Nuclear Instruments and Methods in Physics Research B 269 (2011) 2237–2243

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Nuclear Instruments and Methods in Physics Research B

journal homepage: www.elsevier .com/locate /n imb

Construction of a confocal PIXE set-up at the Jozef Stefan Institute and first results

N. Grlj a,⇑, P. Pelicon a, M. Zitnik a, P. Vavpetic a, D. Sokaras b,e, A.G. Karydas d,e, B. Kanngießer c

a Jozef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Sloveniab Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USAc Institute of Optics and Atomic Physics, Technical University of Berlin, Hardenbergstrasse 36, D-10623 Berlin, Germanyd Nuclear Spectrometry and Applications Laboratory, International Atomic Energy Agency, A-2444 Seibersdorf, Austriae Institute of Nuclear Physics, NCSR ‘‘Demokritos’’, GR-15310 Athens, Greece

a r t i c l e i n f o

Article history:Available online 1 March 2011

Keywords:Micro-PIXEX-ray lensDepth profiling and 3D imagingNuclear microprobe

0168-583X/$ - see front matter � 2011 Elsevier B.V.doi:10.1016/j.nimb.2011.02.072

⇑ Corresponding author. Tel.: +386 1 5885373; fax:E-mail address: natasa.grlj@ijs.si (N. Grlj).

a b s t r a c t

A new confocal PIXE set-up at the Jozef Stefan Institute in Ljubljana was recently designed and built. Itconsists of a silicon-drift detector, a specially designed polycapillary lens and a snout-alignment interfacefor precise positioning. It allows detector movement in all directions and therefore precise alignmentduring the creation of the probing volume and the possibility of simultaneous use of other complemen-tary techniques, including standard l-PIXE measurements with another X-ray detector. A description ofthe new set-up is given, as well as a short presentation of the method itself. Two custom-designed typesof X-ray lenses were tailored and manufactured for this application, a standard semi-lens and a polycap-illary conic collimator; both were characterized and compared within the scope of development of theconfocal PIXE system. First results of depth profiling with the beam scanning mode are shown.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction useful for the characterization of layered material [12,13]. The

Enormous challenges remain for depth-resolved measurementsand three-dimensional elemental tomography using particle in-duced X-ray emission (PIXE). One way to gain layer informationis to change the incident energy or beam impact angle in the so-called differential PIXE method [1–3]. Another way often used isto apply other complementary ion beam methods, like Rutherfordbackscattering spectrometry (RBS), and to measure its spectrasimultaneously with PIXE spectra [4,5]. For three-dimensionalquantitative elemental imaging using PIXE there are even feweroptions. For objects up to about 50 lm in size, a three-dimensionalelemental distribution may be obtained by combined PIXE/STIMtomography, where the sample is sequentially rotated and the en-ergy loss of the primary proton flux is measured at each positiontogether with the emitted X-ray spectrum [6–11]. The problemof such tomography is the need of sample rotation around onefixed axis and the limited achievable size of the analyzed sample.

An alternative approach to three-dimensional analysis with asimple linear sample movement requires restriction of the fieldof view of the X-ray spectrometer. In this so-called confocal set-up, the probing volume is created by overlapping the foci of theproton microbeam with the narrow acceptance solid angle of apolycapillary lens placed in front of the X-ray detector. By thisgeometrical reduction, the signal originating from different depthscan be resolved and the new confocal PIXE method has proven

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+386 1 5885255.

combination of the scanning possibilities and increased lateral res-olution offered by the microbeam [16,17] and the depth-resolvedmeasurements obtained with confocal PIXE leads to possibletomography measurements of micro-particles. Here, the probingvolume is driven through the sample by moving the sample orthe beam, so as to collect X-rays emitted only from the overlappedregion. In general, from a series of spectra recorded at different tar-get and beam positions, three-dimensional images of the investi-gated specimen can be reconstructed [18–20].

In the last years, the confocal PIXE method was developed, theidea followed directly from 3D l-XRF experiments [21,22]. Itscharacteristics were already explored in detail in [12,13,19]. In or-der to create a confocal set-up, the existing system for l-PIXE atthe Jozef Stefan Institute (JSI) in Ljubljana was modified by install-ing a polycapillary lens in front of the regular X-ray spectrometer.The polycapillary lenses were taken from XRF applications and analignment procedure was introduced to create the smallest possi-ble probing volume. Although this is the most important part ofany successful confocal experiment, it was improvised by manuallymoving the detector together with the cryostat that is used forcooling. Without any mechanism that would allow precise move-ments in the direction perpendicular to the detector axis, this taskwas rather demanding.

