefore, the support for the
enamel was made of a massive gold alloy.
The high-energy proton beam exits the vacuum of
the beam line through a 80 µm thick Kapton foil, so
that the objects to be analyzed remain in normal atmosphere. Hence, there are no restrictions in size and
sensitivity of the object. The small beam intensities,
usually 0.1 to 1 pA, depending on the thickness of the
object to be studied, are measured by a commercial
ionization chamber, placed at the beam exit. Typically
counting times of 200 s and beam diameters of 1 mm
are used. As the proton intensities are small, there is
no risk of sample activation. Two X-ray detectors are
mounted at 135° relative to the beam direction: a
12.5 mm2 Si(Li) with a resolution of 155 eV and
a 300 mm2 HPGe with 180 eV at 5.9 keV. The data
are recorded and stored on a commercial PC multichannel analyzer. Reference spectra from thin and
thick samples of known composition and the items to
be studied are measured using an identical setup. This
gives the production ratio for 68 MeV protons of the
various K and L X-rays. Also γ-lines from nuclear
reactions, which might be misleading, are identified
this way.
FIGURE 1. Spectra of the Prussian medal 21.210a (dotted)
and a thin gold foil (solid). The large intensity of the gold K
lines of the medal implies the use of a massive gold alloy, a
gilding can be excluded.
Example: Prussian Medals
For massive metal alloys the composition can be
estimated by comparing the counts in the different
peaks to those measured on similar material. A real
quantitative determination was difficult due to the fact
that the irregular shape of the medals did not allow
measurements with exactly the same geometry for all
spots. In addition, on some spots the gold support was
still covered by remnants of the enamel, which is visible via Pb lines in the spectra. For the medal 21.210a
the composition was estimated to be 78 % gold, 17 %
silver and 5 % copper.
In the frame of the thesis “Gap-Filling on Enamel
Illustrated by Three Medals of the German Historic
Museum” [7] the medals were analyzed by highenergy PIXE. The main topic of the thesis was to find
a suitable material for the completion of the medals in
the restoration process. To examine their usability
under museum conditions, commercial products were
tested as well as materials commonly used in conservation science. Following to a test on the light aging
properties of all materials, a grit section test was carried out on mock-ups to examine the adhesive properties of selected conservation materials on gold and
enamel. To specify the composition of the mock-ups,
the composition of the medals was analyzed using
micro X-ray fluorescence (for the enamel) and highenergy PIXE (for the support). The aim of the PIXE
analysis was the determination of the elemental composition and especially the distinction between massive and gilded parts.
Example: Middle Age Brooches
In the 7th century, noble ladies used so-called disc
brooches to close their coats. These brooches were
made of iron and decorated by an inlay of silver- or
gold-colored wires, gaining a particularly decorative
effect. Inlayed disk brooches are known more or less
from all early middle age grave fields in south-west
Germany. It is noticeable, that certain decorative patterns were used only in the Rhein-Main-area or the
lower Neckar region. Five of these brooches, belonging to the Landesmuseum Mainz, were investigated in
a collaboration with the Phillips-Universität Marburg.
During the restoration process in the sixties, the
brooches were covered by a plastic resin for stabilization and for prevention of further corrosion. The removal of the protective layer would damage the objects. The quality of the brooches and their fragility
On all three medals, different points were measured: At the different spots where the enamel was
missing, on the central medallion as well as on the
suspension eye and suspension ring. Fig. 1 shows the
spectrum of medal 21.210a compared to the results of
a thin gold foil measured with the same setup. The
intensity ratio of the gold Lα/Kα1 lines of the medal
in this spectrum is 1.25, whereas the thin gold foil
yields a ratio of 44. For comparison, a 1 cm thick lead
sample gives a ratio of Lα/Kα1 of 1.3. Hence, a thin
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depth profiles from the measured energy spectra simultaneously for all components of the sample. The
detection sensitivity is almost the same for all elements. For hydrogen the sensitivity is even enhanced
by a factor of four. When using heavy projectiles no
restrictions of the detectable mass exist.
made a non-destructive analysis mandatory, able to
provide information even from behind a 1 mm thick
plastic layer. The energy loss of 68 MeV protons in
this coating is about 1 MeV. Owing to the small lateral
straggling at this incident energy, the size of the beam
spot on the metal surface will be nearly the same as on
the plastic surface, thus allowing the investigation of
fine local structures. The analytical task was mainly
the identification of the metals. For iron X-rays the
transmission through 1 mm plastic is still about 20 %.