Recently, a new system, dedicated entirely to confocal PIXE, wasdeveloped and installed at JSI. Optimal detector and X-ray opticswere selected and a mechanical alignment and load lock systemdesigned. Many of the difficulties that were encountered in thefirst experiments were thus by-passed. This paper is devoted to a

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description of the new system and the first characterizationresults.

2. Experiment and the new CF PIXE set-up

Two parameters describe a confocal PIXE experiment – themicroprobe beam profile and its spatial distribution that character-izes the excitation profile and the X-ray lens field of view thatdefines the collection region. Both parts reach their minimumwidth at a certain point from the end of the corresponding focusingdevice. In a confocal set-up, the probing volume is given by theoverlapping region of the two foci. Thus, the essential characteris-tics of the set-up are a well focused proton beam, a lens with anarrow field of view, mechanical stability of the set-up and goodalignment.

X-ray guiding and focusing can be achieved with a polycapillarylens [14,15]. Its main property is the shape of its acceptance (ifworking in collecting mode) or transmission (if working in thefocusing mode) solid angle. The intensity profile of the envelopeof the collected/transmitted rays can be described reasonably wellby an axially symmetric two-dimensional Gaussian function. A de-tailed analysis of measured intensity profiles shows that a twodimensional Gaussian curve does not describe the profile com-pletely, however, it is a good approximation for the experimentalconditions of a standard confocal PIXE measurement [13]. If weadopt this common description of the lens intensity profile, the fullwidth at half maximum (FWHM) describes its size. The FWHMchanges along the lens axis and is different for different photonenergies transmitted by the lens [15]. After testing several lenses,we concluded that the parameter of focal size is in general approx-imately 20% larger in the collecting mode in comparison with thefocusing mode where these lenses are normally used. Thereforethe data given by the manufacturer can never completely coverthe behaviour of the X-ray optics for any application and the per-formance must therefore be checked for each individual lens be-fore making the desired experiments.

The proton beam current density in the plane that is perpen-dicular to the microbeam axis can also be described by the prod-uct of two Gaussian curves. The corresponding beam FWHM inboth directions orthogonal to the beam direction is kept under1.5 lm at the beam focal point for all confocal measurementsdone at the JSI. The beam diverges outside the focal plane alongthe beam direction and the FWHMs are doubled at the distanceof ±450 lm [12]. This distance is much more than the usualmeasurement interval and thus the beam spread can usuallybe neglected. The proton energy used in most of our experi-ments is 2 or 3 MeV.

Fig. 1. Scheme of the new confocal set-up at JSI. It is composed of a silicon drift

The new system was installed in the microbeam experimentalchamber and the new X-ray detector was attached to the chamberby an alignment interface for precise positioning (Fig. 1). Thedetector was a Peltier-cooled silicon drift X-ray detector with anAP 3.3 ultra-thin window in order to allow detection of light ele-ments. Its crystal has an active volume of 30 mm2 � 450 lm andis positioned 7.20 mm inside the detector snout. The resolutionat 5.9 keV is 135–140 eV at a count rate of 3 kHz. For most of theapplications so far, we used 6 lm thick Mylar coated with Al forthe absorber. As the horizontal plane was occupied with a pair ofX-ray detectors for standard l-PIXE, the detector was positionedat an inclination angle of 30� from the horizontal plane. In such away it gazed on the target from above. Since the beam was nor-mally incident on the target, the angle between the detector axisand the target normal was 135.2�. The device was installed on asupport that allowed small shifts of the whole detector snout per-pendicular to the main translational axis. This is achieved by fourmicro-screws attached to the support that is fixed on a detectorflange. The X-ray optics was mounted on the snout of the detector.Two custom-designed X-ray optical elements were tailored andmanufactured for the application. The first was a standard polycap-illary semi-lens, the other a polycapillary conic collimator (poly-CCC) [23]. The whole system is retractable and can be kept in avacuum when the experimental chamber is vented. The schemeof the new set-up is shown in Fig. 1.