Experimental Setup
At ISL the element respectively mass identification
is done by means of the TOF (time-of-flight) method,
i.e. the coincident measurement of energy and flight
time for each recoil [9]. With this setup (time resolution ~ 150 ps) a mass resolution of one unit up to about
A = 50 is achieved. Since the detection efficiency for
the used channelplate timing detectors is less than
100 % for protons and alpha particles, a second telescope is installed using the RF signal from the cyclotron as timing signal. With the beam pulse width of
about 0.5 ns it is still possible to resolve light elements
but with an efficiency of 100 % even for hydrogen.
The detection limit is in the order of 0.001 at.% for all
elements. The normally used energy of the heavy projectiles of about 1.8 MeV/u enables depth profiling up
to a maximum layer thickness of several micrometers
with a depth resolution of less than 20 nm at the surface. In the last years hundreds of samples from various fields, like semiconductors, metals, and in a few
cases polymers and teeth, are analyzed routinely.
The results obtained on the decorative patterns confirmed the previous measurements on the same kind of
objects from Eltville [8]: For these masterly crafted
objects no gold was used for the gold-colored inlays.
Instead of gold, an alloy containing copper, zinc and
tin was applied. One special feature was observed on
brooch 9./8.05: It is the only brooch containing manganese in the rivets. The art historical meaning of this
result is presently under evaluation.
Example: Chalkopyrite solar cells
The main focus of ERDA at ISL is the study of
photovoltaic materials, mostly produced at the solar
energy research department of the HMI. In general,
the influence of the different deposition methods and
their parameters on the composition (e.g. stoichiometry, impurities, diffusion) of the layers is under investigation. Fig. 3 represents a TOF versus energy graph
(scatterplot) obtained from a chalkopyrite solar cell
structure (glass/Mo/Cu(In,Ga)(S,Se)2/ZnSe).
The
measurement was performed with a 350 MeV gold
beam. Besides the incorporation of the transport agent
I used in the chemical vapor deposition for the growth
of the ZnSe buffer layer, the diffusion between the
absorber and the buffer layer was of special interest.
All components are clearly resolved down to a depth
of about 3 µm. The incorporation of I in the buffer
layer is obvious. The good mass resolution enables the
separation of Cu and Zn via the isotope 68Zn. Thus,
the stoichiometry of both the absorber and the buffer
layer can be determined. Furthermore, the diffusion of
In into the buffer layer is observed [10].
FIGURE 2. Spectra of the rivets (solid) and iron base plate
(dashed) of brooch 9./8.05. The rivets from brooch 9./8.05
contain Mn, which distinguishes this brooch from all other
investigated objects.
HEAVY ION ELASTIC RECOIL DETECTION ANALYSIS
For the ERDA-method the samples are irradiated
with high energetic heavy ions at grazing incidence.
The energy as well as the number of the outscattered
atoms (recoils) of the sample components are measured at a fixed angle relative to the beam direction.
Since the scattering probability is given by the Rutherford cross section and all the experimental parameters
are known, ERDA is a standard free absolute method.
Owing to the computable element specific energy loss
in material it is possible to calculate the concentration
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clearly seen, that the Ar content is below the detection
limit of about 10 ppm, but some H contamination was
detected. A detailed analysis reveals that the H content influences the W:S stoichiometry.
CONCLUSIONS
High-energy PIXE is a well suited method for the
non-destructive analysis of thick layers, especially for
heavy elements. Bulky and sensitive objects can be
analyzed. Heavy ion ERDA provides a standard free
absolute method for the analysis of thin layers.
ACKNOWLEDGEMENTS
FIGURE 3. Scatterplot from the measurement of a
(glass/Mo/Cu(In,Ga)(S,Se)2/ZnSe) multilayer structure, irradiated with 350 MeV Au ions.
The authors would like to thank the ISL crew for
all their help. The authors are indebted to M.C. Blaich
for the interesting collaboration with the brooches.
Example: WS2 layer
Fig. 4 depicts the scatterplot from a measurement
of a WS2 layer, another potential absorber material.
The WS2 film was deposited on a SiO2-covered Si
substrate by DC magnetron sputtering. Ar was used as
the sputter and H2S as the reactive gas. The scatterplot
demonstrates the possibility to detect simultaneously
the lightest element H and heavy elements with masses
of about A=200 (recoiled W and backscattered Au).
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