Apart from the system for confocal measurements, the micro-beam experimental chamber is equipped with a goniometer thatallows sample movement in all three directions, as well asrotations in the polar and azimuthal planes. In order to check theperformance, the stability and/or the accuracy of the final recon-struction, the X-rays emitted from the target atoms are observedsimultaneously by one of two other of the X-ray detectors available– a high purity germanium (HPGe) or lithium-drifted silicon(Si(Li)), as required by each individual experiment. The HPGedetector is positioned at an angle of 135� with respect to the pro-ton beam direction and has a 25 lm thick beryllium window and a100 lm thick kapton absorber, that can again be changed for dif-ferent applications. The Si(Li) detector stands at 125� to the beamdirection in the same horizontal plane as the HPGe and has a 8 lmthick beryllium window. For reference and additional analysis thechamber is equipped with two more detectors – one detects pro-jectile ions that are backscattered from nuclei in the target sample(RBS detector) and the other collects projectiles transmittedthrough the sample (off-axis STIM). Both are partially-depleted im-planted silicon diodes (PIPS, Canberra). Each detected event in theset of detectors is recorded and saved in a list mode together withthe information on the beam position at the moment of the event.

detector, an X-ray lens, a snout-alignment interface and a load-lock system.

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The target is observed with two cameras/microscopes that allow usto monitor the sample movement and perform an easier and moreaccurate manipulation of the target.

The normalization of X-ray images is done via proton dose mea-surements. The fluence is deduced from the RBS spectrum of pro-tons backscattered from a 3 lm thin gold surface layer of agraphite chopper. The chopper intersects the proton beam with afrequency of 10 Hz. The data for the proton fluence maps are re-corded in parallel with the other spectra in list mode by a partiallydepleted silicon detector (PIPS, Canberra). During off-line data pro-cessing, the spectrum accumulated by the in-beam chopper overan arbitrary scanning area can be extracted and used for the doseinformation.

2.1. Alignment procedure

To achieve the best resolution it is essential to align the focal re-gion of the proton beam with the lens focus. In this way, we canensure the smallest possible probing volume and achieve its appro-priate movement through the sample by simple linear movementof the sample or simple scanning of the sample surface by thebeam.

In the alignment procedure we create a confocal situation andbring together the probing volume obtained and the target. Thereference plane in the experimental chamber is determined bythe field of view of the high magnification microscope. The protonbeam is first focused by applying the fluorescence screen and ref-erence meshes, which are positioned in the focal plane. The unde-flected proton beam direction defines the reference point in thereference plane. To position the lens to view the reference point,we first align the detector direction with a lens mock-up mountedon its snout. The lens mock-up has the same dimensions as thesemi-lens, including a pointing tip which marks the lens focal dis-tance. This tip is brought to some marked area at the target wherethe sample is positioned. Since the axis is oriented in the same wayas the lens axis, we can achieve alignment with a precision of a fewhundreds of micrometers. The alignment interface of the new sys-tem is of crucial importance here, since the whole detector can beeasily moved by precise steps. In the next step, the real lens ismounted and thin monoelemental foils are scanned over in thebeam scanning mode. By moving the detector with the lens for-ward and backward from the sample, we are moving along the lens

Fig. 2. Image of Ka line from a 2 lm thick Ni foil measured with a polycapillaryhalf-lens in beam scanning mode at the focal point. The size of the frame is250 � 250 lm2, while the FWHM obtained is 31 lm.

field of view and can measure the FWHM of its cross section. Anexample of a measured image of the Ni Ka line is presented inFig. 2. The obtained curve of FWHM measured at different posi-tions from the focal point look similar to the one in Fig. 5. At itsminimum position, the lens is focused and the confocal set-up isestablished.

3. First measurements

In the last years, the confocal PIXE method has proven its appli-cability. Several examples have been reported so far, from the firstattempts to understand the method [12], through a detailed anal-ysis of layered material [13], a description of the reconstruction ofthe position of micro-particles distributed in a light matrix [18], la-ter offering detailed discussion of the basics of confocal micro-tomography [19], finishing with a total 3D reconstruction of anirregular particle, composed mostly of one visible element [20].CF PIXE can be used in several measurement modes, each contrib-uting something to the analysis of the sample. The main character-istic of all of them is the fact that the analysis is achieved by asimple scan of the target surface by the microbeam, or by simplelinear movement of the sample along the beam direction. The ba-sics of both modes are presented in Fig. 3. Each mode, or a combi-nation of both can therefore be used for the measurement of lensproperties, layer composition, position and thickness in structuredmaterials or for three-dimensional elemental microscopy ofparticles.

3.1. Lens characterization

With the existing set-up, in the beam scanning mode, where thetarget is kept stationary and scanned by the microbeam, any X-rayoptics working in the collecting mode, can be examined and char-acterized. With the microbeam hitting a thin foil, we create a small(�1 lm2) X-ray source with one or more desired energies, that canbe moved with respect to the central axis of the X-ray optics. Inthis way, numerous properties can be extracted. Since knowledgeof the lens characteristics is crucial for any confocal experiment,these were naturally the first measurements performed with thenew set-up. Both new optics were examined and some of the prop-erties are presented here.

By using thin monoelemental foils in the beam scanning mode,we measure the intersection of the lens acceptance volume withthe target. The position and the shape of the measured ‘‘cloud’’ de-pend on the geometry – on the angles between the beam direction,the detector axis and the target normal. Since we adopt the com-mon description of the lens intensity profile [13], properties likethe radial extension can be extracted from the map by fitting a ro-tated and elongated 2-dimensional Gaussian function to the mea-sured data.

The optics we first used was a 58mkl14 polycapillary semi-lens,shown in Fig. 4 (left device). It was 13.1 mm long and had a focaldistance of 4.9 ± 0.1 mm. The manufacturer’s specifications statedthat this lens has a FWHM of around 23 lm for energies below10 keV and around 14 lm for energies above 15 keV when workingin the focusing mode. Our measurements, presented in Fig. 5, give aFWHM of around 31 lm for the Ni Ka line (7.48 keV). The samegraph demonstrates the change of FWHM, which represents theenvelope of the collecting volume, for different positions fromthe lens edge. We can see the drastic change of FWHM far fromthe focus and a smaller dependence close to it. In our previousexperiments, where a different lens was used, we observed a neg-ligible change of FWHM in the interval of approximately 100 lmaround the focal point [12]. This was crucial for successful mea-surements. For this new lens, this interval is much smaller

Fig. 3. The sample (left) and beam (right) scanning mode in confocal PIXE. In the sample scanning mode, the beam is kept stationary in one point while the target is movedalong the incoming beam. The measured photon yield represents the intensity curve of the lens acceptance volume. In the beam scanning mode, the proton beam scans thestationary target – the measured map is a ‘‘snap shot’’ of the target area that is contained in the acceptance volume of the lens.

Fig. 4. Photographs of the two optical devices used: a polycapillary semi-lens (left)and a polycapillary conic collimator (right). Both were designed and manufacturedby IFG, Berlin.

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Fig. 5. FWHM (crossed marks, left axis) at the Ka line of nickel, obtained byscanning a 2 lm thick Ni foil at different positions along the polycapillary half-lensaxis. Zero marks the position of the optical focus. Circles represent the normalizedintensity of the Ni Ka line from the same lens, again at different distances from thefocus. 700 800 900 1000 1100 1200

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(50 lm) and therefore an accurate alignment is even moreimportant.

In Fig. 5 we can also see the normalized photon yield of the NiKa line taken from the scanned surface of a thin nickel foil at differ-ent positions from the focal point. The transmission was alsostrongly position dependent, its maximum being reached at the fo-cal point, where we observed approximately twice as high a trans-mission than ±(250–300) lm away from it. This fact can be usedfor a quicker alternative in the procedure for creating the confocalsituation. Extraction of the FWHM from a general 2D Gaussianplane can sometimes be a difficult or at least a time-consumingtask during the second phase of the alignment procedure, thatcan be replaced by integrating the total intensity of the specificspectroscopic line obtained from the detector with the lens. Whenthe normalized yield reaches a maximum, the confocal set-up isaligned.

The second optics tested was a 2.5 mm long polycapillary coniccollimator (polyCCC) with a focal distance of 1.45 mm (Fig. 4-right). The polyCCC is composed of hollow glass capillaries thatare not bent but straight. Its acceptance angle restriction is purelygeometrical. The estimated FWHM of the device used in focus(again when working in the focusing mode) is 20 lm for the Crand Fe Ka line. In the collecting mode we measured 25 lm forNi Ka line. The normalized yield shown in Fig. 6 shows a similar po-sition dependence as for the standard semi-lens. However, here thedependence is much steeper, because the transmission of the poly-CCC drops much more quickly and reaches one third of the maxi-mum already at a distance of around 150 lm from the focalpoint. The total transmission is also smaller than that of the

Fig. 8. Map of Y Ka line from YF3 deposited on a thin nuclepore polycarbonateaerosol membrane measured with the polyCCC. A strong halo is observed aroundthe otherwise typical ‘‘cloud’’. Scanned area is 500 � 500 lm2.

N. Grlj et al. / Nuclear Instruments and Methods in Physics Research B 269 (2011) 2237–2243 2241

semi-lens. This is interesting, since the polyCCC should have abroader capture angle than the semi-lens. The explanation lies inthe fact that the active crystal of the detector in our set-up ispositioned rather deep inside – 11.2 mm from the bottom end ofthe semi-lens and 14.2 mm from the bottom end of the polyCCC.While the semi-lens is collimating captured X-rays, collection fromthe polyCCC is based entirely on geometry and the transmitted X-rays forms some sort of cone. Therefore, almost 80% of the col-lected photons are lost, due to the smaller effective solid angledetermined by the size of the active crystal. If there was no limita-tion (detector edges, size of the crystal), the solid angle our poly-CCC should cover is 0.028 steradians, while the real solid angleachieved by our system was 0.0064 steradians.

The geometry issue made work with a polyCCC demanding. Weobserved strange intensity profiles of the measured acceptancevolume cross-section when we were far from the focal point, asshown in the profiles on Fig. 7. The shape resembles that of a cometand a 2D Gaussian function cannot be accurately fitted to it. Whenthe foil was brought into the focal distance of the lens, the imagewas again standard, like the one in Fig. 2. Therefore, we do notshow the change of FWHM for different distances along the lensaxis, but only the already mentioned intensity change. Anotherproblem with the polyCCC is the strong halo observed at higherenergies (already above 9 keV), that again broaden the acceptancevolume further. The halo comes from X-rays that are not guidedthrough the channels, but are transmitted directly through the lensmaterial. An example of this effect is shown in the image of Y Ka

(14.88 keV) in Fig. 8.When placing the X-ray optics in front of the detector, a differ-

ence in the background of the spectrum can be observed. Fig. 9shows spectra measured with the new SDD detector induced bythe same target, a 2 lm thin nickel foil. The first is the spectrumtaken with the raw SDD with 7 lm of kapton and 2 lm of alumi-num as absorbers. No optics were placed in front. The next twoare spectra from the same detector with a semi-lens or polyCCCmounted on it, respectively. Both used 6 lm thick mylar coatedwith Al as the absorber. Regardless of the different absorbers,2 lm of Al cuts energies from approximately 1.5–3.0 keV more effi-

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Fig. 7. Image of Au M line, obtained from a 2 lm thick Au foil, measured with the polycannot be described by a Gaussian function are visible.

ciently than just a thin Al coating, the difference in the measuredspectra is radical. The adapted lens adds additional backgroundto the spectrum, due to fluorescence of the lens material inducedby transmitted X-rays. With the polyCCC, this effect is much stron-ger. The reason was already given, the Ni Ka photons have enoughenergy to avoid the guiding channels and simply pass directlythrough the lens. On their way they interfere with the materialatoms and (a) induce their characteristic X-rays, or (b) producephoto-electrons and subsequent bremsstrahlung radiation. Thesame process happens in the polycapillary semi-lens, however,here the majority of the photons are reflected at the channelboundaries and only a small number of them are transmittedthrough the material. The effect is also smaller since the lens itselfis much longer and therefore the absorption is larger.

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CCC approx. 100 lm from the focal point. Non-symmetrical intensity profiles that

Fig. 9. Three different spectra, taken from the same sample – a 2 lm thick Ni foil.The first thick line is the X-ray spectrum from the new SDD detector with no lensand 7 lm of kapton + 2 lm of Al as absorber. The dashed line represents thespectrum from the SDD with the polycapillary semi-lens (or half-lens) mounted and6 lm of Al coated Mylar as an absorber. The normal black line shows the spectrumfrom the SDD with the polyCCC and the same absorber as before. The difference inthe background is significant.

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Our analysis showed that the polycapillary conic collimator isnot a practical optical system and its performance is limited withrespect to a standard polycapillary semi-lens, in spite of its smallerFWHM.

3.2. Depth-resolved measurements

A lot has already been discussed about the analysis of layeredmaterials [12,13]. Naturally, some similar measurements havebeen done with our new system in order to test its performance.In this study we tested the depth-resolved measurement possibil-ities within the beam scanning mode. All depth-resolved measure-ments done so far used the sample scanning mode. There, thebeam stayed at one point and the sample was moved forward

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Fig. 10. Depth profile measurements of an artificially made layered sample, consistedpolycapillary half-lens with no sample movement. Yet, we can deduce the information(275 lm apart) with no numerical analysis at all. Scan size was 500 � 500 lm2.

and backward along the beam axis. In this way, the maximumtransmission probability of the lens is moved through differentpositions inside the sample and the signal from the surface (whichis the strongest and in normal PIXE obscures signals from other re-gions) can be distinguished from the signal coming from differentdepths. The recorded signal as a function of the target positiongives an intensity curve that represents the photon yield and isthe convolution of the lens acceptance probability function withthe X-ray yield obtained from the target by proton irradiation. Thiscurve changes radically due to sample thickness, density, layer po-sition or elemental concentrations – therefore all this informationcan be extracted from the experimental data, even with a qualita-tive analysis only.

However, the same information can be obtained even from thesimple beam scanning mode, as long as the sample has a largelyone-dimensional structure with depth and exhibits feature scalesthat can be resolved given the depth resolution of the confocal sys-tem. Fig. 10 gives an example of the measurement of an artificiallymade structured sample, where we pressed together two thin(2 lm) Ni foils and 25 lm of kapton in between as a sandwich.The foils were simply pressed together between two sample hold-ers and glued slightly. Therefore, a huge gap appeared during theevacuation of the chamber and the real position of the two nickelfoils was not known. From simple scanning of the sample in oneposition, we could distinguish the two Ni layers and calculate thedifference between them to be around 275 lm. No further analysiswas made at this point, however, this showed the possibility to de-duce depth information from just one measurement at one sampleposition. Of course, one should again stress the fact, that for thiskind of measurement, the sample should be composed of homoge-neous plane layers at a large (several hundred lm) extent. Furtherapplications will follow in the future.

4. Conclusion

Recently, a new permanent confocal PIXE set-up was installedat the microbeam experimental chamber at JSI. Its performance

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of parallel and homogeneous Ni/kapton/Ni foils. The image was measured with aabout the number of layers (2), their composition (pure Ni) and relative position

N. Grlj et al. / Nuclear Instruments and Methods in Physics Research B 269 (2011) 2237–2243 2243

was tested and the first measurements were reported here. A basiccomparison between a polycapillary semi-lens and a polycapillaryconic collimator, both working in the collecting mode, was de-scribed. The analysis showed that the polyCCC is not practical forthis type of CF PIXE measurement, mostly due to its small focal dis-tance and large size. The standard polycapillary half-lens is moresuitable for our applications. The radial extension of its field ofview is still not satisfactory. Regardless of the individual design,the lens used here has similar properties to those used before.The FWHM of the X-ray optics defines the depth resolution thatis still several times (and at least an order of magnitude) greaterthan the lateral, defined by the cross section of the microbeam.Obviously, the FWHM of the lens cannot compete with the lateralsize of the microbeam, however it would be very desirable if itwould be reduced further in order to be comparable with thelength of the steps of sample movement. These can now reachfew micrometers, however due to the large lens field of view, suchsmall steps are not used in the experiment. Smaller FWHM wouldincrease depth resolution. Just recently, we have received anotherpolycapillary lens for examination, that should have a smaller ra-dial extension but otherwise similar properties as 58mkl14. Thislens could improve the performance and resolution of our system.Its testing as well as the study of some more properties of otherlenses are planned for the future.

Another feature discussed was depth-resolved measurementsin the beam scanning mode. The number of layers, their elementalcomposition and relative position in the sample could be directlydeduced from just one scan of the sample (with required proper-ties) in one position. More elaborate analysis will follow, however,this fact alone places the confocal PIXE method in the first rankwith other ion-based spectroscopic techniques for depth profiling.

Acknowledgements

The help of the Institute for Scientific Instruments IFG, Berlin[24], is greatly appreciated.

The work was supported by the following programs and projectsof the Slovenian Research Agency: P1-0112, J7-0352, Fellowship for‘‘Young researchers’’ (N.G.), Support of Accelerator Laboratory in

the frame work of Research Infrastructure. The work was also par-tially supported by the European Community as an IntegratingActivity ‘‘Support of Public and Industrial Research Using Ion BeamTechnology (SPIRIT)’’ under EC contract No. 227012.

We thank Dr. A.R. Byrne for editorial assistance.

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